A method for efficiently improving thermal fatigue resistance of lead-free solder interconnection BGA package solder joints

Abstract

The application discloses a method for efficiently improving thermal fatigue resistance of a lead-free solder interconnection BGA package welding spot, and relates to the field of chip connection. Ultrafast laser is used to process a dense arrangement of concentric hexagonal array microstructures on a substrate, solder balls are adhered to chip pins, the solder balls and the chip pins are fused and adhered together, and the substrate and the chip are subjected to BGA packaging. The application improves the connectivity of the solder and the substrate by changing the appearance characteristics of the substrate. The method is simple in operation, wide in application range and capable of satisfying packaging requirements of different materials and processing parameters.

Description

Technical Field

[0001] This invention relates to the field of chip interconnection, and in particular to a method for efficiently improving the thermal fatigue resistance of solder joints in lead-free solder interconnect BGA packages. Background Technology

[0002] As integrated circuits evolve towards smaller size and higher precision, higher demands are placed on the quality and reliability of chip packaging. Among various chip packaging methods, ball grid array (BGA) packaging offers advantages such as high integration density, good conductivity, and heat dissipation. BGA packaging interconnects the chip to the substrate using solder balls, which serve both electrical and mechanical connection functions. However, because the solder balls are hidden at the bottom of the chip, the inspection and repair of BGA solder joints are difficult. Therefore, improving the lifespan of BGA packaged devices and preventing early failure is crucial. Solder joint failure is the most common failure mode in BGA packaging, and the main cause of solder joint failure is thermal fatigue due to temperature. During use, BGA packaged devices are subjected to repeated temperature changes, applying cyclic forces to the solder balls, causing thermal fatigue damage and ultimately leading to thermal fatigue failure of the BGA solder joints.

[0003] Ultrafast laser technology has seen rapid development in the field of material surface processing and modification in recent years. Due to its short pulse width, high peak power, and low heat-affected zone, ultrafast lasers can perform high-precision microstructure processing on various material surfaces. Applying ultrafast laser technology to BGA packaging: using ultrafast lasers to process microstructures on a substrate, the presence of these microstructures increases the connection area and improves the strength of the connection when the substrate is connected to solder balls. It also helps to release some stress and strain during temperature cycling of the solder joint. Furthermore, the increased connection area further improves the heat dissipation of the package.

[0004] In conclusion, the use of ultrafast laser processing technology to prepare substrates with special microstructure surfaces has significant scientific importance and research value in enhancing the thermal fatigue reliability of BGA packages. Summary of the Invention

[0005] This invention provides a method for efficiently improving the thermal fatigue resistance of solder joints in lead-free solder interconnect BGA packages. By using ultrafast laser processing to create a densely arranged concentric hexagonal array microstructure, the surface area and roughness of the substrate are increased, which is beneficial for subsequent solder ball welding and enhances the thermal fatigue reliability of the solder joints.

[0006] A method for efficiently improving the thermal fatigue resistance of solder joints in lead-free solder interconnect BGA packages is characterized by using an ultrafast laser to process a densely arranged concentric hexagonal array microstructure on a substrate, bonding solder balls to the chip pins, heating to melt and bond the solder balls to the chip pins, and then performing BGA packaging on the substrate and the chip.

[0007] The densely packed concentric hexagonal array microstructure is a close-packed array structure composed of many concentric hexagonal structural units arranged closely together. Each concentric hexagonal structural unit is composed of nested multi-level hexagonal line grooves with the same center and a spacing of approximately 30 μm. The outermost six sides of each concentric hexagonal structural unit respectively border and coincide with the outermost six sides of another concentric hexagonal structural unit, and so on. The interior and exterior of the grooves are covered with a large number of microparticles due to the use of ultrafast laser processing. By adjusting the ultrafast laser processing parameters, a suitable groove processing depth is obtained.

[0008] The groove width is approximately 30-50µm. When nesting multi-level hexagonal line grooves, the spacing between two adjacent hexagons is 30-50µm, for example, 40µm. The groove depth is 3-5µm. Each unit nests 5-10 levels of hexagons, meaning there are 5-10 nested hexagons with different side lengths.

