Die bonding apparatus and method utilizing such apparatus

Abstract

Proposed are a die bonding apparatus and a method utilizing one, comprising an attach unit and a clamping unit structured to bond at least one semiconductor die at a particular attach position on at least one lead frame, wherein the at least one lead frame is mounted to a lead frame carrier, the clamping apparatus comprising a support frame structured to receive the lead frame carrier, the support frame defining a reference height dimension, wherein in a first operational condition the clamping unit is structured to contact the lead frame at a height measurement position being next to the attach position and to determine a local height dimension associated with the height measurement position, and in a second operational condition the clamping unit is structured to contact and to fixedly hold the lead frame at a clamping position being next to the attach position and in said second operational condition the attach unit is structured to bond the semiconductor die at the clamping position on the at least one lead frame taking into account at least the reference height dimension and the measured local height dimension.

Description

[0001] TITLE

[0002] Die bonding apparatus and method utilizing such apparatus

[0003] TECHNICAL FIELD

[0004] The present disclosure relates to a die bonding apparatus and a method for bonding at least one semiconductor die to at least one lead frame in a process of manufacturing semiconductor packages.

[0005] BACKGROUND OF THE DISCLOSURE

[0006] In semiconductor package manufacturing a critical step is the bonding of a semiconductor die (the brains of the package) to a substrate or a lead frame (the limbs and / or organs of the package). Issues that could arise during this process may lead to reduced thermal performance of the semiconductor die, mechanical stress which could lead to cracks of the semiconductor die and / or detachment of the bond wires, and lastly warping of the semiconductor die which influences its electrical properties. In the end, the appearance of any of these issues will result in malfunctioning of the semiconductor package.

[0007] Although, several different bonding processes exist like soft solder, eutectic, epoxy, UV, silver sintering, thermocompression and flip-chip die bonding methods, they, in essence, all experience the above-mentioned issues, because a thin layer of adhesive is added in between the semiconductor die and the lead frame, before bringing the semiconductor die in contact with the lead frame. Thereafter, a die bonding apparatus is used to provide adequate pressure and / or force to ensure that the correct adhesive layer thickness and orientation for each semiconductor die are achieved, whereafter the adhesive is allowed to cure.

[0008] Usually, many different dies of various packages are bonded in one continuous process by placing all of them on some sort of lead frame holder. At the beginning of the process the die bonding apparatus is initialized, and all dimensions and locations of die-adhesive-lead frame sandwiches are known, such that the correct operations are performed by the die bonding apparatus, leading to intended thicknesses of the glue layers. However, as the process progresses, dissipated heat causes the lead frame carrier to deform, such that uncertainty in the dimensions and locations of the die-adhesive-lead frame sandwiches arise, leading to incorrect processing, thereby causing the issues mentioned in the beginning.

[0009] Accordingly, it is a goal of the present disclosure to provide an improved die bonding apparatus for the die bonding process, which can actively correct the die bonding as the process progresses by measuring the warpage of the lead frame carrier and / or the warpage of the die bonding apparatus itself.

[0010] SUMMARY OF THE DISCLOSURE

[0011] This disclosure pertains to a die bonding apparatus comprising an attach unit and a clamping unit, structured to bond - during use - at least one semiconductor die at a particular attach position on at least one lead frame, wherein the at least one lead frame is mounted to a lead frame carrier. The die bonding apparatus comprises a support frame structured to receive the lead frame carrier, and the support frame defines a reference height dimension.

[0012] In a first operational condition, the clamping unit is structured to contact the lead frame at a height measurement position being next to the attach position and to determine a local height dimension associated with the height measurement position.

[0013] In a second operational condition, the clamping unit is structured to contact and to fixedly hold the lead frame at a clamping position being next to the attach position and in said second operational condition the attach unit is structured to bond the semiconductor die at the clamping position on the at least one lead frame taking into account at least the reference height dimension and the measured local height dimension.

[0014] Normally, in the bonding process of a semiconductor die on a lead frame, the lead frame is first positioned on a lead frame carrier. Then a clamping unit is used to fixedly hold the lead frame on the lead frame carrier. Thereafter, the lead frame is coated with dots of adhesive by an adhesive depositing apparatus on each attach position for all of the at least one semiconductor dies. Thereafter, an attach unit is used to pick up the semiconductor die from a wafer and position the dies on top of the dots of adhesive. These apparatuses, however, do not correct for layer thickness variations that might occur due to warping of the lead frame carrier. A die bonding apparatus according to this configuration has the benefit that it can determine warpage of the lead frame carrier prior to and during bonding of the at least one die onto the at least one lead frame. This allows for a correct and adequate height positioning of the at least one semiconductor die by the attach unit, such that the layer thickness of the bonding adhesive in between the at least one semiconductor die and the at least one lead frame can be controlled precisely.

[0015] Furthermore such configuration mitigates the known issues outlined in the background information and allows for cost reduction of the die bonding apparatus, since no additional units or elements are needed to measure the warpage.

[0016] Additionally, such configuration allows for a clamping process and a bonding process which are not force controlled, but rather height-information controlled. For the bonding process this is beneficial, since the viscosity properties of the bonding adhesive change during the bonding process. Therefore, a constant force would likely not result in obtaining a precise adhesive layer thickness, whereas heightinformation controlled bonding does allow this. For the clamping process this is beneficial, since too forceful clamping could cause deformation of the at least one lead frame, leading to malfunctioning of the semiconductor package.

