A kind of formic acid solder paste for chip packaging and its preparation method and application

Abstract

The invention provides a formate solder paste for chip packaging, a preparation method, and applications thereof. The solder paste comprises: 5% to 15% flux; and 85% to 95% lead-free solder powder. The lead-free solder powder comprises a first lead-free solder powder having a particle size d1 less than or equal to 8 μm, a second lead-free solder powder having a particle size d2 greater than 8 μm and less than or equal to 15 μm, and a third lead-free solder powder having a particle size d3 greater than 15 μm and less than or equal to 25 μm. The first lead-free solder powder has a particle size distribution of 10% to 20% of the total lead-free solder powder, the second lead-free solder powder has a particle size distribution of 25% to 40% of the total lead-free solder powder, and the third lead-free solder powder has a particle size distribution of 50% to 65% of the total lead-free solder powder. The formate solder paste for chip packaging can improve the soldering quality between the chip and the substrate, as well as packaging efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of solder paste, and in particular to a formic acid solder paste for chip packaging, and a preparation method and application thereof. Background Art

[0002] In the semiconductor packaging process, surface mount technology (SMT) is usually used to solder electronic components (such as chips, etc.) to the pads of the substrate. In the SMT patch processing process, the selection of solder paste is crucial, as it can directly affect the welding quality, packaging efficiency and performance of the final product. However, in the related art, the improvement of the welding quality and packaging efficiency of the solder paste between the chip and the substrate is limited, resulting in the performance of the final product being unable to meet the demand. Therefore, it is urgent to provide a chip packaging formic acid solder paste that can effectively improve the welding quality and packaging efficiency between the chip and the substrate. Summary of the Invention

[0003] In view of this, the present invention provides a formate solder paste for chip packaging, a preparation method and an application thereof, wherein the formate solder paste for chip packaging can improve the welding quality between the chip and the substrate and the packaging efficiency.

[0004] In a first aspect, an embodiment of the present invention provides a formate solder paste for chip packaging, wherein the formate solder paste for chip packaging, based on 100% of the total weight, comprises:

[0005] Flux, 5% to 15%;

[0006] Lead-free solder powder, 85% to 95%,

[0007] The lead-free solder powder includes a first lead-free solder powder having a particle size d1 less than or equal to 8 μm, a second lead-free solder powder having a particle size d2 greater than 8 μm and less than or equal to 15 μm, and a third lead-free solder powder having a particle size d3 greater than 15 μm and less than or equal to 25 μm. The particle size distribution of the first lead-free solder powder in the lead-free solder powder is 10% to 20%, the particle size distribution of the second lead-free solder powder in the lead-free solder powder is 25% to 40%, and the particle size distribution of the third lead-free solder powder in the lead-free solder powder is 50% to 65%.

[0008] According to any of the aforementioned embodiments of the present invention, the lead-free solder powder includes tin-silver-copper alloy particles, wherein the mass content a1 of silver in the first lead-free solder powder, the mass content a2 of silver in the second lead-free solder powder, and the mass content a3 of silver in the third lead-free solder powder satisfy a1; a2:a3=(7-9):(5-6):(3-4).

[0009] According to any of the aforementioned embodiments of the present invention, a1 is 3.75% to 4%;

[0010] and / or, a2 is 3% to 3.2%;

[0011] And / or, a3 is 1.9% to 2.1%.

[0012] According to any of the aforementioned embodiments of the present invention, the mass content b1 of silver and copper in the first lead-free solder powder, the mass content b2 of copper in the second lead-free solder powder, and the mass content b3 of copper in the third lead-free solder powder satisfy b1; b2:b3=(9-10):(7-8):(5-6).

[0013] According to any of the aforementioned embodiments of the present invention, b1 is 0.9% to 1%;

[0014] and / or, b2 is 0.7% to 0.8%;

[0015] And / or, b3 is 0.5% to 0.6%.

[0016] According to any of the aforementioned embodiments of the present invention, the lead-free solder powder further includes an anti-oxidation layer, and the anti-oxidation layer is coated on the surface of the tin-silver-copper alloy particles.

[0017] According to any of the aforementioned embodiments of the present invention, the anti-oxidation layer includes nickel and chromium elements.

[0018] According to any of the aforementioned embodiments of the present invention, the thickness of the anti-oxidation layer is 10 nm to 50 nm.

