Repair of solder bumps

Abstract

A method of circuit manufacturing includes inspecting an array of solder bumps on a circuit board to identify solder bumps having a height above the substrate greater than a predetermined maximum. A first laser beam is directed at the identified solder bump to remove a selected amount of solder material from the identified solder bump. Alternatively or additionally, additional solder bumps having a height above the substrate less than a predetermined minimum are identified, and one or more molten droplets of solder material are deposited on the additional solder bumps. After removing or depositing the solder material, a second laser beam is directed at the identified solder bump with sufficient energy to melt and reflow the solder material of the identified solder bump.

Description

Technical Field

[0001] The present invention generally relates to the manufacture of electronic devices, and more particularly to soldering methods and systems.

Background Art

[0002] A solder bump is a conductive contact element and is used, for example, when flip-chip bonding a semiconductor chip to a circuit board. For this purpose, solder bumps are formed in an array of high density and narrow pitch on a circuit board, for example using photolithography techniques. Solder bump technology has the advantages of small size and short connection length, enabling high connection density, low manufacturing cost, and high package functionality.

[0003] However, the generation of solder bumps needs to be carefully controlled because one defective solder bump may lead to an open circuit or a short circuit when connecting the chip to the substrate. For this reason, many methods have been proposed to repair defects in solder bump arrays. For example, Patent Document 1 describes a method for improving the yield of solder bumps. In one embodiment, the method for improving the yield of solder bumps divides a connected solder bump (i.e., a solder bridge) by laser cutting with a laser head. In another embodiment, reflow is performed by laser at the position of skip printing of the solder bumps.

[0004] Some of the methods for repairing solder bumps involve replacing defective solder balls. For example, Patent Document 2 describes a method for reworking a ball grid array (BGA) of solder balls using a single ball extraction / placement device, which has a heatable capillary pickup head that is reinforced by vacuum suction as needed. A defective solder ball is identified and pulled out by the pickup head and discarded. A non-defective solder ball is picked up by the pickup head and placed at the vacant mounting site and thermally softened for attachment to the work.

[0005] As another example, Patent Document 3 describes a laser soldering repair process. The laser cleaning process is performed by irradiating a repair laser beam onto a repair area of a substrate. A solder ball is provided on the cleaned repair area of the substrate, and the solder ball is heated using a soldering laser beam to attach the solder ball to the repair area.

[0006] In laser direct writing (LDW) technology, a laser beam is used to generate a surface patterned with a spatially resolved three-dimensional structure by controlled material ablation or deposition. Laser-induced forward transfer (LIFT) is an LDW technology that can be applied when depositing micro-patterns onto a surface.

[0007] In LIFT, laser photons provide a driving force to eject a small volume of material from a donor film onto an acceptor substrate. Generally, the laser beam interacts with the inside of the donor film, and the donor film is coated on a non-absorbing carrier substrate. In other words, after the incident laser beam propagates through the transparent carrier substrate, the photons are absorbed by the inner surface of the film. When a specific energy threshold is exceeded, the material is ejected from the donor film onto the surface of the acceptor substrate. By appropriately selecting the donor film and the laser beam pulse parameters, the laser pulse ejects molten droplets of the donor material from the film, lands on the acceptor substrate, and hardens.

[0008] The LIFT system is particularly (but not exclusively) useful for printing conductive metal droplets and wiring for the purpose of manufacturing electronic circuits. This type of LIFT system is described, for example, in Patent Document 4, the disclosure of which is incorporated herein by reference. This patent describes a printing apparatus including a donor supply assembly, the donor supply assembly providing a transparent donor substrate having opposing first and second surfaces, and a donor film formed on the second surface, the donor film being configured to be disposed proximate to a target region on an acceptor substrate. An optical assembly is configured to simultaneously direct a plurality of output beams of laser radiation in a predetermined spatial pattern through the first surface of the donor substrate and onto the donor film to induce the release of material from the donor film onto the acceptor substrate, thereby writing a predetermined pattern onto the target region of the acceptor substrate.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0010] Embodiments of the present invention described below provide improved methods and systems for manufacturing electrical circuits and devices.

Means for Solving the Problems

[0011] Accordingly, according to one embodiment of the present invention, a method of circuit manufacturing is provided, which includes inspecting an array of solder bumps on a circuit board to identify solder bumps on the board having a height higher than a predetermined maximum value. A first laser beam is directed at the identified solder bumps to remove a selected amount of solder material from the identified solder bumps. After removing the solder material, a second laser beam is directed at the identified solder bumps with sufficient energy to melt and reflow the solder material remaining on the identified solder bumps.

[0012] In some embodiments, directing the first laser beam includes directing one or more pulses of laser energy to impinge on the identified solder bumps. In the disclosed embodiments, each pulse has a pulse duration of less than 50 ns, or even less than 10 ns. Additionally or alternatively, inspecting the array includes estimating the amount of solder material to be removed from the identified solder bumps according to the height of the identified solder bumps, and directing one or more pulses includes selecting the number of pulses to apply to the identified solder bumps according to the estimated amount.

[0013] Further additionally or alternatively, directing the first laser beam includes focusing the first laser beam to impinge on the identified solder bumps with a beam diameter smaller than the bump diameter, such that ablation of the solder material creates a cavity in the central region of the identified solder bumps. In the disclosed embodiments, directing the second laser beam at the identified solder bumps melts and reflows the solder material to fill the cavity.

[0014] In one embodiment, directing the first and second laser beams at the identified solder bumps includes repeating the steps of directing the first laser beam to remove the solder material and directing the second laser beam to melt and reflow the solder material multiple times until the height of the solder bumps is below a predetermined maximum value.

[0015] Additionally or alternatively, directing the first laser beam comprises disposing a transparent cover on the substrate in proximity to the identified solder bump and directing the first laser beam to irradiate the identified solder bump through the transparent cover, whereby debris emitted by ablation of the identified solder bump adheres to the cover.

[0016] In some embodiments, directing the second laser beam comprises directing one or more pulses of laser energy to impinge on the identified solder bump. Typically, the pulse duration of each pulse is less than 100 μs. Additionally or alternatively, directing the first and second laser beams comprises generating both the first and second laser beams using a single laser having a variable pulse duration. Further additionally or alternatively, directing the second laser beam comprises focusing the second laser beam to impinge on the identified solder bump with a beam diameter smaller than the bump diameter.

