Method for efficiently improving thermal fatigue resistance of lead-free solder interconnection BGA packaging welding spot

Abstract

The invention discloses a method for efficiently improving thermal fatigue resistance of lead-free solder interconnection BGA packaging welding spots, and relates to the field of chip connection. The method comprises the following steps: processing densely arranged concentric hexagonal array microstructures on a substrate by using ultrafast laser, bonding solder balls at pins of a chip, melting and bonding the solder balls and the pins of the chip together, and carrying out BGA packaging on the substrate and the chip. According to the method, the connectivity of the welding flux and the substrate is improved by changing the morphological characteristics of the substrate. The method is simple to operate and wide in application range, and can meet the packaging requirements of different materials and processing parameters.

Description

Technical Field

[0001] The invention relates to the field of chip connection, in particular to a method for efficiently improving the thermal fatigue resistance of lead-free solder interconnection BGA packaging solder joints. Background Art

[0002] As integrated circuits develop towards small size and high precision, higher requirements are placed on the quality and reliability of chip packaging. Among various forms of chip packaging, ball grid array (BGA) packaging has the advantages of high integration, good conductivity and heat dissipation. BGA packaging interconnects the chip and the substrate through solder balls, and the solder balls serve as both electrical and mechanical connections. However, since the solder balls are hidden at the bottom of the chip, it is difficult to inspect and repair the solder joints of the BGA solder balls. Therefore, it is particularly important to increase the service life of BGA packaged devices and avoid their early failure. Solder joint failure is the most common form of failure in BGA packaging, and the main cause of solder joint failure is thermal fatigue of the solder joint caused by temperature. BGA packaged devices are subjected to repeated temperature changes during use, which exerts cyclic forces on the solder ball solder joints, causing thermal fatigue damage to the solder joints, and ultimately leading to thermal fatigue failure of the BGA solder joints.

[0003] Ultrafast laser technology has developed rapidly in the field of material surface processing and modification in recent years. Due to the characteristics of ultrafast laser pulse width, high peak power and small thermal impact, high-precision microstructure processing can be performed on the surface of various materials. Ultrafast laser technology is applied to BGA packaging: microstructures are processed on the substrate using ultrafast lasers. When the processed substrate is connected to the solder ball, the presence of the microstructure can increase the connection area and improve the strength of the connection. It can also release some stress and strain when the solder joint is subjected to temperature cycling. In addition, the increase in the connection area also further improves the heat dissipation of the package.

[0004] In summary, the use of ultrafast laser processing technology to prepare substrates with special microstructure surfaces has important scientific significance and research value in the field of enhancing the thermal fatigue reliability of BGA packages. Summary of the invention

[0005] The present invention provides a method for efficiently improving the thermal fatigue resistance of lead-free solder interconnected BGA package solder joints. The surface area and roughness of the substrate surface are increased by ultrafast laser processing of densely arranged concentric hexagonal array microstructures, which is beneficial to the subsequent welding of solder balls and the enhancement of the thermal fatigue reliability of solder joints.

[0006] A method for efficiently improving the thermal fatigue resistance of lead-free solder interconnect BGA packaging solder joints is characterized in that a densely arranged concentric hexagonal array microstructure is processed on a substrate using an ultrafast laser, solder balls are bonded to chip pins, the solder balls and chip pins are heated to melt and bond together, and the substrate and the chip are packaged in a BGA.

[0007] The densely packed concentric hexagonal array microstructure is a close-packed array structure, which is composed of many concentric hexagonal structural units closely arranged in an array structure, each concentric hexagonal structural unit is composed of nested multi-level hexagonal line grooves with the same center and a spacing of about 30um, and the outermost six sides of each concentric hexagonal structural unit are respectively bordered and overlapped with the outermost six sides of a concentric hexagonal structural unit, and so on; the inside and around 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 about 30-50um. When the multi-level hexagonal line grooves are nested, the distance between two adjacent hexagons is 30-50um, for example, 40um, and the groove depth is 3-5um. Each unit is nested with 5-10 levels of hexagons, that is, there are 5-10 hexagons with different side lengths nested.

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

[0010] Preferably, the material of the chip is the chip material used for power devices.

[0011] Preferably, the substrate and the chip are arranged in parallel, and the surfaces of the substrate and the chip facing each other are planes.

[0012] To achieve the above content, the present invention provides a close-packed concentric hexagonal array microstructure substrate surface, which is prepared by an ultrafast laser processing process, comprising the following steps:

[0013] (1) Chip trimming and cleaning: remove burrs on the chip surface or edge to ensure the smooth progress of subsequent processes, clean impurities and dirt on the surface, and ensure welding quality;

[0014] (2) Solder ball array arrangement: The selection of solder balls is determined by the number and size of chip pins. The position of the solder balls corresponds to the chip pins and the alignment accuracy is maintained. After the solder ball array arrangement is completed, the chip is placed on a preheating table for preheating. Then, a hot air gun is used to heat the solder balls so that the solder balls and chip pins are melted and bonded.