[0019] In a second aspect, an embodiment of the present invention provides a method for preparing a formate solder paste for chip packaging as described in any embodiment of the first aspect of the present invention, comprising:

[0020] a preparation step of mixing a first lead-free solder powder having a particle size d1 less than or equal to 8 μm, a second lead-free solder powder having a particle size d2 greater than 8 μm and less than or equal to 15 μm, and a third lead-free solder powder having a particle size d3 greater than 15 μm and less than or equal to 25 μm to obtain a lead-free solder powder, wherein the particle size distribution of the first lead-free solder powder in the lead-free solder powder is 10% to 20%, the particle size distribution of the second lead-free solder powder in the lead-free solder powder is 25% to 40%, and the particle size distribution of the third lead-free solder powder in the lead-free solder powder is 50% to 65%;

[0021] In a mixing step, the flux and the lead-free solder powder are mixed to obtain a formate solder paste for chip packaging, wherein, based on 100% of the total weight of the formate solder paste for chip packaging, the weight proportion of the flux is 5% to 15%, and the weight proportion of the lead-free solder powder is 85% to 95%.

[0022] In a third aspect, an embodiment of the present invention provides a use of a formate solder paste for chip packaging according to any embodiment of the first aspect of the present invention in chip packaging, comprising:

[0023] In the coating process, the chip packaging formate solder paste is coated on the pad of the substrate to form a solder layer

[0024] In the welding process, the chip is mounted on the solder layer and formic acid gas is introduced to perform reflow welding so as to weld the chip on the welding pad.

[0025] An embodiment of the present invention provides a formate solder paste for chip packaging, a preparation method and an application thereof. Based on 100% of the total weight of the formate solder paste for chip packaging, the solder paste includes: a flux, 5% to 15%; and a lead-free solder powder, 85% to 95%. The lead-free solder powder includes a first lead-free solder powder having a particle size d1 less than or equal to 8 μm, a second lead-free solder powder having a particle size d2 greater than 8 μm and less than or equal to 15 μm, and a third lead-free solder powder having a particle size d3 greater than 15 μm and less than or equal to 25 μm. The particle size distribution of the first lead-free solder powder in the lead-free solder powder is 10% to 20%, the particle size distribution of the second lead-free solder powder in the lead-free solder powder is 25% to 40%, and the particle size distribution of the third lead-free solder powder in the lead-free solder powder is 50% to 65%. In the above technical solution, a reasonable weight ratio of flux and lead-free solder powder can help improve soldering quality. The flux weight ratio effectively removes oxides from the surface of the soldered object, achieving the desired surface cleanliness. It also prevents re-oxidation during soldering, reduces surface tension, and improves soldering quality. The weight ratio of the lead-free solder powder, as well as the relative arrangement of the first lead-free solder powder, the second lead-free solder powder, and the third lead-free solder powder, can effectively reduce cracks and pores in the solder layer formed by the solder paste, further contributing to improved soldering quality.

[0026] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are specifically listed below. DETAILED DESCRIPTION

[0027] As used herein, "a," "the," "at least one," and "one or more" are used interchangeably. Thus, for example, a coating composition comprising "a" additional additive can be interpreted to mean that the coating composition comprises "one or more" additional additives.

[0028] For simplicity, some numerical ranges are disclosed herein. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, and likewise, any upper limit can be combined with any other upper limit to form an unspecified range. In addition, although not explicitly stated, each point or individual value between the endpoints of a range is included in the range. Thus, each point or individual value can serve as its own lower limit or upper limit and be combined with any other point or individual value or with other lower limits or upper limits to form an unspecified range.

[0029] The above summary of the invention is not intended to describe every disclosed embodiment or every implementation of the present invention. The following description more specifically illustrates exemplary embodiments. In many places throughout the application, guidance is provided by a series of examples, which can be used in various combinations. In each example, the enumeration is intended only as a representative group and should not be construed as exhaustive.

[0030] An embodiment of the present invention provides a formate solder paste for chip packaging. Based on 100% of the total weight of the formate solder paste for chip packaging, the solder paste includes: 5% to 15% of flux; 85% to 95% of lead-free solder powder, wherein the lead-free solder powder includes a first lead-free solder powder having a particle size d1 less than or equal to 8 μm, a second lead-free solder powder having a particle size d2 greater than 8 μm and less than or equal to 15 μm, and a third lead-free solder powder having a particle size d3 greater than 15 μm and less than or equal to 25 μm. The particle size distribution of the first lead-free solder powder in the lead-free solder powder is 10% to 20%, the particle size distribution of the second lead-free solder powder in the lead-free solder powder is 25% to 40%, and the particle size distribution of the third lead-free solder powder in the lead-free solder powder is 50% to 65%.

