Chip soldering method

Abstract

The application discloses a chip welding method, wherein the chip welding method comprises the following steps: printing tin paste on a pad of a substrate to be processed; wherein the height difference between the highest point of the substrate to be processed and the lowest point of the substrate to be processed is less than or equal to 45 microns; flattening the tin paste on the pad by a pressing block to compensate for the height difference of the substrate to be processed; and welding a solder ball on a chip with the tin paste. The method widens the limit of the substrate warpage in the traditional process, and effectively improves the welding yield.

Description

Technical Field

[0001] This application relates to the field of chip mounting technology, and in particular to a chip soldering method. Background Technology

[0002] In the field of semiconductor packaging, the interconnection between the substrate and the chip is the core link to achieve electrical conduction and physical load-bearing. Current mainstream chip packaging technologies (such as flip chip and ball grid array packaging) generally rely on high-precision surface mount and reflow soldering processes. To achieve reliable soldering, the warpage of the substrate is controlled at a high level to ensure its dimensional stability during subsequent processing and use.

[0003] However, the pursuit of low warpage directly leads to a significant increase in packaging costs. High-performance low-warpage substrates are not only expensive in themselves, but the complex processes used to achieve low warpage also reduce production efficiency and yield. In subsequent assembly, testing, and use, uncontrollable warpage may still occur due to factors such as thermal stress and mechanical stress, putting the high initial investment at risk of failure. Summary of the Invention

[0004] This application provides a chip soldering method that can compensate for the height difference on the substrate surface, realize the planarization of the solder pads, make the contact area of ​​the chip solder balls uniform, improve the soldering reliability and increase the chip packaging yield.

[0005] To address the aforementioned technical problems, this application provides a chip soldering method, comprising: printing solder paste on pads of a substrate to be processed; wherein the height difference between the highest point and the lowest point of the substrate to be processed from the horizontal plane is less than or equal to 45 micrometers; leveling the solder paste on the pads using a pressure block to compensate for the height difference of the substrate to be processed; and soldering the solder balls on the chip to the solder paste.

[0006] The step of leveling the solder paste on the pads by pressing down with a pressure block includes pressing down the pressure block in a direction perpendicular to the surface of the substrate to be processed, so that the solder paste on the pads is at the same horizontal height.

[0007] The solder paste must be at least 20 micrometers thick.

[0008] The solder paste is in a semi-cured state.

[0009] The thickness difference of the solder paste is less than or equal to 40 micrometers.

[0010] The step of printing solder paste on the pads of the substrate to be processed includes: curing the solder paste by reflow process.

[0011] The step of soldering the solder balls and solder paste on the chip includes: aligning and mounting the solder balls and solder paste on the chip.

[0012] The step of aligning and mounting solder balls and solder paste includes: reflowing and curing the mounted chip and the substrate to be processed to solder the solder balls and solder paste.

[0013] The length of the pressing block is greater than or equal to the length of the substrate to be processed.

[0014] The solder balls correspond to the positions of the solder pads.

[0015] The beneficial effects of this application are as follows: By printing solder paste on the pads of the substrate to be processed; wherein the height difference between the highest point and the lowest point of the substrate to be processed from the horizontal plane is less than or equal to 45 micrometers; by using a pressure block to level the solder paste on the pads to compensate for the height difference of the substrate to be processed; and by soldering the solder balls on the chip to the solder paste, the limitation of the warpage of the substrate to be processed is expanded, which not only reduces the procurement cost, but also reduces the possibility of defects such as local cold solder joints and cracks caused by poor contact or stress concentration, making the metallurgical bonding of the solder interface more complete and firm, and improving the soldering yield. Attached Figure Description

[0016] Figure 1 This is a schematic flowchart of one embodiment of the chip bonding method provided in this application; Figures 2a-2e yes Figure 1 Structural diagrams corresponding to each step of the welding method. Detailed Implementation

[0017] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0018] It should be noted that if the embodiments of this application involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indicators will also change accordingly.

[0019] Furthermore, if the embodiments of this application involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, features defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the technical solutions of various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. If the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed in this application.

[0020] Please see Figure 1 as well as Figures 2a-2e , Figure 1 This is a schematic flowchart of an embodiment of the chip bonding method provided in this application. Figures 2a-2e yes Figure 1 Structural diagrams corresponding to each step of the welding method.

[0021] S11: Print solder paste on the pads of the substrate to be processed; wherein the height difference between the highest point and the lowest point of the substrate to be processed from the horizontal plane is less than or equal to 45 micrometers.