[0017] In one embodiment, directing the second laser beam comprises applying sufficient energy to the identified solder bump using the second laser beam to melt the entire volume of the identified solder bump. Alternatively, directing the second laser beam comprises applying an amount of energy to the identified solder bump using the second laser beam, which is selected to melt only a portion of the identified solder bump.

[0018] In some embodiments, inspecting an array of solder bumps includes identifying yet another solder bump on the substrate having a height lower than a predetermined minimum value, the method including depositing one or more molten droplets of solder material onto yet another solder bump and directing a second laser beam at yet another solder bump with sufficient energy to melt the deposited solder material and reflow it onto yet another solder bump. In such an embodiment, emitting one or more molten droplets includes applying a first laser beam to direct one or more pulses of laser energy through a donor substrate to induce the emission of the molten droplets.

[0019] According to one embodiment of the present invention, a method of circuit manufacturing is further provided, which includes inspecting an array of solder bumps on a circuit substrate to identify a solder bump on the substrate having a height lower than a predetermined minimum value. One or more molten droplets of solder material are deposited onto the identified solder bump, whereby the droplets adhere to and solidify on the identified solder bump. After depositing the solder material, a laser beam is directed at the identified solder bump with sufficient energy to melt the deposited solder material and reflow it onto the identified solder bump.

[0020] In some embodiments, depositing one or more molten droplets comprises ejecting one or more molten droplets from a donor substrate proximate to the identified solder bumps by a process of laser-induced forward transfer (LIFT). Typically, the donor substrate is transparent and has opposing first and second surfaces, and on the second surface, a donor film containing a solder material, the donor film being proximate to the identified solder bumps, and ejecting one or more molten droplets comprises directing one or more pulses of laser radiation to pass through the first surface of the donor substrate and impinge on the donor film, inducing the ejection of one or more molten droplets of the solder material from the donor film onto the identified solder bumps. In one embodiment, directing one or more pulses of laser radiation in the LIFT process and directing the laser beam with respect to the identified solder bumps comprises using a single laser having a variable pulse duration to perform both ejecting molten droplets and melting and reflowing the deposited solder material.

[0021] Additionally or alternatively, inspecting the array comprises estimating the amount of solder material to add to the identified solder bumps according to the height of the identified solder bumps, and depositing one or more molten droplets comprises selecting the number of droplets to deposit on the identified solder bumps according to the estimated amount.

[0022] In the disclosed embodiments, depositing one or more molten droplets and directing the laser beam with respect to the identified solder bumps comprises repeating the steps of depositing a molten droplet of the solder material and directing the laser beam to melt and reflow the solder material a plurality of times until the height of the solder bump exceeds a predetermined minimum value.

[0023] According to one embodiment of the present invention, an apparatus for circuit manufacturing is additionally provided. The apparatus includes an inspection module configured to acquire image data regarding an array of solder bumps on a circuit board. The laser module is configured to output a first laser beam configured to remove solder material from the solder bumps and a second laser beam configured to melt and reflow the solder material at the solder bumps. The control circuit is configured to process the image data to identify solder bumps in the array on the substrate having a height higher than a predetermined maximum value, control the laser module to direct the first laser beam at the identified solder bumps to remove a selected amount of solder material from the identified solder bumps, and after removing the solder material, direct the second laser beam at the identified solder bumps with sufficient energy to melt and reflow the solder material remaining at the identified solder bumps.

[0024] According to one embodiment of the present invention, an apparatus for circuit manufacturing is further provided. The apparatus includes an inspection module configured to acquire image data regarding an array of solder bumps on a circuit board. The deposition module is configured to emit molten droplets of solder material. The laser module is configured to output a laser beam configured to melt and reflow the solder material at the solder bumps. The control circuit is configured to process the image data to identify solder bumps in the array on the substrate having a height lower than a predetermined minimum value, control the deposition module to deposit one or more of the molten droplets of solder material onto the identified solder bumps, whereby the droplets adhere to and solidify on the identified solder bumps, and after removing the solder material, control the laser module to direct the laser beam at the identified solder bumps with sufficient energy to melt the deposited solder material and reflow it into the identified solder bumps. The present invention will be more fully understood from the following detailed description of its embodiments in conjunction with the following drawings.

Brief Description of the Drawings

[0025]

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DETAILED DESCRIPTION OF THE INVENTION

[0026] Overview When generating an array of solder bumps on a substrate, it is important that all bumps are present and not only electrically isolated from each other, but also that all solder bumps are approximately the same size. For example, in a solder bump array used for flip chip mounting, if the volume of the solder bump is too small, the height on the substrate will be lower than that of adjacent ones, and there is a possibility that the circuit will remain open when the chip is mounted on the array. On the other hand, if the volume of the solder bump is too large, the height of the bump will simultaneously increase, and the excess solder material will melt and spread during the reflow stage after chip mounting, potentially causing a short circuit with other solder bumps and connecting to the circuit pads. One defective solder bump, whether it is too small or too large, may impair the function of the entire circuit.

[0027] In order to avoid a reduction in yield due to such solder bump defects, it is necessary to inspect the solder bumps on the circuit board and repair the defective solder bumps identified during the inspection. The repair steps should desirably include both the removal of excess solder material from bumps that are too large and the addition of solder material to bumps that are too small. In some embodiments of the present invention, both of these steps are performed at the same repair station. Alternatively, the step of removing solder material from over-sized bumps and the step of adding solder material to under-sized bumps can be performed separately and independently of each other. The embodiments of the present invention described herein provide a solution for repairing both over-sized and under-sized solder bumps.

[0028] In some embodiments, the inspection module inspects an array of solder bumps on the circuit board to identify solder bumps on the substrate that are higher than a predetermined maximum height. When such a solder bump is identified, the laser module directs a laser beam at the solder bump to remove a selected amount of solder material from the solder bump. This ablation typically changes not only the size of the solder bump but also its shape relative to those adjacent in the array. Thus, after removing the solder material, the laser module directs another laser beam at the solder bump with sufficient energy to melt and reflow the solder material remaining on the identified solder bump, thereby giving it a rounded shape similar to the other solder bumps in the array. This ablation technique can also be used to remove excess solder material from solder bumps that do not have the desired rounded shape, even if their height is not excessive.