[0017] In an example, the clamping unit comprises an elongated clamping arm having a first end and a free second end. The clamping arm is structured to contact with the free second end the lead frame in the first operational condition and in the second operational condition.

[0018] Whenever the clamping unit comprises an elongated clamping arm, which is performing the local height measurement and the die clamping process, the elongated clamping arm can minimize the influence of vibrations or movements of the clamping unit’s base. The elongated clamping arm may act as a damped spring, thereby offering greater stability and as a result greater resolution can be achieved at the free second end of the clamping arm.

[0019] In another example the elongated clamping arm substantially extends in a direction parallel to a plane defined by the support frame.

[0020] This configuration allows for better vibration mitigation in the Z-direction, being defined perpendicular to a plane defined by the support frame, opposed to a clamping arm which extends perpendicularly to the support frame, which would mostly mitigate vibrations in the XY-plane (the plane defined by the support frame). One may generally understand that the Z-direction is substantially parallel to the direction of gravitational pull, such that the lead frame carrier including the at least one lead frame remain on the holder by gravity. Therefore, the configuration according to the example allows for vibration and movement mitigation in the direction in which the local height measurement is being performed, resulting in greater resolution and sensitivity of the apparatus.

[0021] In another example the elongated clamping arm is structured to pivot around a point near or at its first end.

[0022] A pivoting mechanism of the elongated clamping arm ensures that less stress is generated on the mechanism driving the elongated clamping arm, since with a pivoting mechanism the weight of the elongated clamping arm no longer has to be carried by the force mechanism itself, but is rather supported by the housing of the frame of the clamping unit. This could result in greater accuracy since more sensitive driving mechanisms could be used, which would only need to provide less force to induce movements.

[0023] In another example the free second end of the elongated clamping arm has a forked shape or a Y-shape. In another example the free second end is shaped in a direction towards the support frame.

[0024] A Y-shape can be understood to be a fork with only 2 teeth, whereas a fork shape can have multiple teeth. Multiple teeth ensure multiple points of contact between the elongated clamping arm and the at least one lead frame. Having more than just one points of contact, especially separated from each other allows the clamping unit to clamp the at least one lead frame onto the lead frame carrier more evenly. In particular, skewed lead frames could skew even more during the clamping process if only one point of contact is used.

[0025] To further improve the clamping behavior of the free end of the elongated clamping arm, the free end may be shaped towards the support frame, such that the points of contact, that are created with the at least one lead frame, move substantially in a direction perpendicular to the plane defined by the support frame.

[0026] In yet another example the die bonding apparatus comprises a XY-stage for displacing within a XY-plane defined by a plane parallel to the plane formed by the support frame. This XY-stage allows displacement of the clamping unit between the various height measurement positions and clamping positions on the at least one lead frame. In another example, it may also be used to move from lead frame to lead frame.

[0027] In a further example the clamping unit further comprises a Z-stage, wherein a Z-direction is defined perpendicular to the XY-plane. Alternatively, the clamping unit further comprises a rotation stage for rotating the elongated clamping arm around the pivot point.

[0028] Both configurations have benefits in different situations. A linear Z-stage allows the clamping unit to move perfectly perpendicular to the support frame, such that the die is also compressed perfectly perpendicularly. Whereas the use of a rotation stage allows the clamping unit to operate needing less force, since the elongated clamping arm may be supported with a pivot rod.

[0029] The clamping unit of the die bonding apparatus, as an example, may also comprise an encoder structured - during use - to map a position of the elongated clamping arm to the local height dimension measured at the height measurement position.

[0030] This example of a clamping unit allows for exact determination of the position of the free second end of the elongated clamping arm. Knowing the exact position of the free second end allows for precise local height dimensions measurement in the first operational use and positioning of the free second end for forceless clamping in the second operational use.

[0031] The local height dimension may be understood to be the position at which the free second end contacts the lead frame. In relation to the position of the free second end that has contacted the support frame, known as the reference dimension, of the same XY location, deviations in the local height dimension can be tracked over time, which are expected to change due to heating leading to warping.

[0032] The force or pressure needed to be provided to the semiconductor die by the attach unit during the bonding process changes as the adhesive cures. At the beginning of the bonding process the adhesive has very low viscosity and constant pressure would likely press away all the adhesive from underneath the semiconductor die and cause the semiconductor die to crash into lead frame. Whereas at a later stage in the process, the force would increase due to the increasing viscosity of the hardening adhesive. In that case, providing constant pressure and having an expanding or contracting adhesive, the correct glue layer thickness could never be precisely controlled. Therefore, not force controlled, but rather height-information controlled bonding can achieve greater resolution in the adhesive layer thickness, which can be achieved with the apparatus according to the disclosure. This height information can be supplied to the attach unit by the clamping unit prior to bonding or even during the bonding process as the clamping unit is clamping the at least one lead frame at the clamping position. As described earlier, the height measurement position is next to the attach position of the at least one semiconductor die on the at least one lead frame. Even so, the clamping position is next to the attach position. It may be understood that the clamping position and the height measurement position are the same position, however it is not limited thereto. In particular cases, it could be beneficial to measure the local height dimensions closer to at least one semiconductor die than it is to clamp, for instance due to adhesive contamination or the like.