[0031] In the above technical solution, a reasonable combination of the weight ratio of flux and lead-free solder powder can help improve the welding quality. Among them, the weight ratio setting of the flux can effectively remove oxides on the surface of the welded objects, so that their surfaces reach the required cleanliness, and can also prevent their surfaces from re-oxidation during welding, reduce their surface tension, and improve welding quality.

[0032] The weight ratio of the lead-free solder powder is set within the above range, which can help improve the welding effect and mechanical properties. In addition, the particle size d1 of the first lead-free solder powder is less than or equal to 8μm, the particle size d2 of the second lead-free solder powder is greater than 8μm and less than or equal to 15μm, and the particle size d3 of the third lead-free solder powder is greater than 15μm and less than or equal to 25μm. In addition, the particle size distribution of the first lead-free solder powder in the lead-free solder powder is 10% to 20%, the particle size distribution of the second lead-free solder powder in the lead-free solder powder is 25% to 40%, and the particle size distribution of the third lead-free solder powder in the lead-free solder powder is 50% to 65%. The particle size distribution of the above lead-free solder powder can improve the wetting effect of the solder paste, and the particle size and its reasonable distribution can reduce cracks and pores in the solder layer formed by the solder paste, thereby further helping to improve the welding quality.

[0033] In the above embodiment, the particle size and distribution of the lead-free solder powder can be measured using instruments and methods well known in the art, such as using a laser particle size analyzer to measure and obtain the particle size and distribution of the lead-free solder powder.

[0034] In an embodiment of the present invention, the metal components and their contents in the lead-free solder powder can also be reasonably configured to further reduce cracks and pores in the solder layer formed by the solder paste, while also helping to form a highly uniform solder layer, thereby further improving the welding quality.

[0035] In some embodiments of the present invention, the lead-free solder powder includes tin-silver-copper alloy particles, wherein the mass content a1 of silver in the first lead-free solder powder, the mass content a2 of silver in the second lead-free solder powder, and the mass content a3 of silver in the third lead-free solder powder satisfy a1; a2: a3 = (7-9): (5-6): (3-4).

[0036] In the above embodiment, the ratio setting of a1, a2 and a3 can help to further reduce cracks and pores in the solder layer formed by the solder paste, and can also help to improve the height uniformity of the solder layer, thereby further improving the welding quality.

[0037] Furthermore, a1 is 3.75% to 4%; and / or a2 is 3% to 3.2%; and / or a3 is 1.9% to 2.1%.

[0038] In some embodiments of the present invention, the mass content b1 of silver and copper in the first lead-free solder powder, the mass content b2 of copper in the second lead-free solder powder, and the mass content b3 of copper in the third lead-free solder powder satisfy b1; b2:b3=(9-10):(7-8):(5-6).

[0039] In the above embodiment, the ratio of b1, b2 and b3 can help improve the strength and corrosion resistance of the solder layer, thereby helping to improve the welding quality.

[0040] Furthermore, b1 is 0.9% to 1%; and / or b2 is 0.7% to 0.8%; and / or b3 is 0.5% to 0.6%.

[0041] In the lead-free solder powder of the above embodiment, the mass content of tin is the remaining ratio excluding the above-mentioned components, and it can also be appropriately adjusted according to the content of each component.

[0042] In addition, the structure of lead-free solder powder can be improved to further enhance its anti-oxidation and anti-cracking properties to improve welding quality.

[0043] In some embodiments of the present invention, the lead-free solder powder further includes an anti-oxidation layer, and the anti-oxidation layer is coated on the surface of the tin-silver-copper alloy particles.

[0044] In some embodiments of the present invention, the anti-oxidation layer includes nickel and chromium. The addition of nickel and chromium can improve the corrosion resistance and crack resistance of the solder layer, thereby further improving the welding quality.

[0045] In some embodiments of the present invention, the thickness of the anti-oxidation layer is 10 nm to 50 nm. When the thickness of the anti-oxidation layer is within the above range, the corrosion resistance and crack resistance can be further enhanced.

[0046] The embodiments of the present invention are not particularly limited to the solder flux, and any solder flux known in the art may be used. For example, the solder flux may include main components such as a resin (e.g., rosin, acrylic resin, etc.), an activator (e.g., ethylamine, citric acid, propylamine, benzoic acid, lactic acid, etc.), and a thixotropic agent (e.g., beeswax, carnauba wax, etc.).