[0022] The substrate to be processed can be a multilayer circuit board, a ceramic substrate, or a silicon interposer or redistribution layer used for packaging, etc., with pads used to achieve electrical connections. The substrate to be processed exhibits a certain degree of deformation in its free state, and the vertical distance between the highest and lowest points on its surface relative to a reference horizontal plane is less than or equal to 45 micrometers. Solder paste can be prepared by mixing alloy solder powder such as SAC305 tin-silver-copper alloy with flux carriers such as activators, thixotropic agents, and solvents in a certain proportion, and is used as a soldering material. Specifically, the substrate to be processed is placed on the printing equipment table and can be stably fixed by clamps such as side clamps, vacuum suction cups, or contour supports to minimize its movement during the printing process. The printing tool, such as a laser-cut stainless steel stencil or electroformed stencil that matches the substrate pad pattern, is kept parallel to the aligned substrate and maintains the set printing gap. Then, an appropriate amount of solder paste is placed on one end of the stencil, and a squeegee pushes the solder paste forward on the stencil surface at a set speed, angle, and pressure. At this time, the solder paste is squeezed into each opening of the stencil, with the opening corresponding to the pad of the substrate to be processed. After the squeegee passes, the stencil separates from the substrate to be processed and is demolded. The solder paste adheres to the pad of the substrate to be processed due to its own adhesiveness, forming solder paste deposition points with regular shape and controllable volume. Alternatively, solder paste can be directly sprayed onto the pads in the form of tiny droplets using piezoelectric or pneumatic micro-droplet ejection valves, following a digital path. For example, a digital inkjet printer equipped with piezoelectric-driven micro-droplet valves can be used. This equipment generates a motion path based on the CAD (Computer-Aided Design) coordinate data of the pads, and applies a high-frequency pulse voltage to deform the piezoelectric ceramic, precisely ejecting the solder paste from the cavity in the form of discrete droplets with a diameter of 30-100 micrometers, depositing them sequentially onto the center of each pad. A spiral valve or piston-type dispensing head, for example, moves the system above each pad according to a preset program, and precisely extrudes a predetermined volume of solder paste, such as 0.01 to 0.5 microliters, through air pressure or mechanical drive to form a highly uniform conical deposition point; or a layer of pure tin or tin alloy is directly deposited on the substrate pads through electroplating or chemical plating. For example, on a substrate that has completed the seed layer preparation, it is immersed in an electroplating solution containing tin ions as a cathode, and by controlling the current density and time, a dense tin layer of 5 to 20 micrometers thick is selectively deposited in the pad area.

[0023] like Figure 2aAs shown, the substrate 11 to be processed has a plurality of pads 12 for electrical connections. These pads 12 can be arranged in a fixed-pitch array to form a regular grid pattern; or they can be arranged with non-uniform, variable pitch according to specific circuit design, chip pin layout, or thermodynamic simulation requirements to achieve optimal signal integrity, heat dissipation performance, or structural adaptability. The lower surface of the substrate 11 to be processed, i.e., the surface opposite to where the pads 12 are located, has its highest and lowest points selected, and the difference in vertical distance between these two points and an ideal reference horizontal line is ΔX. The value of ΔX is less than or equal to 45 micrometers.

[0024] like Figure 2b As shown, solder paste 13 is printed on the pads 12 on the substrate 11 to be processed. A metal stencil with an aperture pattern that perfectly corresponds to the layout of the pads 12 can be visually aligned and tightly attached to the substrate 11 to be processed. Then, a squeegee is used to push the solder paste 13 onto the surface of the stencil with constant pressure, forcing the paste to fill the mesh openings. After the squeegee passes through, the stencil separates from the substrate 11 to be processed, and the solder paste 13 remains on the surface of the pads due to its own adhesiveness, forming a regularly shaped and controllable volume of solder paste 13 deposited on the corresponding pads 12.

[0025] S12: The solder paste on the pads is leveled by pressing the pads to compensate for the height difference of the substrate to be processed.