[0033] In a further example of the die bonding apparatus according to the disclosure, the Z- or rotation stage, respectively, comprises an electric motor, such as a stepper motor, a brushless motor, or a three-phase motor. In an alternative example, the Z- and / or rotation stage, respectively, comprises an electromagnetic plunger.

[0034] The choice of the exact force generator depends on the exact local height information resolution that is desired, and the desired clamping force needed to fixedly hold the at least one lead frame. Some of the above-mentioned examples may provide greater accuracy, but as a result can only provide little force, whereas others can provide a lot of force, but do this rather inaccurately.

[0035] This disclosure also pertains to a method of bonding at least one semiconductor die at a particular attach position on at least one lead frame, utilizing a die bonding apparatus according to the disclosure, wherein the at least one lead frame is mounted to a lead frame carrier, the method comprising the steps: i) defining a reference height dimension from a support frame which is structured to receive the lead frame carrier; ii) contacting, in a first operational condition, the lead frame with the clamping unit at a height measurement position being next to the clamping position and to determine a local height dimension associated with the height measurement position, iii) bonding, in a second operational condition, the semiconductor die with the attach unit at the attach position on the at least one lead frame taking into account at least the reference height dimension and the measured local height dimension, while clamping the lead frame with the clamping unit at a clamping position being next to the attach position.

[0036] Optionally, the method comprises the step iv) of continuously performing steps ii) and iii) by moving to the next one of the at least one semiconductor die until all semiconductor dies are bonded. The method as described here allows for semiconductor die bonding to at least one lead frame at various positions on a lead frame carrier. Furthermore, the method allows for the bonding to be adjusted based on a measurement of a local height dimension, thereby offering a way to correct for warpage.

[0037] Because the clamping unit is used for both measuring and clamping, the die bonding apparatus is more cost efficient, and the combined warpage of the holder and the lead frame carrier can be measured. In case an additional unit is used to measure the warpage typically only one isolated warpage is measured, thereby still not obtaining the correct local height information, resulting in providing incorrect height information towards the attach unit, such that bonding of the at least one semiconductor die would still be error prone.

[0038] In an example of the method, step ii) of contacting the lead frame to determine the local height dimension is at least performed twice in succession. In another example of the method, the successive steps of performing step ii) of contacting the lead frame to determine the local height dimension are performed on different height measurement positions next to the clamping position.

[0039] Performing multiple local height measurements in succession allows for performing mathematical analysis on the obtained information, for instance the mean or the median could be taken as the desired local height information. In case an operator of the method would be more hesitant because of the potential of bringing the semiconductor die too close the lead frame, the maximum obtained local height information could be used as well. To further improve the local height measurement, the successive measurements could be performed on other locations close to the attach position. This allows to correct for warpage that is non-uniform across the semiconductor die. This becomes especially useful in case large semiconductor dies are used.

[0040] In the last example of the method, contacting the lead frame to determine the local height dimension of step ii) comprises the sub steps of: a) starting a force-distance measurement to obtain force-distance data; b) bringing the clamping unit into contact with the at least one lead frame; c) fitting the force-distance data with a mathematical model to extract the local height dimension.

[0041] Measuring the local height dimension based on one single point measurement could lead to inconsistencies and incorrect measurements due to vibrations, surface roughness, or the like. Therefore, the inventors have found that measuring a graph and fitting the data of said graph results in better consistency and higher accuracy. Furthermore, it allows them to utilize multiple fitting method dependent of the desired specifications of the die bonding apparatus. For instance, the force-distance curves could simply be fitted with linear fits to obtain intersect points, or with more advanced fitting models, which have a local height dimension unit as one of the fitting parameters.

[0042] All in all, this the method and apparatus as disclosed in this document, provide a way to obtain local height information during the semiconductor die bonding process, such that corrections can be made that account for warpage (for instance because of heating). These corrections ensure that greater accuracy and resolution is achieved for the layer thickness of the adhesive which connects the semiconductor die to its lead frame. By doing so, many issues that could arise during the bonding process can be mitigated and malfunctioning of the final semiconductor package can be greatly reduced.

[0043] BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The disclosure will now be discussed with reference to the drawings, which show in:

[0045] Figure 1A-B-C show two operational conditions of the die bonding apparatus.

[0046] Figure 2A and Figure 2B show one operational conditions of a different example of a die bonding apparatus.

[0047] Figure 3 depicts another example of a die bonding apparatus with open housing.

[0048] Figure 4A and Figure 4B show two examples of elongated clamping arms.

[0049] Figure 5 shows a 3D representation of a die bonding apparatus.

[0050] Figure 6 displays a block scheme of a method utilizing a die bonding apparatus.

[0051] Figure 7 shows a force-distance curve measured with a die bonding apparatus. DETAILED DESCRIPTION OF THE DISCLOSURE

[0052] For a proper understanding of the disclosure, in the detailed description below corresponding elements or parts of the disclosure will be denoted with identical reference numerals as in the figures.