[0009] Preferably, the substrate is a PCB substrate.

[0010] Preferably, the chip is made of the same chip material used in power devices.

[0011] Preferably, the substrate is arranged parallel to the chip, and the surface of the substrate opposite to the chip is a plane.

[0012] To achieve the above, the present invention provides a surface of a closely packed concentric hexagonal array microstructure substrate, which is prepared by an ultrafast laser processing technology, including the following steps:

[0013] (1) Chip trimming and cleaning: Remove burrs from the chip surface or edges to ensure smooth subsequent processes, and clean impurities and dirt from the surface to ensure welding quality;

[0014] (2) Solder ball array arrangement: The selection of solder balls is determined according to the number and size of chip pins. The position of the solder balls corresponds to the chip pins to maintain alignment accuracy. After the solder ball array arrangement is completed, the chip is placed on the preheating stage for preheating. Then, the solder balls are heated with a hot air gun to melt and bond the solder balls to the chip pins.

[0015] (3) Start the laser control system and adjust the parameters of the ultrafast laser processing equipment, such as laser frequency, current, processing speed and number of times, in preparation for processing the densely arranged concentric hexagonal array microstructure.

[0016] (4) Substrate preparation: Place the part of the substrate that is soldered to the chip within the irradiation range of the laser beam in step (3), adjust the displacement stage of the laser control system to ensure that the processing part is at the laser focus, then control the ultrafast laser processing equipment to process according to the target pattern, optimize the laser processing parameters, and the shape processed on the substrate surface is a regular hexagonal close-packed filling.

[0017] (5) Chip and substrate soldering: Place the chip with solder balls from step (2) onto the substrate processed in step (4), so that the pins on the chip correspond to the soldering part on the substrate. Then place it on the preheating table for preheating, and then use a hot air gun to heat the solder balls to melt them. Control the soldering temperature and time to ensure the connection quality between the solder balls and the substrate.

[0018] (6) Clean the solder slag and residual flux generated during the soldering process to ensure product reliability; check the quality of chip packaging, including soldering quality, pin connectivity and appearance.

[0019] A comparative test of thermal fatigue reliability was conducted after packaging substrates that were not processed by laser and substrates that were processed by laser.

[0020] Preferably, lead-free solder balls with a diameter of 0.3 mm are selected, with a specific composition of Sn 96.5% / Ag 3% / Cu 0.5%; the preheating table temperature is 200-400℃ during the above process, and the hot air gun temperature parameters include a preheating range of 200-300℃, a heating range of 300-400℃, and a reflow range of 400-500℃.

[0021] Compared with the prior art, the present invention has the following advantages and technical effects:

[0022] Currently, in the field of ball grid array (BGA) packaging, methods to enhance package connection reliability mainly focus on the selection of soldering materials, such as choosing substrate materials with good coefficients of thermal expansion, and lead-free solder or silver-containing solder with higher reliability. This invention improves the connection between the solder and the substrate by changing the substrate morphology. This method is simple to operate, has a wide range of applications, and can meet the packaging requirements of different materials and processing parameters. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this patent. For those skilled in the art, other drawings can be obtained from the following drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the chip and substrate packaging.

[0025] Wherein, 1: substrate, 2: pin, 3: chip, 4: solder ball

[0026] Figure 2 A schematic diagram illustrating the process of processing the surface of a BGA substrate using an ultrafast laser.

[0027] Figure 3This is a surface morphology image of a BGA substrate after processing with an ultrafast laser.

[0028] Figure 4 This is a cross-sectional microstructure of a lead-free solder joint fabricated on a BGA substrate after ultrafast laser processing.

[0029] Figure 5 This is a microscopic morphology image of a sample cross-section that developed initial cracks after a thermal shock test.

[0030] (a) Example (b) Comparative Example 1 (c) Comparative Example 2 (d) Comparative Example 3.