[0026] A pressure block is used to level the solder paste, which has been printed and reflowed to form a semi-cured state, in order to actively compensate for any height differences in the substrate itself. For example, the pressure block applies uniform pressure to make the solder paste flow rapidly and level it to a coplanar state, ensuring that the contact area between the chip solder balls and the solder paste is completely uniform and reducing the risk of cold solder joints during soldering. The pressure block can be made of high-hardness, high-rigidity materials, such as hardened tool steel, tungsten carbide, or alloys with a wear-resistant coating. The contact surface with the solder paste should at least cover the entire area of ​​the leveled pads on the substrate, and its length should be greater than or equal to the length of the substrate. Specifically, the substrate with printed solder paste is first sent to a reflow oven, such as a reflow oven with a stepped temperature profile. The preheating stage, holding stage, reflow stage, and cooling stage are precisely controlled. Heating is performed according to the set temperature profile. After the solder paste melts, the surface tension causes it to flow to a uniform surface. After cooling, it forms a semi-cured state of solder paste, providing a stable foundation for subsequent pressure block leveling and preventing uneven surface caused by uncured solder paste flowing during the leveling process. Specifically, the substrate to be processed is heated uniformly from room temperature to 120-150°C to evaporate the solvent in the solder paste and complete the initial preheating; then the temperature is further increased to 150-180°C and held for 60-90 seconds to fully activate the flux and clean the pads and solder surface; then the temperature is rapidly increased to above the liquidus temperature of the solder alloy, for example, 235-245°C for SAC305 alloy, and this peak temperature is maintained for 10-30 seconds to cause local melting of the solder powder particles and their bonding to form a cohesive porous structure; finally, it is cooled at a controlled rate below the solder solidification point. A semi-cured solder paste with stable mechanical strength but still retaining remeltable properties is formed. Finally, a pressure block is used to level the semi-cured solder paste. Under the control of a drive mechanism such as a precision cylinder or servo motor, the pressure block applies stable and uniform downward pressure to the substrate along a direction perpendicular to a reference horizontal line. At this time, the semi-cured solder paste undergoes controllable compression and flow deformation, thereby dynamically offsetting the undulations of the substrate's pad surface, making the solder paste on the pads tend to a common horizontal plane to meet the coplanarity requirements for subsequent chip soldering. The minimum thickness of the solder paste on the pads is at least 20 micrometers to ensure the minimum amount of solder required to form high-strength solder joints. The difference in thickness between any two pads must be less than or equal to 40 micrometers to avoid insufficient leveling or residual unevenness due to excessive height difference. For example, taking a warped substrate with a central depression as an example, after leveling, the solder paste height on the pads on both sides (relatively low points) of the substrate is the minimum value, while the solder paste height on the pad in the middle (highest point) is the maximum value. The thickness difference between these two must also be controlled within 40 micrometers, such as 30 micrometers, 35 micrometers, or 40 micrometers.

[0027] like Figure 2cAs shown, the pressure block 20 applies vertical downward pressure to the semi-cured solder paste 13 on the pads 12 of the substrate 11 to deform the solder paste 13 while avoiding damage to the structure of the pads 12 or the substrate 11. Under the action of the pressure block 20, the solder paste 13, which was originally uneven due to the warping of the substrate 11, undergoes differentiated compression deformation. The solder paste on the higher pads is significantly compacted, while the solder paste on the lower pads undergoes micro-flow and material redistribution under pressure. Ultimately, the top of the solder paste 13 on the pads 12 is adjusted to the same target height plane. After leveling, the minimum thickness of the solder paste 13 on each pad 12 is at least 20 micrometers to ensure the minimum solder volume required to form reliable solder joints. At the same time, the difference in thickness of the leveled solder paste 13 on any two pads 12 must be less than or equal to 40 micrometers to reduce the risk of cold solder joints during soldering.

[0028] S13: Solder the solder balls on the chip to the solder paste.

[0029] The solder balls on the chip are aligned and soldered to the leveled solder paste to ensure uniform contact between the solder balls and the solder paste. Specifically, the chip solder balls and solder paste are first aligned and mounted. For example, an optical alignment system can be used, such as using a camera to capture feature points on the solder paste surface and calculate the offset, driving the chip stage for fine-tuning; or a mechanical fixture can be used to fix the chip, using positioning pins to align with preset holes on the substrate pads for physical alignment; or automated equipment can be combined with software algorithms to control servo motors to drive the chip for high-speed and precise mounting. Subsequently, after the solder paste is leveled by a pressure block to form a plane, the chip is lifted to a preset height. The system automatically identifies the solder paste points and adjusts the chip position, applying gentle pressure to ensure the solder balls are tightly adhered to the solder paste surface, ensuring complete coverage of the contact points. After alignment and mounting are completed, a reflow curing process is used for heat treatment, causing the solder balls and solder paste to melt and bond together to form solder joints. Specifically, the aligned and mounted chip and the substrate to be processed are fed into a reflow soldering machine, where nitrogen gas is introduced to create a protective atmosphere. The equipment uniformly heats the components from room temperature to 150-180 degrees Celsius at a rate of 1-3 degrees Celsius per second, fully activating the flux and cleaning the solder surface. Then, it rapidly heats the components above the liquidus line of the solder alloy, for example, heating SAC305 alloy to 235-245 degrees Celsius, and holds this peak temperature for 20-60 seconds, allowing the chip solder balls and the solder paste on the substrate to be processed to completely melt and form a homogeneous liquid solder. This liquid solder uniformly wets the metal surfaces of the chip and the substrate pads under the action of surface tension. Finally, it cools the components below the solder solidification point at a controlled rate of 1-4 degrees Celsius per second, allowing the molten solder to crystallize and solidify, forming a metallurgical bonding layer with an intermetallic compound core at the interface, thus completing the welding of the chip and the substrate. In addition, localized energy application methods such as thermocompression welding, laser welding, or infrared focused heating can be used. For example, a thermocompression welding machine with a heating head can be used to apply pressure and localized high temperature only to the chip area after precise alignment, so that the solder balls and solder paste can be quickly melted and bonded in a tiny area.