[0053] For general understanding of the figures shown, only part of the semiconductor die bonding process is shown. For the figures shown in this disclosure, we simply assume that an adhesive 30 might already have been dispensed on top of a lead frame 40. Please note that the adhesive 30 has not hardened (unless mentioned otherwise). Only with those assumptions the semiconductor die bonding process with a die bonding apparatus 10 may be considered to obtain the correct layer thickness of the adhesive 30.

[0054] The bonding process may therefore be understood to be the process of positioning a semiconductor die 20 on top of a lead frame 40 with an intended layer thickness of an adhesive 30. Whereafter the adhesive is hardened, such that the lead frame, adhesive, and semiconductor die become unison. This hardening is initiated and / or sped-up by applying heat to the adhesive (further not discussed in this disclosure). The exact workings of the attach unit are understood to be known to the person skilled in the art. Only it should be noted that this attach unit can operate forceless and position the semiconductor die at a particular height.

[0055] In Figures 1A-1 B-1C a die bonding apparatus 10 comprising an attach unit 200 and a clamping unit 100 is disclosed. The bonding apparatus 10 is structured to bond - during use - at least one semiconductor die 20 at a particular attach position 300 on at least one lead frame 40, wherein the at least one lead frame 40 is mounted to a lead frame carrier 50. The bonding apparatus 10 comprises a support frame 120 structured to receive the lead frame carrier 50. Due to its stiff configuration and mounting within the die bonding apparatus 10, the support frame 120 defines a reference height dimension 500, which is determined prior to the sequence of bonding the various semiconductor dies 20 at various locations on the lead frame 40.

[0056] During operation of the die bonding apparatus 10, heat is generated causing warping of the support frame 120 which subsequently deforms the lead frame 40 mounted thereon. Accordingly, this warping phenomenon causes local deviations in the height dimension along the lead frame surface and may cause undesired and incorrect bonding of a semiconductor die 20 at its intended attach position, causing malfunction of the semiconductor package. The die bonding apparatus 10 according to the disclosure provides a solution for this disadvantage by tracking the local deviations in the height dimension prior to the die bonding. In Figure 1 A and Figure 1 B a first operational condition of the die bonding apparatus 10 is shown, in which it is structured to contact the at least one lead frame 40 at a height measurement position 310 being next to the attach position 300 and to determine a local height dimension 400 associated with the height measurement position 310.

[0057] To do so, both the attach unit 200 and the clamping unit 100 are in a retracked state (not touching the at least one semiconductor die 20 or the at least one lead frame 40, respectively), wherein the clamping unit 100 is positioned next to the attach position 300 above the height measurement position 310. This is depicted in Figure 1A. Thereafter, the clamping unit 100 is brought into contact with the at least one lead frame 40 to measure the local height dimension 400 of said height measurement position 310, see Figure 1 B.

[0058] In Figures 1C a second operational condition is shown, in which the clamping unit 100 is structured to contact and fixedly hold the lead frame 40 at a clamping position 320 being next to the attach position 300 and the attach unit 200 is structured to bond the at least one semiconductor die 20 at the attach position 300 on the at least one lead frame 40 taking into account at least the reference height dimension 500 and the measured local height dimension 400.

[0059] To do so, the clamping unit 100 is brought into contact with the at least one lead frame 40 at the clamping position 320. Furthermore, the attach unit 100 is brought into contact with the at least one semiconductor die 20, as seen in Figure 1 C, to bond the at least one semiconductor die 20 to the at least one lead frame 40 by means of an adhesive layer 30 which is located between them. The thickness of the at least one semiconductor die 20 is known to the operator and therefore the clamping unit 100 can be used to position the at least one semiconductor die 20, such that the desired layer thickness of the adhesive layer 30 is achieved. For this the local height dimension obtained in the first operational mode is used to account for warping of the lead frame carrier 50.

[0060] The clamping position 320 may be the same as the height measurement position 310, but is not required to. For instance, it might be beneficial to measure the local height dimension 400 closer to the semiconductor die than for clamping the lead frame 40 onto the lead frame carrier 50. However, it may be understood that in most common cases the clamping position 320 and the height measurement location 310 may overlap, such that correct local height dimension information 400 is obtained, which can directly and unambiguously be used during the clamping.

[0061] It should be noted though, that the attach unit 100 is not pressing against the at least one semiconductor die 20 with constant force, but presses the at least one semiconductor die 20 close enough to the at least one lead frame 40 based on the local height dimension of the at least one lead frame and of the reference (holder). It may even further take thickness information of the at least one semiconductor die 20 into account during the bonding of the at least one semiconductor die 20 to the at least one lead frame 40. That way, the desired position of the at least one semiconductor die 20 can be chosen with the attach unit 200, such that a thin layer of adhesive 30 will remain, and its layer thickness can be controlled with great accuracy.

[0062] This is beneficial, since the viscosity properties of the bonding adhesive 30 change during the bonding process as it hardens. Bonding the at least one semiconductor die 20 with constant force would therefore not likely result in a precise thickness of the adhesive layer 30, whereas height-information controlled bonding does allow this. Having such precise control over the thickness of the adhesive layer 30 ensures that less damage and less malfunctioning of the final semiconductor package product occurs.