[0029] Typically, the laser beam used in both the ablation step and the reflow step is a pulsed beam, but the pulse duration and, if possible, other beam parameters are different. They can be generated by the same laser or by different lasers with appropriate characteristics. In the ablation step, a plurality of consecutive pulses can be used, and the number of pulses is adjusted according to the initial height of the solder bump, i.e., the amount of material to be removed. In some cases, the ablation step and the reflow step are repeatedly applied in a plurality of cycles until the height of the solder bump falls below a predetermined maximum value.

[0030] Additionally or alternatively, the inspection module identifies solder bumps on the substrate that are lower than a predetermined minimum height. In this case, one or more molten droplets of the solder material are deposited on each of the undersized solder bumps so that the droplets adhere to and harden on the solder bumps. After depositing the solder material, the laser beam is directed at the solder bumps with sufficient energy to melt the deposited solder material and reflow it onto the identified solder bumps. The number of droplets deposited on each such solder bump depends on the height of the bump. The droplet deposition step and the reflow step can be repeatedly applied in a plurality of cycles until the height of the solder bump exceeds a predetermined minimum value. The same technique can be applied to fill solder bumps that are completely missing from the array.

[0031] In the embodiments described below, the LIFT process is used to deposit droplets onto undersized solder bumps, although other means for ejecting the droplets may alternatively be applied. In the LIFT process, a pulse of laser radiation is focused onto a donor film of solder material, which is formed on the surface of a donor substrate in proximity to the solder bumps, and the molten droplets of solder material are ejected from the donor film onto the solder bumps. The same laser is used for LIFT ejection as is used for ablation of over-sized solder bumps, as described above, and the laser beam focusing and possibly other parameters can be appropriately adjusted. Additionally or alternatively, the laser used in the LIFT process can also be used in the reflow step. Alternatively, different lasers may be used for different process steps.

[0032] Accordingly, the present embodiment provides a comprehensive solution to the problem of solder bump defects. By using laser technology for ablation and deposition of solder materials, the techniques described herein are applicable to any type of solder bump array, including high-density arrays of very small solder bumps with diameters of 20 μm or less, as well as large-scale solder bumps with diameters of 150 μm or more. These techniques are equally applicable to conventional low-temperature solders such as tin-based solder and high-temperature solders such as silver alloys.

[0033] System Description FIG. 1 is a schematic side view of a system 20 for solder bump repair according to an embodiment of the present invention. In the example shown, the system 20 is applied to inspect and repair an array of solder bumps 22 on a circuit board 24, for example, on a semiconductor, dielectric, or ceramic substrate, as is well known in the art. The over-sized bumps 26 protrude higher above the substrate 24 than the other bumps 22, while the under-sized bumps 28 are at a lower height than the other bumps. During the repair process, the substrate 24 is held on a suitable mount, typically an adjustable mount such as a translational stage 58.

[0034] The inspection module 30 acquires image data regarding the array of solder bumps 22. The inspection module 30 typically includes one or more optical sensors with depth sensing capabilities, as is well-known in the art. For example, the inspection module may include a pair of image sensors with an optical system suitable for stereoscopic imaging, or may include a pattern projector that projects structured light onto the substrate 24 and an image sensor that acquires an image of the pattern for triangulation. Alternatively, the inspection module may include an interferometer or a time-of-flight sensor that scans over the solder bumps 22 to measure their respective dimensions. The term "image data" as used in the context of this specification and the claims is to be broadly understood to include any type of data that can be used in reconstructing the three-dimensional (3D) profile of the features on the substrate 24.

[0035] The control circuit 32 processes the image data output by the inspection module 30 to measure the height of the solder bumps 22 and identify bumps such as bumps 26 and 28 whose height is higher than a predetermined maximum value or lower than a predetermined minimum height. The control circuit 32 typically includes a general-purpose computer processor, which is programmed with software to execute the functions described herein in conjunction with a suitable interface for communicating with and controlling the other components of the system 20. Alternatively or additionally, at least some of the functions of the control circuit 32 may be performed by a digital signal processor (DSP) or hardware logic components, which may be hard-wired or programmable.

[0036] For solder bump repair, system 20 includes a laser module 33 that includes one or more lasers, and an appropriate optical system for directing an appropriate laser beam at substrate 24. In the illustrated embodiment, laser module 33 includes both an ablation laser 34 and a reflow laser 38, as well as a LIFT laser 36 that also functions as part of a deposition module 37 described below. For simplicity, the functions and characteristics of these lasers are described herein as if the lasers were all separate units (this is one possible implementation of laser module 33). Alternatively, a single laser that emits short high-energy pulses may perform the functions of both ablation laser 34 and LIFT laser 36. The same laser may also be configured to function as reflow laser 38. Lasers 34, 36, and 38 emit optical radiation in the visible, ultraviolet, and / or infrared ranges at appropriate wavelengths, with appropriate temporal pulse lengths and focus qualities, to perform the functions described herein, which are further detailed in the following description.

[0037] Ablation laser 34 typically emits short pulses, for example pulses with a pulse length of about 50 ns, and high fluence, for example in the range of 5 - 15 J / cm 2 . Laser 34 can operate at any wavelength in the visible, ultraviolet, or near-infrared range that is absorbed by solder bump 22. Alternatively, even shorter laser pulses, for example less than 10 ns, or even in the range of 1 ns, may be used. Beam scanner 40 directs one or more pulses from laser 34 to impinge on a solder bump, such as bump 26, from which solder material is to be removed. Each pulse removes a certain amount of solder material from the bump. Thus, control circuit 32 can select the number of pulses directed at bump 26 based on the total amount of material to be removed, as indicated by the height of the bump. Focusing optical system 46 focuses the beam onto bump 26, typically with a spot diameter smaller than the diameter of the solder bump, for example a spot diameter of about 10 μm or less.