[0031] Table 1 compares the time it takes for initial cracks to form in each connection structure. Detailed Implementation

[0032] It should be noted that, unless otherwise specified, the features in the embodiments and comparative examples of this invention can be combined with each other. The described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0033] This invention provides a substrate for BGA packaging with a microstructured surface fabricated by ultrafast laser to improve the thermal fatigue reliability of solder joints. For example... Figure 1 The system includes a substrate 1, a pin portion 2 for ultrafast laser processing, a chip 3 above the pin portion 2, and pre-installed solder balls 4 on the chip 3. The substrate 1 and the chip 3 form an interconnect structure with the pins through the solder balls.

[0034] Furthermore, the substrate is a PCB substrate.

[0035] Furthermore, the chip is made of the same chip material used in power devices.

[0036] Furthermore, the substrate is arranged parallel to the chip, and the surfaces of the substrate and the chip facing each other are planar.

[0037] Example 1: The following describes the implementation method of the patent in detail.

[0038] 1. Chip and substrate preparation: Remove burrs from the surface or edges of the chips and substrates to be packaged to ensure smooth subsequent processes. Then, immerse them in alcohol for ultrasonic cleaning, and dry them after cleaning to remove contaminants at the connection points.

[0039] 2. Chip Ball Placement: Based on the chip pin size, select 0.3mm diameter lead-free solder balls with a composition of Sn 96.5% / Ag 3% / Cu 0.5%. Apply BGA-specific solder paste to the chip pin area, then place the solder balls at the corresponding pins. After completion, use tweezers to place them on the preheating stage. Preheat at 300℃ for one minute, then start the hot air gun with the following parameters: preheating zone 250℃ for 60 seconds, heating zone 350℃ for 60 seconds, reflow zone 450℃ for 120 seconds, and cooling zone for 60 seconds. Set the airflow speed to 2 for each zone. After cooling, observe the ball placement quality. If defects are found, rework and repair the balls for future use.

[0040] 3. Substrate Preparation: The substrate is placed on an ultrafast laser processing stage, leveled and focused using a laser rangefinder, and then positioned at the processing location. The laser parameters are adjusted as follows: laser power 6W, scanning speed 300mm / s, laser repetition frequency 400kHz, pulse width 12ps, 5 scans, as per... Figure 2 The pattern shown is processed. After processing, the size of each packaging unit pattern is approximately 0.3mm × 0.3mm, the trench spacing is approximately 40μm, and the trench depth is approximately 3-5μm. Then, it is cleaned with anhydrous ethanol and set aside for later use.

[0041] 4. Chip Packaging: Apply flux evenly to the chip solder balls obtained in step 2, then place the chip with the solder balls facing down and the substrate processing surface from step 3 facing up, aligning the solder balls with the pin positions; then place it on a preheating stage, preheat at 300℃ for two minutes, then turn on the hot air gun and adjust the air speed to 4, keeping other parameters the same as in step 2; after cooling, observe whether the soldering positions are consistent and whether the soldering is firm.

[0042] 5. Solder Joint Morphology Observation: Observe the morphology of the processed substrate using a metallographic microscope. Polish the unprocessed and patterned substrates along the connection points to expose the solder joints, then clean them. First, observe the solder joint morphology using a metallographic microscope, then further observe the soldering quality using a scanning electron microscope.

[0043] 5. Thermal shock test: After observing the morphological characteristics, the sample is placed in the thermal shock test equipment. The temperature range is set to -40 to 125℃. The test is repeated for 15 minutes at -40℃ and 125℃ respectively. After every 100 cycles, the sample is taken out to observe the weld quality, compare the crack initiation and propagation, and evaluate the thermal fatigue reliability of the weld.

[0044] Comparative Example 1: The following details the implementation method of the patent.

[0045] Except for not performing ultrafast laser processing and directly attaching balls to the pretreated substrate for welding, the rest is basically the same as in Example 1.

[0046] The connection structure underwent thermal shock testing, and the results are shown below. Figure 5 .

[0047] Comparative Example 2: The following details the implementation methods of the patent.

[0048] Except for the ultrafast laser processing parameters in step 3, the rest is basically the same as in Example 1.