[0063] Additionally, a die bonding apparatus 10 according to this configuration has the benefit that it can determine warpage of the lead frame carrier 50 prior to or even during the bonding the at least one semiconductor die 20 to the at least one lead frame 40. This warpage would arise during the semiconductor die bonding process, since typically heat is used to cure the adhesive 30. The excess heat causes the lead frame carrier 50 to expand and deform.

[0064] In case the warpage is not taken into account during later bonding of the other semiconductor dies 20 to the lead frame 30, the results will be incorrect thickness of the adhesive layer 30 and even more detrimental non-uniform layer thicknesses. This will typically cause an incorrect bonding of the die 20 to the lead frame 30 and subject the semiconductor die 20 to undesirable forces during bonding, which may cause malfunctioning of the final semiconductor package and thus the rejection of large batches of processed semiconductor packages. With the die bonding apparatus 10 according to the disclosure, the layer thickness of the bonding adhesive 30 in between the at least one semiconductor die 20 and the at least one lead frame 40 can be controlled precisely. In particular, a more accurate height measurement and control is achieved and a proper height compensation or correction for the observed (detected) warpage is now possible, thus improving the quality of the final semiconductor packages and reducing rejection.

[0065] Not only can the known issues outlined in the background information be mitigated, but also cost of the die bonding apparatus 10 itself are reduced, since no additional units or elements are needed to measure the warpage. With the configuration as disclosed in this application, the warpage can be measured, and more precise control and height compensation can be achieved.

[0066] As further can be seen in the example of a die bonding apparatus 10 in Figures 1A-B-C, the clamping unit 100 comprises an elongated clamping arm 110 having a first end 111 (which is accommodated inside the clamping unit 100) and a free second end 112. The clamping arm 110 is structured to contact with the free second end 112 the lead frame 40 in the first operational condition and in the second operational condition.

[0067] The local height dimension 400 may be understood to be the point at which the free second end 112 contacts the lead frame 40. In a similar fashion, the reference dimension 500 may be understood to be the point of the free second end 112 that has contacted the support frame 120, at the same XY location seen in the plane of the support frame 120. Knowing the reference dimension 500, measurements of the local height dimension 400 can show deviations which can be tracked over time. These deviations represent the effects that occurred due to warping of the lead frame carrier 50.

[0068] Utilizing an elongated clamping arm 110 for performing the local height measurement and the clamping process, minimizes the influence of vibrations or movements of the die bonding apparatus 100 itself on the free second end 112 of the clamping arm 110. The elongated clamping arm 110 may act as a damped spring, thereby offering greater stability, such that greater resolution can be achieved for the measurement of the local height dimension 400 and subsequently for the bonding of the at least one semiconductor die 20.

[0069] Moreover, an elongated clamping arm 110 may offer greater control over contact point(s) for the measurement of the local height dimension 400 or for the clamping of the at least one lead frame 40. This will be discussed in greater detail when reference is made to Figures 4A-B.

[0070] It should be mentioned that the die bonding apparatus 10 of Figures 1A- B-C comprises a XY-stage, wherein a XY-plane is defined by a plane oriented parallel to the plane of the support frame 120. This XY-stage allows the die bonding apparatus to move from the height measurement position 310 (Figures 1A-B) to the clamping position 320 (Figures 1 B-C). Furthermore, the XY-stage allows the operator of the die bonding apparatus 100 to move from lead frame 40 to lead frame 40.

[0071] In Figures 2A-2B another example of a die bonding apparatus 100 is shown, which is in the first operational condition. Herein is the die bonding apparatus 100 shown in retracked status in Figure 2A and the clamping unit is in contact with the at least one lead frame 40 for the die local height dimension measurement process in Figure 2B. Additionally, in Figure 5, a 3D representation of a similar die bonding apparatus 100 is shown measuring the warpage of the lead frame carrier 50. In all three Figures, the elongated clamping arm 110 substantially extends in a direction parallel to a plane defined by the support frame 112. This design offers greater vibration mitigation in the Z-direction, which is the direction of the local height dimension measurement. Thereby greater accuracy can be achieved in the local height dimension 400. The Z-direction is defined perpendicular to a plane defined by the support frame 120. Furthermore, in the example shown in Figures 2A-B gravity can be understood to keep the at least one semiconductor die on the lead frame carrier.

[0072] It should be noted that in Figures 2A-B and Figure 5, glue has not yet been dispensed on the at least one lead frame 40. Furthermore, in Figure 5 the at least one lead frame has even not been provided. It should be clear to the person skilled in that art that the operation of the clamping unit is independent of what object is positioned underneath it, and independent of whether glue has been dispensed or not for local height dimension measurements.

[0073] In Figures 2A-2B and Figure 5 it is shown that the free second end 112 of the elongated clamping arm 110 may be shaped towards the support frame 120, such that contact points with the at least one semiconductor die 20 are created that move substantially in a direction perpendicular to the plane defined by the support frame 120. Especially in the case of a substantially extending clamping arm 110 in a direction parallel to the support frame 120. In that case, if the free second end 112 were not shaped towards the support frame 120, a non-uniform thickness for the adhesive layer 30 would be obtained upon bonding, which would lead to malfunctioning of the final semiconductor package.