[0038] Adding solder material to undersized solder bumps is performed using deposition module 37, which in this example includes LIFT donor substrate 52. LIFT laser 36 emits short pulses, typically with a pulse duration of about 1 ns, under the control of control circuit 32 towards donor substrate 52. Donor substrate 52 typically includes a thin flexible sheet of a transparent material, and the side facing circuit substrate 24 is coated with a donor film 54 containing a specific solder material or a plurality of solder materials. Alternatively, donor substrate 52 may include a rigid or semi-rigid material. Beam deflector 42 and focusing optics 48 direct the radiation pulses from LIFT laser 36 to pass through the upper surface of donor substrate 52 and thus impinge on the donor film 54 on the lower surface in a spatial pattern determined by control circuit 32.

[0039] Each laser pulse induces one or more droplets 56 of solder material from donor film 54 to be emitted onto solder bumps identified as undersized, solder bump 28 in the illustrated example. Droplets 56 adhere to and harden on the target solder bumps. Each droplet adds a certain amount of solder material to the bump. Thus, control circuit 32 may select the number of droplets deposited on bump 28 based on the total amount of material to be added, as indicated by the height of the bump.

[0040] After either ablation to remove excess solder material or deposition of droplets for the purpose of adding solder material to a predetermined target solder bump, the reflow laser 38 irradiates the solder bump with sufficient energy to melt and reflow the solder material, thereby returning the solder bump to a desired rounded shape and the normal height of the adjacent solder bumps 28. The beam deflector 44 and the focusing optics 50 direct the radiation from the reflow laser 38 to impinge on the target solder bump. The beam energy and other parameters of the reflow laser 38 are selected to melt the solder bump while minimizing thermal damage to the substrate 24. The beam may have sufficient energy to melt the entire volume of the solder bump or only a portion of the solder bump (e.g., if a preceding ablation or deposition step affects only the upper portion of the solder bump and reflow of the entire solder bump is not required).

[0041] To ensure that the thermal effects of solder bump reflow are sufficiently localized to minimize the impact on the substrate 24 and surrounding solder bumps, in some embodiments of the present invention, the reflow laser 38 emits pulses of laser energy rather than a continuous wave (CW) beam. The optics 50 focus the beam to impinge on the target solder bump with a beam diameter small enough so as not to melt adjacent bumps, and the beam diameter may be smaller than the bump diameter. However, the beam diameter is large enough to melt the entire area removed or covered with melted droplets. For example, the beam diameter used in the reflow step may be between about half and two-thirds of the bump diameter. For solder bumps with a diameter less than 100 μm, it is desirable that the pulse duration be less than 100 μs, and for very small solder bumps, e.g., those with a diameter less than 40 μm, the pulse may be even shorter, e.g., on the order of 10 μs. These short and powerful laser pulses are also beneficial in reducing oxidation of the solder material during the reflow process and enabling the reflow process to be performed under ambient atmospheric conditions. The use of short laser pulses is also advantageous in reducing the sensitivity of the process to small misalignments of the laser beam and the thermal dissipation characteristics of the solder bump.

[0042] In order to make the pulse duration adjustable with respect to the sizes of different solder bumps and the melting depth, the reflow laser 38 may include, for example, an appropriate fiber laser or a high-power diode laser. When the laser is adjusted to have a sufficiently wide range of pulse durations up to the nanosecond range, it can also function as the ablation laser 34 and, if possible, the LIFT laser 36.

[0043] Method for repairing solder bumps FIG. 2 is a flowchart schematically showing a method for repairing solder bumps according to an embodiment of the present invention. This method will be described with reference to the elements of the system 20 shown in FIG. 1 for the sake of convenience and clarity. However, instead, the principle of this method may be implemented in other system configurations, and all of them are considered to be within the scope of the present invention. For example, separate subsystems may be used for ablation of over-sized solder bumps and for addition of material to under-sized solder bumps.

[0044] This method starts at the inspection step 60, where the inspection module 30 acquires image data regarding an array of solder bumps (SB) 22, 26, 28,.... As described above, the term "image data" in this context refers not only to a two-dimensional image in the plane of the substrate 24 but also to depth data in a direction perpendicular to the substrate. The control circuit 32 processes the image data to determine the respective height H i of each solder bump B i i The control circuit 32 compares the measured bump height with a reference design height H0 in the bump classification step 62. For each solder bump, the control circuit 32 calculates a height deviation ΔH i i =H i i -H0. Bumps with a relative deviation exceeding a specific threshold δ, that is, |ΔH i i | / H0>δ are classified as defective, while deviations below the threshold are ignored. In other words, the value of δ defines a specific maximum height and minimum height, and if a corresponding bump exceeds or is less than that, it is identified as defective.

[0045] The control circuit 32 selects, in the bump selection step 64, one of these defective bumps B i for repair. When the height deviation ΔH i of the bump is negative, it is assumed that the volume V i of the solder material in the bump is also smaller than the designed volume, that is, ΔV i is also negative. For example, the bump 28 in FIG. 1 satisfies this criterion. In this case, the control circuit 32 sends the bump to the solder deposition branch 66. On the other hand, like the bump 26, when the height deviation ΔH i (therefore, ΔV i ) is positive, the control circuit 32 sends the bump to the solder removal branch 74.

[0046] In the solder deposition branch 66, the control circuit 32 determines, in the deposition amount estimation step 68, the volume ΔV i of the solder material added to the solder bump B +i . The volume ΔV +i can be estimated by comparing the measured height and diameter of the solder bump with the designed height. Based on this volume and other characteristics, such as the diameter of the bump and the type of solder material, the control circuit 32 also selects a recipe to apply to the repair of the solder bump. The recipe may indicate, for example, the number of droplets deposited on the solder bump, the position where each droplet is deposited within the bump region, and whether all the droplets are deposited at once or deposited in two or more steps with reflow of the deposited droplets after each step.

[0047] Based on the selected recipe, the control circuit 32 places the donor substrate 52 at an appropriate position close to the solder bump (e.g., the bump 28), and then, in the LIFT step 70, fires the LIFT laser 36 one or more times to eject the droplets 56 onto the solder bump. The number of droplets is selected such that the volume of the solder material ejected from the donor film 54 onto the bump 28 cumulatively reaches the volume set in step 68. In other words, when each droplet has a volume δV, the number N of laser pulses is such that N×δV is ΔV +iIt is selected to be approximately equal to. After the selected number of droplets are deposited on the solder bump 28, the control circuit 32 directs the beam from the reflow laser 38 and irradiates the bump in the local laser reflow step 72.