[0049] The parameters of the ultrafast laser processing equipment in this comparative example are: laser power 6W, scanning speed 300mm / s, laser repetition frequency 400KHz, pulse width 12ps, one scan, as follows. Figure 1 The shown graphic was processed; thermal shock tests were performed on the connection structure, and the results are shown in [the table below]. Figure 5 .

[0050] Comparative Example 3: The following details the implementation methods of the patent.

[0051] Except for the ultrafast laser processing parameters in step 3, the rest is basically the same as in Example 1.

[0052] The parameters of the ultrafast laser processing equipment in this comparative example are: laser power 6W, scanning speed 300mm / s, laser repetition frequency 400KHz, pulse width 12ps, 10 scans, as follows... Figure 1 The shown graphic was processed; thermal shock tests were performed on the connection structure, and the results are shown in [the table below]. Figure 5 .

[0053] Table 1

[0054]

Claims

1. A method for improving thermal fatigue resistance of solder joints of lead-free solder interconnect BGA packages, comprising: The dense arrangement of concentric hexagonal array microstructure is processed on the substrate by using ultrafast laser, the solder balls are adhered to the chip pins, the solder balls are adhered to the chip pins by melting, and the substrate and the chip are BGA packaged; ​ The dense arrangement of concentric hexagonal array microstructure is a dense array structure, which is formed by many concentric hexagonal structure units arranged in an array, each concentric hexagonal structure unit is formed by a plurality of nested hexagonal line grooves with the same center and a pitch of 30-50 um, and the six outermost sides of each concentric hexagonal structure unit are respectively in contact with the six outermost sides of another concentric hexagonal structure unit, and so on; the inside and around the groove are covered with a large number of micro-particles due to the use of ultrafast laser processing; The groove width is 30-50 um, and the groove depth is 3-5 um; each unit is nested with 5-10 levels of hexagons, i.e. 5-10 hexagons with different side lengths are nested; The substrate is a PCB substrate; the material of the chip is a chip material used by a power device; The substrate and the chip are arranged in parallel, and the opposite surfaces of the substrate and the chip are flat surfaces; The diameter of the solder ball is 0.3 mm.

2. The method of claim 1, wherein, The specific components of the solder ball are Sn96.5% / Ag3% / Cu0.5%.

3. The method of claim 1, wherein, Specifically, the following steps are included: (1) Chip trimming and cleaning: remove burrs on the surface or edge of the chip to ensure smooth subsequent process, clean impurities and dirt on the surface to ensure welding quality; (2) Solder ball array arrangement: the selection of solder balls is determined according to the number and size of chip pins, the position of solder balls corresponds to the chip pins, the alignment accuracy is maintained, after the solder ball array arrangement is completed, the chip is placed on the preheating table for preheating, then the solder ball is heated by using a hot air gun, so that the solder ball is adhered to the chip pin by melting; (3) Start the laser control system, adjust the parameters of the ultrafast laser processing equipment, including laser frequency, current, processing speed and times, prepare to process the dense arrangement of concentric hexagonal array microstructure; (4) Substrate preparation: the part of the substrate to be welded with the chip is located in the irradiation range of the laser beam in step (3), the displacement table of the laser control system is adjusted to ensure that the processing part is at the laser focal point, then the ultrafast laser processing equipment is controlled to process according to the target pattern, and the laser processing parameters are optimized, and the shape processed on the surface of the substrate is a regular hexagonal dense packing; (5) Chip and substrate welding: the chip with solder balls in step (2) is placed on the substrate processed in step (4), so that the pins on the chip correspond to the parts to be welded on the substrate, then it is placed on the preheating table for preheating, then the solder ball is heated by using a hot air gun, so that it is melted, the welding temperature and time are controlled to ensure the connection quality of the solder ball and the substrate; (6) Clean the welding slag and residual flux generated during the welding process to ensure product reliability; check the quality of the chip package, including welding quality, pin connectivity and appearance.

4. The method of claim 3, wherein, In the above process, the preheating table temperature is 200-400℃, and the hot air gun temperature parameters include preheating interval 200-300℃, heating interval 300-400℃, and reflow interval 400-500℃.

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