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

[0016] (4) Substrate preparation: The portion of the substrate to be welded to the chip is placed within the irradiation range of the laser beam in step (3), and the translation stage of the laser control system is adjusted to ensure that the processing portion is at the laser focus. Then, the ultrafast laser processing equipment is controlled to process according to the target pattern, and the laser processing parameters are optimized. The shape processed on the substrate surface is a close-packed regular hexagon;

[0017] (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 of the substrate to be welded, and then placed on a preheating table for preheating. Then, a hot air gun is used to heat the solder balls to melt them, and the welding temperature and time are controlled to ensure the quality of the connection between the solder balls and the substrate;

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

[0019] After packaging the substrates without laser processing and the substrates with laser processing, a comparative test of thermal fatigue reliability was conducted.

[0020] Preferably, a lead-free solder ball with a diameter of 0.3 mm is selected, and the specific composition is Sn96.5% / Ag3% / Cu0.5%; in the above process, the preheating table temperature is 200-400°C, and the hot air gun temperature parameters include a preheating range of 200-300°C, a heating range of 300-400°C, and a reflow range of 400-500°C.

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

[0022] At present, in the field of ball grid array (BGA) packaging, the method of enhancing the reliability of packaging connection mainly focuses on the selection of soldering materials, such as selecting substrate materials with good thermal expansion coefficients, and more reliable lead-free solder or silver-containing solder. The present invention improves the connectivity between solder and substrate by changing the morphological characteristics of the substrate. The method is simple to operate and has a wide range of applications, and can meet the packaging requirements of different materials and processing parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of this patent. For ordinary technicians in this field, other drawings can be obtained based on the following drawings without creative work.

[0024] Figure 1 It is a schematic diagram of chip and substrate packaging;

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

[0026] Figure 2 Schematic diagram of the process of processing the surface of a BGA substrate using an ultrafast laser;

[0027] Figure 3This is the surface morphology of the BGA substrate after ultrafast laser processing;

[0028] Figure 4 This is a cross-sectional microscopic image of a lead-free solder joint made on a BGA substrate after processing it using an ultrafast laser.

[0029] Figure 5 This is the microscopic morphology of the cross section of the sample with initial cracks after thermal shock test;

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

[0031] Table 1 is a comparison of the initial crack generation time of each connection structure. DETAILED DESCRIPTION

[0032] It should be noted that, in the absence of conflict, the features in the embodiments of the present invention and the comparative examples can be combined with each other. The described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0033] The present invention provides a BGA packaging substrate with a microstructure surface prepared by ultrafast laser to improve the thermal fatigue reliability of solder joints. Figure 1 , including a substrate 1, a pin portion on the substrate 1 is an ultrafast laser processed pin portion 2, a chip 3 is arranged above the pin portion 2, and a pre-implanted solder ball 4 is arranged on the chip 3. The substrate 1 and the chip 3 form an interconnection structure through the solder ball and the pin.

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

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

[0036] Furthermore, the substrate and the chip are arranged in parallel, and the surfaces of the substrate and the chip facing each other are planes.

[0037] Example 1: The following specifically describes the implementation method of the patent

[0038] 1. Chip and substrate preparation: Remove burrs on the surface or edges of the chip and substrate to be packaged to ensure smooth subsequent processes, then place them in alcohol for ultrasonic cleaning, dry them after cleaning to remove contaminants at the connection, and set them aside.

[0039] 2. Chip ball planting: According to the chip pin size, select a lead-free solder ball with a diameter of 0.3mm, with a specific composition of Sn96.5% / Ag3% / Cu0.5%. Use BGA package special flux paste to apply to the chip pin area, and then place the solder ball at the corresponding pin. After completion, use tweezers to place it on the preheating table. After preheating at 300℃ for one minute, start the hot air gun, and set the parameters to preheating interval 250℃, lasting 60s, heating interval 350℃, lasting 60s, reflow interval 450℃, lasting 120s, cooling interval lasting 60s, and the wind speed in each interval is set to 2. Observe the quality of ball planting after cooling. If there are defects, re-solder and set aside.

[0040] 3. Substrate preparation: Place the substrate on the ultrafast laser processing table, use a laser rangefinder to level and focus, and then place it at the processing position. Adjust the laser parameters to: laser power 6W, scanning speed 300mm / s, laser repetition frequency 400KHz, pulse width 12ps, scan 5 times, as shown below Figure 2 After processing, the size of each package unit pattern is about 0.3mm×0.3mm, the groove spacing is about 40μm, and the processing groove depth is about 3-5μm. Then it is cleaned with anhydrous ethanol and set aside.

[0041] 4. Chip packaging: Apply flux evenly to the chip solder balls obtained in step 2, then place the chip with the ball-planting surface facing down and the substrate processing surface facing up in step 3, so that the solder balls are aligned with the pin positions; then place it on the preheating table, preheat it at 300°C for two minutes, start the hot air gun, adjust the wind speed to 4, and keep other parameters the same as in step 2; after cooling, observe whether the welding positions are consistent and whether the welding is firm.