[0048] In the solder removal branch 74, it sometimes happens that the solder bump is too high, not because it contains excess solder material, but because it contains one or more air bubbles. In this case, ablation of the bump followed by reflow may cause the height of the bump to be lower than the desired minimum value. To avoid this type of situation, oversize solder bumps are optionally irradiated by the reflow laser 38 in a pre-reflow step 75 to melt the bump and release the trapped air. Next, the ablation laser 34 is applied only if the height of the bump still exceeds the desired maximum value after this pre-reflow step to remove the solder material. Alternatively or additionally, if it is found that the height of the solder bump has become too low after ablation, the bump may then be returned to the deposition branch 66.

[0049] Regardless of whether the pre-reflow step 75 is performed, the control circuit 32 then determines, in the ablation volume estimation step 76, the volume ΔV of the solder material removed from the solder bump B i (e.g., bump 26). -i This volume in this case can also be estimated by comparing the measured height and diameter of the solder bump with the reference design height. Similar to the case of the solder deposition branch, the control circuit 32 selects the recipe to apply when repairing the solder bump based on the volume ΔV of the solder bump -i and other characteristics. In this case, the recipe indicates the number of ablation pulses to apply to the solder bump, and in some cases the pulse duration and intensity, and the pattern of ablation (e.g., a circle with a diameter approximately equal to half the diameter of the bump). The recipe may also indicate whether the excess solder material is removed all at once or in two or more steps with reflow of the remaining solder material after each step.

[0050] Based on the selected recipe, in ablation step 78, control circuit 32 fires ablation laser 34 one or more times to remove material from solder bump 26. The number of pulses is selected such that the volume of solder material removed from the solder bump cumulatively reaches the volume ΔV set in step 76. -i After the selected number of ablation pulses, control circuit 32 directs the beam from reflow laser 38 and irradiates the bump in local laser reflow step 72.

[0051] Following step 72, inspection module 30 is restarted in verification step 80 to measure the height of the repaired solder bump. (Alternatively, control circuit 32 may delay step 80 until multiple bumps are repaired, and then inspect all of these bumps together as in step 60). The height deviation ΔH of the bump i needs to be reduced at this point compared to the height before the repair process. If the relative deviation is below the threshold δ, i.e., |ΔH i | / H0 < δ, the solder bump is considered to be in a satisfactory state in repair completion step 82. Control circuit 32 then returns to step 64 here, selects the next solder bump for repair, and continues until no defective solder bumps remain on substrate 24.

[0052] Alternatively, if the relative deviation measured in step 80 still exceeds the threshold δ, i.e., |ΔH i | / H0 > δ, the solder bump is still considered to be defective in defective bump detection step 84. In this case, control circuit 32 returns this bump to either solder deposition branch 66 or solder removal branch 74 as needed. LIFT deposition step 70 or ablation step 78 and the subsequent reflow step 72 are repeated one or more times as needed until |ΔH i | / H0 < δ.

[0053] Solder Bump Ablation Technology Figures 3A and 3B are schematic cross-sectional views of solder bump 26 before and after laser ablation, respectively, according to an embodiment of the present invention. As shown in Figure 3A, the solder bump 26 has an initial height H1 that is greater than the nominal value H0. The radius of the solder bump 26 is R1, which is greater than the nominal radius R. Since the diameter D of the base (pad) is known, the initial volume of the solder bump 26 is given by the equation for a hemisphere.

Number

[0054] To reduce the volume of the solder bump 26, the control circuit 32 estimates the extra volume to be removed, ΔV = V1 - V0. In some cases, it may be advantageous to remove the extra volume in multiple steps and / or with different ablation patterns, as will be described with reference to the following figures. However, in this example, the height of the solder bump 26 is simply reduced by an amount h by ablation of the cap 90, as shown in Figure 3B, to produce a solder bump 92 having a flat circular surface with diameter d.

[0055] The top parameters are determined based on the measured height H1 and base diameter D of the solder bump 26. The bump radius is given below.

Number

Number

Number

[0056] Based on the above formula, the control circuit 32 calculates the parameters of one or more ablation pulses directed by the ablation laser 34 at the solder bump 26 to obtain the solder bump 92. Following ablation, the reflow laser 38 is fired to melt the solder bump 92 to obtain a rounded bump having a height of approximately H0 and a radius of approximately R.

[0057] FIG. 4 is a schematic cross-sectional view of a solder bump 96 after laser ablation according to another embodiment of the present invention. In this case, the optical system 48 focuses the beam from the ablation laser 34 onto a solder bump with a more sharply oversized diameter such that the beam impinges on the solder bump with a beam diameter smaller than the bump diameter. Thus, ablation of the solder material creates a cavity 94 of diameter d and depth L in the central region of the identified solder bump. The top of diameter d and height h (smaller than the removed top of FIG. 3B) is likewise removed. Subsequently, melting by the reflow laser 38 causes the solder material to reflow and fill the cavity 94, and the solder bump returns to the desired rounded shape.

[0058] The energy and diameter of the ablation laser beam are selected as in the previous embodiments to remove a volume of solder material corresponding to the dimensions of the top and cavity calculated by the control circuit 32. The approach shown in FIG. 4 is particularly advantageous for reducing the amount of debris scattered around the area of the solder bump during ablation. Since the debris is conductive, it may cause a short circuit if not completely removed. In this case, depending on the depth of the cavity 94, a substantial portion of the debris is trapped within the cavity and then simply reflows onto the solder bump when melted by the reflow laser 38. The laser pulse parameters, as well as the depth and aspect ratio of the cavity 94, can be optimized to even extend the cavity down to the underlying pad, achieving the desired ablation volume while minimizing debris scatter.

[0059] FIG. 5 is a plot schematically showing the volume of material removed from a solder bump as a function of the number of laser pulses applied to remove the material. This plot demonstrates that the amount of solder material removed from the solder bump increases approximately proportionally to the number of laser pulses applied. Thus, the amount of solder material removed per laser pulse can be calibrated, and the number of ablation pulses applied to a given solder bump can be selected according to the amount of solder material to be removed.