[0047] In a second aspect, an embodiment of the present invention provides a method for preparing a formic acid solder paste for chip packaging according to any embodiment of the first aspect of the present invention, comprising:

[0048] a preparation step of mixing a first lead-free solder powder having a particle size d1 less than or equal to 8 μm, a second lead-free solder powder having a particle size d2 greater than 8 μm and less than or equal to 15 μm, and a third lead-free solder powder having a particle size d3 greater than 15 μm and less than or equal to 25 μm to obtain a lead-free solder powder, wherein the first lead-free solder powder has a particle size distribution of 10% to 20% of the lead-free solder powder, the second lead-free solder powder has a particle size distribution of 25% to 40% of the lead-free solder powder, and the third lead-free solder powder has a particle size distribution of 50% to 65% of the lead-free solder powder;

[0049] In a mixing step, the flux and the lead-free solder powder are mixed to obtain a formate solder paste for chip packaging, wherein, based on 100% of the total weight of the formate solder paste for chip packaging, the weight proportion of the flux is 5% to 15%, and the weight proportion of the lead-free solder powder is 85% to 95%.

[0050] In a third aspect, an embodiment of the present invention provides a use of a formate solder paste for chip packaging according to any embodiment of the first aspect of the present invention in chip packaging, comprising:

[0051] A coating process is to coat the chip packaging with formate solder paste on the pads of the substrate to form a solder layer;

[0052] In the welding process, the chip is mounted on the solder layer and formic acid gas is introduced to perform reflow welding to weld the chip to the welding pad.

[0053] In the above embodiment, formic acid gas can react with metal oxides generated during welding, that is, hydroxyl groups replace oxygen atoms to form formates, and formates decompose at high temperatures to form metal elements, eliminating the adverse effects of metal oxides on the welds.

[0054] The following embodiments describe the present disclosure in more detail, and these embodiments are intended to be illustrative only, as various modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are by mass, and all reagents used in the embodiments are commercially available or synthesized according to conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0055] The following embodiments describe the present disclosure in more detail, and these embodiments are intended to be illustrative only, as various modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are by mass, and all reagents used in the embodiments are commercially available or synthesized according to conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0056] Example 1

[0057] This embodiment provides a method for preparing formate solder paste, comprising:

[0058] The first lead-free solder powder Sn95.29Ag3.8Cu0.91, the second lead-free solder powder Sn96.18Ag3.1 Cu0.72 and the third lead-free solder powder Sn97.36Ag2.1 Cu0.54 in Table 1 were mixed to obtain lead-free solder powder;

[0059] Based on a total weight of 100% formate solder paste, 2% rosin, 3% citric acid, 1% beeswax, 3% diethylene glycol ethyl ether and 91% lead-free solder powder were placed in a disperser and stirred at a stirring speed of 25 rpm / min for 25 minutes. The disperser was then evacuated and the stirring speed was maintained under a vacuum environment for another 20 minutes to obtain the formate solder paste.

[0060] Example 2

[0061] The difference between this embodiment and embodiment 1 lies in the particle size distribution of the first lead-free solder powder, the second lead-free solder powder and the third lead-free solder powder.

[0062] Example 3

[0063] The difference between this embodiment and embodiment 1 lies in the particle size distribution of the first lead-free solder powder, the second lead-free solder powder and the third lead-free solder powder.

[0064] Example 4

[0065] The difference between this embodiment and embodiment 1 lies in the particle size distribution of the first lead-free solder powder, the second lead-free solder powder and the third lead-free solder powder.

[0066] Example 5

[0067] The difference between this embodiment and embodiment 1 lies in the particle size distribution of the first lead-free solder powder, the second lead-free solder powder and the third lead-free solder powder.

[0068] Example 6

[0069] The difference between this embodiment and embodiment 1 lies in: the first lead-free solder powder is Sn95.34Ag3.75Cu0.91, the second lead-free solder powder is Sn96.28Ag3.0Cu0.72, and the third lead-free solder powder is Sn97.46Ag2.0Cu0.54.

[0070] Example 7

[0071] The difference between this embodiment and embodiment 1 lies in: the first lead-free solder powder is Sn95.09Ag4.0Cu0.91, the second lead-free solder powder is Sn96.08Ag3.2Cu0.72, and the third lead-free solder powder is Sn97.56Ag1.9Cu0.54.