[0042] 5. Observation of solder joint morphology: Use a metallographic microscope to observe the morphology of the processed substrate. Grind and polish the unprocessed and processed substrates and the chip after welding along the joint until the solder joint is exposed and cleaned. First use a metallographic microscope to observe the solder joint morphology, and then use a scanning electron microscope to further observe the welding quality.

[0043] 5. Hot and cold shock test: Place the samples after observing the morphological characteristics into the hot and cold shock test equipment, set the temperature range to -40-125℃, keep them at -40℃ and 125℃ for 15 minutes respectively, and take them out after every 100 cycles to observe the quality of the solder joints, compare the generation and expansion of cracks, and judge the thermal fatigue reliability of the solder joints.

[0044] Comparative Example 1: The following specifically describes the implementation method of the patent

[0045] Except that the ultrafast laser processing is not performed and the ball bonding is performed directly on the pre-treated substrate, the rest is basically the same as that of Example 1.

[0046] The connection structure is subjected to thermal shock test, and the results are shown in Figure 5 .

[0047] Comparative Example 2: The following specifically describes the implementation method 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, scanning 1 time, as shown in Figure 1 The connection structure is processed by hot and cold shock test, and the results are shown in Figure 5 .

[0050] Comparative Example 3: The following specifically describes the implementation method 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, scanning 10 times, as shown in Figure 1 The connection structure is processed by hot and cold shock test, and the results are shown in Figure 5 .

[0053] Table 1

[0054]

Claims

1. A method for efficiently improving the thermal fatigue resistance of lead-free solder interconnect BGA package solder joints, characterized in that: An ultrafast laser is used to process a densely arranged concentric hexagonal array microstructure on the substrate, and solder balls are bonded to the chip pins so that the solder balls and the chip pins are melted and bonded together, and the substrate and the chip are packaged in BGA. The densely packed concentric hexagonal array microstructure is a close-packed array structure, which is composed of many concentric hexagonal structural units tightly arranged in an array structure, each concentric hexagonal structural unit is composed of a nested multi-level hexagonal line groove with the same center and a spacing of about 30um, and the outermost six sides of each concentric hexagonal structural unit are respectively bordered and overlapped with the outermost six sides of a concentric hexagonal structural unit, and so on; the inside and around the grooves are covered with a large number of microparticles due to the use of ultrafast laser processing.

2. The method according to claim 1, characterized in that The groove width is 30-50um, and when multi-level hexagonal line grooves are nested, the spacing between two adjacent hexagons is 30-50um, and the groove depth is 3-5um; each unit nests 5-10 levels of hexagons, that is, there are 5-10 hexagons with different side lengths nested.

3. The method according to claim 1, characterized in that The substrate is a PCB substrate; the material of the chip is the chip material used in power devices.

4. The method according to claim 1, characterized in that The substrate is arranged in parallel with the chip, and the surface of the substrate opposite to the chip is a plane.

5. The method according to claim 1, characterized in that The solder ball is a lead-free solder ball with a diameter of 0.3 mm, and the specific composition is Sn96.5% / Ag3% / Cu0.5%.

6. The method according to claim 1, characterized in that The specific steps include: (1) Chip trimming and cleaning: remove burrs on the chip surface or edge to ensure the smooth progress of subsequent processes, clean impurities and dirt on the surface, and ensure welding quality; (2) Solder ball array arrangement: The selection of solder balls is determined by the number and size of chip pins. The position of the solder balls corresponds to the chip pins and the alignment accuracy is maintained. After the solder ball array arrangement is completed, the chip is placed on a preheating table for preheating. Then, a hot air gun is used to heat the solder balls so that the solder balls and chip pins are melted and bonded. (3) starting the laser control system and adjusting the parameters of the ultrafast laser processing equipment, such as laser frequency, current, processing speed, and number of times, in preparation for processing densely arranged concentric hexagonal array microstructures; (4) Substrate preparation: The portion of the substrate to be welded to the chip is placed within the irradiation range of the laser beam in step (3), and the translation stage of the laser control system is adjusted to ensure that the processing portion is at the laser focus. Then, the ultrafast laser processing equipment is controlled to process according to the target pattern, and the laser processing parameters are optimized. The shape processed on the substrate surface is a close-packed regular hexagon; (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 of the substrate to be welded, and then placed on a preheating table for preheating. Then, a hot air gun is used to heat the solder balls to melt them, and the welding temperature and time are controlled to ensure the quality of the connection between the solder balls and the substrate; (6) Clean the solder slag and residual flux generated during the welding process to ensure product reliability; check the quality of chip packaging, including welding quality, pin connectivity and appearance.

7. The method according to claim 6, characterized in that In the above process, the temperature of the preheating table is 200-400°C, and the temperature parameters of the hot air gun include a preheating range of 200-300°C, a heating range of 300-400°C, and a reflow range of 400-500°C.

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