[0060] Referring again to FIG. 4, in order to reduce the range in which debris scatters during ablation, it is desirable that the cavity removed in the solder bump be as narrow as possible. However, if the aspect ratio of the cavity is too high, bubbles may remain in the solder bump after reflow. Further, when the aspect ratio is high, the amount of the cavity removed may be less than the actual amount of solder material removed from the over-sized solder bump. To minimize the diameter of the cavity removed while alleviating these difficulties, in some embodiments, the control circuit 32 repeats the ablation step and the reflow step two or more times to reduce the height and volume of the solder bump within desired limits. This iterative approach keeps the volume removed at each step small enough to enable local ablation at the center of the bump and prevent over-deep removal. Subsequent local reflow steps cause the solder bump to return to a spherical shape, after which the next ablation is performed.

[0061] Figures 6A - 6D are schematic cross - sectional views of solder bump 102 at successive stages of a repetitive process of this kind of laser ablation and reflow, according to one embodiment of the present invention. In Figure 6A, a small cavity 100 is removed by the solder bump 102. The reflow laser 38 is applied to melt the solder bump, creating a solder bump 104 with reduced height and volume as shown in Figure 6B. The ablation laser 34 removes yet another cavity 106 in the solder bump 104 as shown in Figure 6C. Finally, as shown in Figure 6D, the reflow laser 38 melts the solder material again, which reflows to form a rounded solder bump 108 with the desired height and volume.

[0062] Figures 7A - 7C are schematic cross - sectional views of solder bump 26 at successive stages of laser ablation and reflow, according to another embodiment of the present invention. In contrast to the previous embodiment where the reflow laser 38 is applied with sufficient energy to melt the entire volume of the solder bump after each stage of ablation, in this case, the energy is reduced, and as a result, only a part (the upper part in this example) of the solder bump melts and reflows. This approach is advantageous in reducing the dissipation of heat to the substrate 24 and the surroundings of the solder bump. It is less susceptible to the influence of changes in the internal structure of the solder bump, and thus less susceptible to the influence of associated changes in thermal conductivity that could affect the entire melted volume and temperature.

[0063] FIG. 7A shows an oversize solder bump 26 of height H1 from which a specific volume ΔV is removed to reduce the bump to a nominal volume and height H0. For simplicity, as shown in FIG. 7B, an ablation laser 34 is operated in this example to remove the cleanly cut top 90, leaving a flattened bump 92. The height h of the top 90 is selected based on the diameter d1 such that the volume of the top is exactly equal to the excess volume (ΔV = V1 - V0). High-speed laser reflow continues depending on the duration and energy of the laser pulses, melting the solder only to a depth L. Since the melted phase depth L is less than the bump height after ablation (H1 - h), reflow occurs only on the upper portion 110 of the bump volume. The lower portion 112 remains solid. As shown in FIG. 7C, the resulting restored shape of the solder bump 114 has a diameter d2 and does not exactly match the nominal bump shape, whereby the height H2 of the bump is less than the nominal height (H2 < H0), but the bump volume is approximately equal to the nominal volume V0.

[0064] As described above, laser ablation of metals typically generates scattered metal droplets and other energy debris, as well as metal gas and plasma. The scattered debris can contaminate the surrounding area and can also cause inaccuracies in the ablation process if the debris returns to the solder bump. Oxidation of the debris can also potentially affect the electrical characteristics of the solder bump.

[0065] Figure 8 is a schematic cross-sectional view of solder bump 120 during the ablation process, showing a debris capture technique according to an embodiment of the present invention. In this embodiment, a transparent cover 124, such as a suitable slide glass, is placed on the circuit board in proximity to solder bump 120. Beam 122 is directed by ablation laser 34 (FIG. 1) and irradiates solder bump 120 through transparent cover 124, thereby removing cavity 126 to a depth L below the transparent cover. Debris 128 emitted by ablation adheres to cover 124, thereby capturing the debris and preventing it from redepositing on the solder bump and the surrounding substrate. Cover 124 is placed in proximity to solder bump 120 to maximize the collection volume and collect the removed residue before they cool by interaction with the ambient air.

[0066] Capturing debris using this type of transparent cover is beneficial not only in the ablation of solder bumps, but also in other laser micromachining applications, particularly when removing metal.

[0067] Solder deposition technique FIG. 9A is a micrograph showing the deposition of solder droplets 132 to increase the volume of solder bump 130 according to an embodiment of the present invention. As can be seen from this figure, the droplets released from donor film 54 (FIG. 1) adhere to the solder bump. The droplet volume and the number of droplets deposited are selected to constitute the total volume of solder material added to the solder bump.

[0068] FIG. 9B is a micrograph showing solder bumps 134 following the reflow stage subsequent to the deposition stage of FIG. 9A, according to one embodiment of the present invention. After deposition of the droplets 132, the reflow laser 38 is activated to melt the droplets, along with some or all of the volume of the solder bump 130 itself, such that the solder bump is reflowed into a suitably rounded shape. This cycle of droplet deposition and reflow can be repeated multiple times to reach the total volume of solder bumps required. In the example shown in FIGS. 7A-7C, the energy applied during the reflow stage can be limited, such that the melt depth is similarly limited, i.e., only the upper portion of the solder bump 130 is melted together with the droplet 132.

[0069] By finely tuning the LIFT of the solder material to provide a stable ejection regime, each pulse from the LIFT laser 36 can result in a single droplet of a selected volume. For example, with a donor substrate 52 comprising a donor film 54 having a thickness in the range of 300-800 nm including the solder material, and laser pulses having a pulse duration in the range of about 1 ns to 20 ns, a pulse energy in the range of 1-5 μJ, and a laser spot diameter on the donor film in the range of 30-50 μm, the droplet volume can be controlled in the range of about 50-300 fL. The direction of droplet ejection under these conditions is well controlled, such that the donor substrate 52 can be positioned 0.3-0.5 mm away from the circuit board 24 and still achieve accurate deposition as shown in FIG. 9. Alternatively, smaller or larger solder droplets can be deposited provided that the donor structure and laser parameters are appropriately adjusted (however, it may be desirable to position the donor substrate close to the circuit board as the ejection quality may be compromised).