[0072] Example 8

[0073] The difference between this embodiment and embodiment 1 lies in: the first lead-free solder powder is Sn95.24Ag3.8Cu0.96, the second lead-free solder powder is Sn96.15Ag3.1Cu0.76, and the third lead-free solder powder is Sn97.34Ag2.1Cu0.56.

[0074] Example 9

[0075] The difference between this embodiment and embodiment 1 lies in: the first lead-free solder powder is Sn95.2Ag3.8Cu1.0, the second lead-free solder powder is Sn96.1Ag3.1Cu0.8, and the third lead-free solder powder is Sn97.3Ag2.1Cu0.6.

[0076] Example 10

[0077] The difference between this embodiment and embodiment 1 is that the surfaces of the first lead-free solder powder, the second lead-free solder powder, and the third lead-free solder powder are coated with a 25 nm thick anti-oxidation layer containing nickel and chromium elements. The specific method is as follows:

[0078] Argon gas was introduced at a flow rate of 120 sccm. After the bias voltage was 110 V, the duty cycle was 45%, the vacuum degree reached 0.003 Pa, and the temperature reached 400°C, the chromium-nickel target power supply was turned on, and deposition processing was performed under the condition that the current of each chromium-nickel target was 50 A to form an anti-oxidation layer on the surface of the lead-free solder powder.

[0079] Comparative Example 1

[0080] The difference between this comparative example and Example 1 is that the first lead-free solder powder is not included.

[0081] Comparative Example 2

[0082] The difference between this comparative example and Example 1 is that the second lead-free solder powder is not included.

[0083] Comparative Example 3

[0084] The difference between this comparative example and Example 1 is that the third lead-free solder powder is not included.

[0085] Comparative Example 4

[0086] The difference between this comparative example and Example 1 lies in the mass content of silver in the first lead-free solder powder, the second lead-free solder powder and the third lead-free solder powder.

[0087] Comparative Example 5

[0088] The mass content of copper in the first lead-free solder powder, the second lead-free solder powder and the third lead-free solder powder.

[0089] Test section

[0090] 1) Crack resistance test

[0091] 0.3 g of formic acid solder paste prepared in Examples 1-10 and Comparative Examples 1-5 was coated on the central part of a 0.5 mm thick substrate (an area of approximately 4 mm × 4 mm) to prepare a sample, and then the sample was placed in a reflow oven and heated by introducing formic acid gas. The reflow conditions were as follows: preheating temperature of 190°C for 90 seconds, peak temperature of 260°C for 100 seconds, and finally a cooling rate of 2°C / s from the peak temperature to 200°C to obtain a reflowed sample.

[0092] After reflowing, the sample was cut and the cross-section was polished. The polished cross-section was then observed using a scanning electron microscope. The size of the intermetallic compound structure precipitated in the reflowed solder was measured and evaluated according to the following criteria. The smaller the size of the intermetallic compound structure, the better the crack resistance. The test results are shown in Table 2.

[0093] Very good: The largest observed structure size is less than 30 μm.

[0094] Good: The maximum observed structure size is 30 μm or more and 80 μm or less.

[0095] Poor: The largest observed tissue size exceeds 80 μm.

[0096] 2) Pore area test

[0097] The formic acid solder paste prepared in Examples 1-10 and Comparative Examples 1-5 was printed onto a substrate for chip packaging, and the chip was mounted by a reflow soldering method, wherein a metal mask with a thickness of 200 μm was used to adjust the printed film thickness of the solder paste. After printing the solder paste, a chip with a length of 20 mm and a width of 12 mm was mounted on a specified position of the above-mentioned substrate and heated in a reflow soldering furnace. The reflow soldering conditions were: a preheating temperature of 190°C, a peak temperature of 250°C, and a heating time of 150 s.

[0098] After the substrate cooled, the solder surface was observed using X-rays. The ratio of the total area of pores to the area where solder was formed (pore area ratio) was measured. The pore formation was evaluated by averaging the pore area ratios at 10 solder pads on the substrate using the following criteria. The smaller the average pore area ratio, the better the soldering quality. The test results are shown in Table 2.

[0099] Very good: The average value of the pore area ratio is 4% or less.

[0100] Good: The average value of the pore area ratio is more than 4% and 8% or less.

[0101] Poor: The average value of the porosity area ratio exceeds 8%.