[0074] It may be understood that the die bonding apparatus 10 of Figures 2A- 2B has the same two operational conditions as the die bonding apparatus 10 of Figures 1 A-B-C, even though the second operational condition is not shown in Figures 2A-B. Additionally, it may be understood that the die bonding apparatus 10 of Figure 2A-B also comprises an XY-stage to move from lead frame 40 to lead frame 40. To not repeat features and to be more concise, individual unique features of the die bonding apparatuses 10 will be discussed in each Figure. The expert in the field may thus understand that most of these features are interchangeable between examples disclosed in this application.

[0075] In Figure 3 yet another example of a die bonding apparatus 10 is shown in the first operational condition, wherein the housing of the clamping unit 100 is made transparent to describe the internal components. Here the die bonding apparatus 10 is structured to pivot around or near its first end 111. In an alternative example, the clamping unit 100 comprises a pivot rod or pivot point 105 positioned perpendicular in abutment in between the two ends 111-112 of the elongated clamping arm 110, wherein the clamping unit 100 is structured to pivot around the pivot rod I pivot point 105.

[0076] Due to the pivoting mechanism of the clamping unit 100 less stress is generated on the driving mechanism (rotation stage) 102 of the clamping unit 100. Namely, the pivoting mechanism allows most of the weight of the elongated clamping arm 110 to rest on the pivot rod 105. This way only little force has to be produced by the rotation stage 102, allowing for the use of more sensitive mechanisms, which in turn result in greater accuracy for the height compensation and die clamping procedure.

[0077] As mentioned, the die bonding apparatus of Figure 3 also comprises a rotation stage 102, wherein a rotation axis is defined as a direction perpendicular to the elongated clamping arm 110 in between the two ends 111-112 of the elongated clamping arm 110.

[0078] The rotation stage 102 is connected to the first end 111 of the clamping arm 110 onto which it provides force, and the clamping arm 111 is resting and pivoting around a pivot rod 105 connected to the housing of the clamping unit 100. As an alternative to this mechanism, the clamping unit 100 could also comprise a Z-stage 102’, wherein a Z-direction is defined perpendicular to the XY-plane, which was inside the housing of the clamping unit 100 of Figures 1A-B-C and Figures 2A-B.

[0079] Both configurations have benefits in different situations. A linear Z-stage allows the clamping unit to move perfectly perpendicular to the support frame 120, such that the lead frame 40 is also compressed perfectly perpendicularly. Whereas the use of a rotation stage allows the clamping unit to operate needing less force, since the elongated clamping arm 110 may be supported by the pivot rod 105.

[0080] The Z- or rotation stage 102 may be automated with an electric motor, such as a stepper motor, brushless motor, or three-phase motor. Alternatively, an electromagnetic plunger may also be used.

[0081] The choice of the exact automation of the driving mechanism 102 depends on the lead frame 40 that is being used, and the exact local height information resolution that is desired. Some of the above-mentioned examples may provide greater accuracy, but as a result can only provide little force, whereas others can provide a lot of force, but do this rather inaccurately.

[0082] Furthermore, the clamping unit 100 of the die bonding apparatus 10, as shown in the example of Figure 3, also comprises an encoder 101. The encoder 101 is structured to map during use a position of the elongated clamping arm 110 to the local height dimension 400 measured at the height measurement position 310. The mapping may encompass mapping a position of the first end 111 of the elongated clamping arm 110 or an angular rotation of the elongated clamping arm 110 around its pivot point 105.

[0083] Comprising an encoder 101 allows for exact determination of the position of the free second end 112 of the elongated clamping arm 110 by relating it to either a position of the first end 111 of the elongated clamping arm 110 or by the angular / rotational position of the Z- or rotation stage. The encoder 101 information can be used to achieve precise clamping and communicate exact local height dimension information 400 to the attach unit 200 for forceless bonding.

[0084] Forceless die bonding is important to obtain a uniform layer thickness of the adhesive layer. The bonding process may be understood to comprise the positioning of the semiconductor die 20 on top of the adhesive 30 and lead frame 30 to subsequently harden the adhesive layer 30.

[0085] Figures 4A-B show two examples of elongated clamping arms 110a- 110b. In Figure 4A the free second end 112 of the elongated clamping arm 110a only has one larger contact point, which is structured to contact the at least one lead frame 40.

[0086] In Figure 4B the free second end 112 of the elongated clamping arm 110b has a forked or a Y-shape. This way two contact points are formed, which are structured to contact the at least one lead frame 40. It may be understood that the choice of two contact points is arbitrary and that this may be any number of contact points or fork teeth 113. These teeth 113 allow for even and uniform clamping of the at least one lead frame 20. Furthermore, the teeth 113 are shown to lie along one direction, but it may be understood that multiple teeth 113 could also be provided. For example, a triangular or other shaped configuration of teeth 113 could be beneficial to achieve more stability during local height dimension measurements or clamping.

[0087] Having a larger or more than just one contact points, especially spread out over a larger area allows the clamping unit 100 to clamp the at least one lead frame 40 more evenly. In particular lead frames 40 which are large could skew during the compression process if only one (small) point of contact is used, especially when the underlying lead frame carrier 50 is warped. Without multiple points of contact, a non-uniform thickness of the adhesive layer 30 would be achieved, potentially resulting in the issues raised in the background information.