[0070] A judicious choice of the injection method also helps to minimize the amount of metal debris that scatters in the vicinity of the deposition site and facilitates the cleaning around the substrate after LIFT deposition. Following the reflow step, the circuit board can be cleaned using, for example, ultrasonic treatment in water. Thus, debris that was not melted during the reflow step is detached from the substrate during the cleaning process. Alternatively or additionally, the debris can be reflowed using an accurate and delicate laser ablation process after being cleaned from the circuit board.

[0071] Figure 10 is a plot schematically showing the increase in the height of solder bumps as a function of the volume of solder droplets added to the bumps, according to one embodiment of the present invention. This plot shows the measured values obtained with actual bumps having a diameter of 70 μm. The continuous bars and boxes show the average height and standard deviation measured as a function of the added volume, i.e., the number of deposited droplets. The curve passing through the average values shows that the height increases proportionally to the volume up to about 170% of the initial volume. Thus, LIFT-based solder deposition can be used to accurately repair undersized solder bumps.

[0072] Another way to measure the total volume of solder deposited at a given location can be based on in-line imaging of the diameter of the holes remaining in the donor film 54 (FIG. 1) after the release of each droplet 56. The droplet volume can be calculated based on the diameter of the holes shown in the image and the known thickness of the donor film. Experimental measurements can be performed to find the correction factors necessary, for example, to account for the thickness of the rim around the holes.

[0073] Further details regarding the accurate LIFT printing of solder materials, such as the characteristics of suitable donor films and solder materials, as well as the laser pulse parameters for the injection of solder droplets and the reflow of solder bumps, are described in U.S. Provisional Patent Application No. 63 / 034,422, filed on June 4, 2020, the disclosure of which is incorporated herein by reference.

[0074] The above embodiments are cited as an example, and it should be understood that the present invention is not particularly limited to what is shown and described above. Rather, the scope of the present invention includes both the various combinations and sub-combinations of the above features, as well as those variations and modifications that can be conceived by those skilled in the art upon reading the above description and that are not disclosed in the prior art.

Claims

Claim 1 A method of manufacturing a circuit, comprising: inspecting an array of solder bumps on a circuit board to identify solder bumps on the board having a height greater than a predetermined maximum value; directing a first laser beam at the identified solder bumps to remove a selected amount of solder material from the identified solder bumps; after removing the solder material, directing a second laser beam at the identified solder bumps with sufficient energy to melt and reflow the solder material remaining on the identified solder bumps; wherein the identified solder bumps have a bump diameter, and directing the first laser beam includes focusing the first laser beam to impinge on the identified solder bumps with a beam diameter smaller than the bump diameter, creating a cavity in a central region of the identified solder bumps by ablation of the solder material; method. Claim 2 Directing the first laser beam includes directing one or more pulses of laser energy to impinge on the identified solder bumps, wherein each of the pulses has a pulse duration of less than 50 ns. The method according to claim 1. Claim 3 Inspecting the array includes estimating the amount of solder material to be removed from the identified solder bumps according to the height of the identified solder bumps, and directing the one or more pulses includes selecting the number of pulses to be applied to the identified solder bumps according to the estimated amount. The method according to claim 2. Claim 4 Directing the first and second laser beams at the identified solder bumps includes repeating the step of directing the first laser beam to remove the solder material and the step of directing the second laser beam to melt and reflow the solder material multiple times until the height of the solder bumps is below the predetermined maximum value. The method according to claim 1. Claim 5 Directing the first laser beam includes disposing a transparent cover on the substrate in proximity to the identified solder bump and directing the first laser beam to irradiate the identified solder bump through the transparent cover, whereby debris emitted by ablation of the identified solder bump adheres to the cover. The method according to claim 1.

6. Directing the second laser beam includes directing one or more pulses of laser energy to impinge on the identified solder bump, each pulse duration of the pulses being less than 100 μs. The method according to claim 1.

7. Directing the first and second laser beams includes generating both the first and second laser beams using a single laser having a variable pulse duration. The method according to claim 6.

8. The identified solder bump has a bump diameter, and directing the second laser beam includes focusing the second laser beam to impinge on the identified solder bump with a beam diameter smaller than the bump diameter. The method according to claim 1.

9. Directing the second laser beam includes applying sufficient energy to the identified solder bump using the second laser beam to melt the entire volume of the identified solder bump. The method according to claim 1.

10. Directing the second laser beam includes applying an amount of energy to the identified solder bump using the second laser beam selected to melt only a portion of the identified solder bump. The method according to claim 1.

11. Inspecting the array of solder bumps includes identifying yet another solder bump on the substrate having a height lower than a predetermined minimum value. Depositing one or more molten droplets of the solder material onto the further separate solder bump, and directing the second laser beam with sufficient energy to the further separate solder bump to melt the deposited solder material and reflow it onto the further separate solder bump, wherein depositing the one or more molten droplets includes ejecting the one or more molten droplets from a donor substrate proximate to the further separate solder bump by a process of laser-induced forward transfer (LIFT). The method according to claim 1.

12. The method according to claim 11, wherein ejecting the one or more molten droplets includes applying the first laser beam to direct one or more pulses of laser energy through the donor substrate to induce ejection of the molten droplets.

13. A method of circuit manufacturing, inspecting an array of solder bumps on a circuit board to identify solder bumps on the substrate having a height lower than a predetermined minimum value, depositing one or more molten droplets of solder material onto the identified solder bumps such that the droplets adhere to and solidify on the identified solder bumps, after depositing the solder material, directing a laser beam with sufficient energy to the identified solder bumps to melt the deposited solder material and reflow it onto the identified solder bumps, comprising wherein the identified solder bumps have a bump diameter, and directing the laser beam includes focusing the laser beam to impinge on the identified solder bumps with a beam diameter smaller than the bump diameter to create a cavity in a central region of the identified solder bumps by ablation of the solder material. Method.

14. The method according to claim 13, wherein depositing the one or more molten droplets includes ejecting the one or more molten droplets from a donor substrate proximate to the identified solder bumps by a process of laser-induced forward transfer (LIFT).