[0030] like Figure 2d As shown, the solder balls 32 on the chip 31 correspond one-to-one with the solder paste 13 on the substrate 11 below, achieving aligned mounting. Precise positioning holes and guide pins can be pre-set on the substrate 11 and the chip 31 carrier to achieve coarse positioning through physical mating, followed by final fine-tuning using a micro-motion mechanism; or a laser displacement sensor can be used to scan the height contours of the solder balls 32 and solder paste 13, achieving spatial alignment through three-dimensional coordinate matching, ultimately ensuring that the center vertical projection of each solder ball 32 falls within the effective contact area of ​​the corresponding solder paste 13, establishing an accurate spatial correspondence for subsequent reflow soldering.

[0031] like Figure 2eAs shown, chip 31 and substrate 11 are electrically connected via solder joint 40. Chip 31 and substrate 11 are placed together in a reflow soldering machine and uniformly heated to the active temperature of the solder alloy, for example, 150-180 degrees Celsius, to fully activate the flux residue on the solder paste on chip 31. The temperature is then rapidly increased above the solder liquidus line; for example, for SAC305 alloy, the peak temperature is approximately 235-245 degrees Celsius, and this temperature is maintained for 20-60 seconds. Finally, the solder balls on chip 31 and the solder paste on substrate 11 completely melt, forming solder joint 40.

[0032] Through the above steps, this embodiment compensates for the flatness of the substrate surface by using solder paste and flattening the solder paste by pressing, resulting in a highly consistent coplanar soldering plane in the pad area. This ensures that all solder balls of the chip can obtain uniform initial contact pressure and contact area during mounting. As a result, during the subsequent reflow soldering process, the molten solder can synchronously and uniformly wet each solder joint interface. Finally, the warpage limit of the substrate is widened to 45 micrometers, expanding the process window of the qualified substrate and reducing the possibility of defects such as local cold solder joints and cracks caused by poor contact or stress concentration. This makes the metallurgical bonding of the solder interface more complete and firm, thereby improving the soldering yield.

[0033] The above are merely embodiments of this application and do not limit the scope of this patent application. Any equivalent structural or procedural changes made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of this application.

Claims

1. A chip bonding method, characterized in that, include: Solder paste is printed on the pads of the substrate to be processed; wherein the height difference between the highest point and the lowest point of the substrate to be processed from the horizontal plane is less than or equal to 45 micrometers. The solder paste on the pads is leveled by a pressure block to compensate for the height difference of the substrate to be processed; The solder balls on the chip are soldered to the solder paste.

2. The welding method according to claim 1, characterized in that, The step of leveling the solder paste on the pads using a pressure block includes: The pressure block is pressed down in a direction perpendicular to the surface of the substrate to be processed, so that the solder paste on the pads is at the same horizontal height.

3. The welding method according to claim 2, characterized in that, The thickness of the solder paste is at least 20 micrometers.

4. The welding method according to claim 2, characterized in that, The thickness difference of the solder paste is less than or equal to 40 micrometers.

5. The welding method according to claim 2, characterized in that, The solder paste is in a semi-cured state.

6. The welding method according to claim 1, characterized in that, The step of printing solder paste on the pads of the substrate to be processed includes: The solder paste is cured by a reflow process.

7. The welding method according to claim 1, characterized in that, The step of soldering the solder balls on the chip to the solder paste includes: The solder balls of the chip are aligned and mounted with the solder paste.

8. The welding method according to claim 6, characterized in that, The step of aligning and mounting the solder balls and the solder paste includes: After the chip and the substrate to be processed are mounted, they are reflowed and cured to solder the solder balls to the solder paste.

9. The welding method according to claim 1, characterized in that, The length of the pressing block is greater than or equal to the length of the substrate to be processed.

10. The welding method according to claim 1, characterized in that, The solder balls correspond to the positions of the solder pads.

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