[0088] Additionally, as hinted at in Figures 2A-2B the free end of the elongated clamping arm 110 may be shaped towards the support frame 120. This way the contact points can be created that would move substantially in a direction perpendicular to the plane defined by the support frame 120. This helps the clamping process, since a parallel to the support frame 120 extending clamping arm 110 in combination with a rotation stage 102 would typically result in an arced movement of the free second end 112. By designing the shape of the teeth 113 or the free second end 112 in such a way, the at least one lead frame 40 can be pressed nearly perpendicularly, improving the local height dimension measurement process or the clamping process.

[0089] One could understand this latter as, that when the contact point at the free second end 112 of the elongated clamping arm 110 would be followed, a linear motion perpendicular to the support frame would be obtained. This ensures that the lead frame 40 are evenly and uniformly bonded even if there are height differences, due to differences in the lead frame thickness or lead frame carrier warpage effects.

[0090] In Figure 6 a method of bonding at least one semiconductor die 20 at a particular attach position 300 on at least one lead frame 40, utilizing a die bonding apparatus 10 according to the disclosure is shown, wherein the at least one lead frame 40 is mounted to a frame carrier 50.

[0091] The method comprises the steps: i) defining a reference height dimension 500 from a support frame 120 which is structured to receive the lead frame carrier 50; ii) contacting, in a first operational condition, the lead frame 40 with the clamping unit 100 at a height measurement position 310 being next to the attach position 300 and to determine a local height dimension 400 associated with the height measurement position 310, iii) bonding in a second operational condition the semiconductor die 20 with the attach unit 200 at the attach position 300 on the at least one lead frame 40 taking into account at least the reference height dimension 500 and the measured local height dimension 400, while clamping the lead frame 40 with the clamping unit 100 at a clamping position 320 being next to the attach position 300.

[0092] Furthermore, the method according to the disclosure may comprise the step iv) of continuously performing steps ii) and iii) by moving to the next one of the at least one semiconductor die 20 until all semiconductor dies 20 are bonded.

[0093] As shown in Figure 6 and described above, the method allows for semiconductor die bonding to at least one lead frame 40 at various positions on a lead frame carrier 50. Furthermore, the method allows for the bonding to be performed precisely without the need of additional units or elements, such that only the clamping unit 100 and the attach unit 200 are needed to measure a local height dimension 400 and to bonding of the semiconductor die 20 using the pre-measured reference height dimension 500 from the support frame 120 and the measured local height dimension 400 of the warped lead frame 40, respectively. As the latter local height dimension 400 might be affected due to possible warpage of the lead frame carrier 50, the combination of both measurements can be effectively used to compensate for the observed warpage and to correctly bond the semiconductor die 20 to the lead frame 40 with the free second end 112 of the clamping arm 110 at the correct height and with the correct height position. Incorrect bonding of the at least one semiconductor die 20 is herewith prevented, and damage and semiconductor package rejection is minimized.

[0094] Because both height measuring and clamping is performed by the clamping unit 100, the die bonding apparatus 10 is more cost efficient, has a smaller form factor, and warpage can be measured and corrected for during the subsequent semiconductor die bonding.

[0095] As further shown in Figure 6, step ii) of contacting the at least one lead frame 40 to determine the local height dimension 400 may optionally also be performed multiple times in succession. This allows the die bonding apparatus to perform some type of mathematical analysis on the obtained local height dimensions 400, such as mean or median determination. Alternatively, the successive steps of performing step ii) of contacting the lead frame 40 to determine the local height dimension 400 may be performed on different height measurement positions 310 next to the attach position 300.

[0096] For example, if the at least one lead frame 40 or the at least one semiconductor die 20 is rather large, a local height measurement may be measured on the left, right, top, and bottom next to the semiconductor die 20, such that mathematical averaging would give the operator information of the local height dimension 400 of the center of the semiconductor die 20.

[0097] In Figure 7 a force-distance measurement graph is shown, which was measured to obtain the local height dimension 400 with the die bonding apparatus 10. Namely, step ii) of contacting the lead frame 40 to determine the local height dimension 400 of the method may for example comprise the sub steps of: a) starting a force-distance measurement to obtain force-distance data; b) bringing the clamping unit 100 into contact with the at least one lead frame 40; c) fitting the force-distance data with a mathematical model to extract the local height dimension 400.

[0098] The force-distance measurement graph in Figure 7 shows that as the distance towards the lead frame 40 is reduced, only little force is needed up to a point where the distance is almost no longer reduced, but at that point the force increases a lot. These two regimes can for example be subdivided into two linear domains, which can be fitted with linear fits. The intersect of both linear fits may be taken as the value for the local height dimension 400. Alternatively, a more advanced fitting model could be used to extract the local height dimension 400, taking into account surface roughness, material hardness etc. for example.

[0099] These force-distance curves and thus the extracted local height information 400 depend on the speed, starting height, and starting force with which the elongated clamping arm 110 is moved. The inventors have found that operations within a range of 10mm / s to 100mm / s for the speed and starting heights in a range of 1 pm to 50 pm resulted in very low standard deviation of the local height dimension 400 for repeated measurements of only 0.8 pm.