15. The donor substrate is transparent and has opposing first and second surfaces and a donor film containing the solder material on the second surface such that the donor film is proximate to the identified solder bumps. Releasing the one or more molten droplets includes directing one or more pulses of laser radiation to pass through the first surface of the donor substrate and impinge on the donor film, and inducing the release of the one or more molten droplets of the solder material from the donor film onto the identified solder bumps. The method according to claim 14. **Claim 16** Directing the one or more pulses of laser radiation in the LIFT process and directing the laser beam at the identified solder bumps includes using a single laser having a variable pulse duration to perform both releasing the molten droplets and melting and reflowing the deposited solder material. The method according to claim 15. **Claim 17** Inspecting the array includes estimating the amount of solder material applied to the identified solder bumps according to the height of the identified solder bumps, and depositing the one or more molten droplets includes selecting the number of droplets to deposit on the identified solder bumps according to the estimated amount. The method according to claim 13. **Claim 18** Depositing the one or more molten droplets and directing the laser beam at the identified solder bumps includes repeating depositing the molten droplets of the solder material and directing the laser beam to melt and reflow the solder material multiple times until the height of the solder bump exceeds the predetermined minimum value. The method according to claim 13. **Claim 19** Directing the laser beam includes applying sufficient energy to the identified solder bumps using the laser beam to melt the entire volume of the identified solder bumps including the deposited solder material. The method according to claim 13. **Claim 20** Directing the laser beam includes applying a certain amount of energy to the identified solder bumps using the laser beam, and the certain amount of energy is selected to melt only a part of the identified solder bumps in addition to the deposited solder material. The method according to claim 13. **Claim 21** Directing the laser beam includes directing one or more pulses of laser energy to impinge on the identified solder bump, each of the pulses having a pulse duration of less than 100 μs, the method of claim 13.

22. An inspection module, an inspection module configured to acquire image data regarding an array of solder bumps on a circuit board, A laser module, a first laser beam configured to remove solder material from the solder bump, and a second laser beam configured to melt and reflow the solder material at the solder bump, a laser module configured to output A control circuit, which processes the image data to identify solder bumps in the array on the substrate having a height higher than a predetermined maximum value, controls the laser module to direct the first laser beam towards the identified solder bumps, removes a selected amount of the solder material from the identified solder bumps, and after removing the solder material, directs the second laser beam towards the identified solder bumps with sufficient energy to melt and reflow the solder material remaining at the identified solder bumps. A control circuit configured as Comprising The identified solder bumps have a bump diameter, and the laser module is configured to focus the first laser beam and cause it to impinge on the identified solder bumps with a beam diameter smaller than the bump diameter, creating a cavity in the central region of the identified solder bumps by ablation of the solder material An apparatus for circuit manufacturing.

23. The control circuit is configured to estimate the amount of solder material to be removed from the identified solder bumps according to the height of the identified solder bumps, and select the number of pulses to be applied to the identified solder bumps according to the estimated amount, the apparatus of claim 22.

24. The apparatus according to claim 22, comprising a transparent cover disposed on the substrate and proximate to the identified solder bump, wherein the laser module is configured to direct the first laser beam to irradiate the identified solder bump through the transparent cover, such that debris emitted by ablation of the identified solder bump adheres to the cover.

25. The apparatus according to claim 22, wherein the laser module includes a single laser having a variable pulse duration and is configured to generate both the first and second laser beams.

26. The apparatus according to claim 22, wherein the identified solder bump has a bump diameter, and the laser module is configured to focus the second laser beam to impinge on the identified solder bump with a beam diameter smaller than the bump diameter.

27. The control circuit is configured to identify yet another solder bump on the substrate having a height lower than a predetermined minimum value. The apparatus comprises a deposition module configured to deposit one or more molten droplets of the solder material onto the yet another solder bump. The laser module is configured to direct the second laser beam with sufficient energy onto the yet another solder material to melt the deposited solder material and reflow it onto the yet another solder bump. The apparatus according to claim 22.

28. An inspection module configured to acquire image data regarding an array of solder bumps on a circuit board, a deposition module configured to emit molten droplets of solder material, and a laser module configured to output a laser beam configured to melt and reflow the solder material at the solder bump. A control circuit that processes the image data to identify solder bumps in the array on the substrate having a height lower than a predetermined minimum value, controls the deposition module to deposit one or more of the molten droplets of the solder material onto the identified solder bumps, whereby the droplets adhere to and harden on the identified solder bumps, and after removing the solder material, controls the laser module to direct the laser beam at the identified solder bumps with sufficient energy to melt the deposited solder material and reflow it into the identified solder bumps. Comprising The identified solder bumps have a bump diameter, and the laser module is configured to focus the laser beam and collide it with the identified solder bumps with a beam diameter smaller than the bump diameter, creating a cavity in the central region of the identified solder bumps by ablation of the solder material. An apparatus for circuit manufacturing.

29. The deposition module is configured to release the one or more molten droplets from a donor substrate adjacent to the identified solder bumps by a laser-induced forward transfer (LIFT) process, according to the apparatus of claim 28.

30. The donor substrate is transparent and has opposing first and second surfaces and a donor film containing the solder material on the second surface, such that the donor film is adjacent to the identified solder bumps. The laser module is configured to direct one or more pulses of laser radiation through the first surface of the donor substrate and onto the donor film to collide with it, inducing the release of the one or more molten droplets of the solder material from the donor film onto the identified solder bumps, according to the apparatus of claim 29.

31. The laser module includes a single laser having a variable pulse duration and is configured to perform both releasing the molten droplets and melting and reflowing the deposited solder material, according to the apparatus of claim 30.

32. The control circuit is configured to estimate the amount of the solder material applied to the identified solder bump according to the height of the identified solder bump, and to select the number of the droplets deposited on the identified solder bump according to the estimated amount, the apparatus according to claim 28.

33. The control circuit is configured to cause the deposition module and the laser module to repeat the steps of depositing the molten droplets of the solder material and directing the laser beam to melt and reflow the solder material a plurality of times until the height of the solder bump exceeds the predetermined minimum value, the apparatus according to claim 28.

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