[0100] All in all this the method and apparatus 10 as disclosed in this document, provide a way to obtain local height information prior to and during the semiconductor die bonding process, such that corrections can be made that account for warpage (for instance because of heating). These corrections ensure that greater accuracy, resolution and control are achieved for the layer thickness of the adhesive 30 which connects the semiconductor die 20 to its lead frame 40. By doing so, many issues that could arise during the bonding process can be mitigated and malfunctioning of the final semiconductor package can be greatly reduced.

[0101] LIST OF REFERENCE NUMERALS USED

[0102] 10 die bonding apparatus

[0103] 100 clamping unit

[0104] 101 encoder

[0105] 102 Z-stage / rotation stage / driving mechanism

[0106] 105 pivot rod / pivoting point

[0107] 110 elongated clamping arm

[0108] 110a first version of elongated clamping arm

[0109] 110b second version of elongated clamping arm

[0110] 111 first end of the elongated clamping arm

[0111] 112 free second end of the elongated clamping arm

[0112] 113 fork tooth of free second end

[0113] 120 support frame

[0114] 20 at least one semiconductor die

[0115] 200 attach unit

[0116] 30 adhesive

[0117] 300 die clamping position

[0118] 310 height measurement position

[0119] 320 clamping position

[0120] 400 local height dimension

[0121] 500 reference height dimension

[0122] 40 at least one lead frame

[0123] 50 lead frame carrier

Claims

CLAIMS1. A die bonding apparatus comprising an attach unit and a clamping unit, structured to bond - during use - at least one semiconductor die at a particular attach position on at least one lead frame, wherein the at least one lead frame is mounted to a lead frame carrier, the die bonding apparatus comprising: a support frame structured to receive the lead frame carrier, the support frame defining a reference height dimension, wherein in a first operational condition, the clamping unit is structured to contact the lead frame at a height measurement position being next to the attach position and to determine a local height dimension associated with the height measurement position, and in a second operational condition the clamping unit is structured to contact and to fixedly hold the lead frame at a clamping position being next to the attach position and in said second operational condition the attach unit is structured to bond the semiconductor die at the clamping position on the at least one lead frame taking into account at least the reference height dimension and the measured local height dimension.

2. The die bonding apparatus according to claim 1 , wherein the clamping unit comprises an elongated clamping arm having a first end and a free second end, the clamping arm being structured to contact with the free second end the lead frame in the first operational condition and in the second operational condition.

3. The die bonding apparatus according to claim 2, wherein the elongated clamping arm substantially extends in a direction parallel to a plane defined by the support frame.

4. The die bonding apparatus according to claim 3, wherein the elongated clamping arm is structured to pivot around a point near or at its first end.

5. The die bonding apparatus according to any of the claims 2-4, wherein the free second end has a forked shape or a Y-shape.

6. The die bonding apparatus according to any of claims 2-5, wherein the free second end is shaped in a direction towards the support frame.

7. The die bonding apparatus according to any of the previous claims, wherein die bonding apparatus comprises a XY-stage for displacing within a XY-plane defined by a plane parallel to the plane formed by the support frame.

8. The die bonding apparatus according to claim 7, wherein the die bonding apparatus further comprises a Z-stage, wherein a Z-direction is defined as being perpendicular to the XY-plane.

9. The die bonding apparatus according to claim 7, wherein the clamping unit further comprises a rotation stage for rotating the elongated clamping arm around the pivot point.

10. The die bonding apparatus according to any of the claims 2-9, wherein the clamping unit further comprises: an encoder structured - during use - to map a position of the elongated clamping arm to the local height dimension measured at the height measurement position.

11. The die bonding apparatus according to any of the claims 8-10, wherein the Z- and / or rotation stage comprises an electric motor, such as a stepper motor, a brushless motor, or a three-phase motor.

12. The die bonding apparatus according to the claims 8-10, wherein the Z- or rotation stage, respectively, comprises an electromagnetic plunger.

13. A method of bonding at least one semiconductor die at a particular attach position on at least one lead frame, utilizing a die bonding apparatus according to any of the previous claims, wherein the at least one lead frame is mounted to a lead frame carrier, the method comprising the steps: i) defining a reference height dimension from a support frame which is structured to receive the lead frame carrier; ii) contacting, in a first operational condition, the lead frame with the clamping unit at a height measurement position being next to the attach position and to determine a local height dimension associated with the height measurement position, iii) bonding, in a second operational condition, the semiconductor die with the attach unit at the attach position on the at least one lead frame taking into account at least the reference height dimension and the measured local height dimension, while clamping the lead frame with the clamping unit at a clamping position being next to the attach position.

14. The method according to claim 13, wherein step ii) of contacting the lead frame to determine the local height dimension is at least performed twice in succession.

15. The method according to claim 14, wherein the successive steps of performing step ii) of contacting the lead frame to determine the local height dimension are performed on different height measurement positions next to the attach position.

16. The method according to any of the claims 13-15, wherein contacting the lead frame to determine the local height dimension of step ii) comprises the sub steps of: a) starting a force-distance measurement to obtain force-distance data; b) bringing the clamping unit into contact with the at least one lead frame; c) fitting the force-distance data with a mathematical model to extract the local height dimension.

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