[0102] 3) Solder layer height test

[0103] The formic acid solder paste prepared in Examples 1-10 and Comparative Examples 1-5 was coated on the central part of a 0.5 mm thick substrate (an area of approximately 4 mm × 4 mm) to prepare a sample, and then the sample was placed in a reflow oven and heated by introducing formic acid gas. The reflow conditions were as follows: preheating temperature of 190°C for 90 seconds, peak temperature of 260°C for 100 seconds, and finally a cooling rate of 2°C / s when cooling from the peak temperature to 200°C to obtain a reflowed sample.

[0104] The distance from the bottom to the top of the solder layer was measured using a focal depth meter and used as the solder layer height. The solder layer height was measured at 100 random locations. The average value (average solder layer height) and height variation (standard deviation) were calculated from these measurements. The test results are shown in Table 2.

[0105] Table 1 Particle size distribution and component content of lead-free solder powder in Examples 1-10 and Comparative Examples 1-5

[0106]

[0107] Table 2 Test results of Examples 1-10 and Comparative Examples 1-5

[0108]

[0109]

[0110] According to Table 1, by comparing the test results of Examples 1-10 and Comparative Examples 1-5, it can be seen that the embodiments of the present invention, through the related arrangement of the first lead-free solder powder, the second lead-free solder powder, and the third lead-free solder powder included in the lead-free solder powder, can not only effectively reduce cracks and pores in the solder layer formed by the solder paste, but also make the height of the solder layer formed by the solder paste more uniform, thereby further helping to improve the welding quality.

[0111] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and description of the present invention. In particular, as long as there is no structural conflict, the various technical features mentioned in the various embodiments can be combined in any way. The present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions that fall within the scope of the claims.

Claims

1. A formic acid solder paste for chip packaging, characterized in that: The formate solder paste for chip packaging, based on 100% of the total weight, comprises: Flux, 5% to 15%; Lead-free solder powder, 85% to 95%, The lead-free solder powder includes a first lead-free solder powder having a particle size d1 less than or equal to 8 μm, a second lead-free solder powder having a particle size d2 greater than 8 μm and less than or equal to 15 μm, and a third lead-free solder powder having a particle size d3 greater than 15 μm and less than or equal to 25 μm, wherein the particle size distribution of the first lead-free solder powder in the lead-free solder powder is 10% to 20%, the particle size distribution of the second lead-free solder powder in the lead-free solder powder is 25% to 40%, and the particle size distribution of the third lead-free solder powder in the lead-free solder powder is 50% to 65%; The lead-free solder powder includes tin-silver-copper alloy particles, wherein the mass content a1 of silver in the first lead-free solder powder, the mass content a2 of silver in the second lead-free solder powder, and the mass content a3 of silver in the third lead-free solder powder satisfy a1; a2:a3=(7-9):(5-6):(3-4); a1 is 3.75% to 4%; and / or, a2 is 3% to 3.2%; and / or, a3 is 1.9% to 2.1%; The mass content b1 of silver and copper in the first lead-free solder powder, the mass content b2 of copper in the second lead-free solder powder, and the mass content b3 of copper in the third lead-free solder powder satisfy b1; b2:b3=(9-10):(7-8):(5-6); b1 is 0.9% to 1%; and / or, b2 is 0.7% to 0.8%; and / or, b3 is 0.5% to 0.6%; The lead-free solder powder further includes an anti-oxidation layer, which is coated on the surface of the tin-silver-copper alloy particles; The anti-oxidation layer includes nickel and chromium elements.

2. The formate solder paste for chip packaging according to claim 1, wherein The thickness of the anti-oxidation layer is 10 nm to 50 nm.

3. A method for preparing a formic acid solder paste for chip packaging according to any one of claims 1 to 2, characterized in that: include: a preparation step of mixing a first lead-free solder powder having a particle size d1 less than or equal to 8 μm, a second lead-free solder powder having a particle size d2 greater than 8 μm and less than or equal to 15 μm, and a third lead-free solder powder having a particle size d3 greater than 15 μm and less than or equal to 25 μm to obtain a lead-free solder powder, wherein the particle size distribution of the first lead-free solder powder in the lead-free solder powder is 10% to 20%, the particle size distribution of the second lead-free solder powder in the lead-free solder powder is 25% to 40%, and the particle size distribution of the third lead-free solder powder in the lead-free solder powder is 50% to 65%; In a mixing step, the flux and the lead-free solder powder are mixed to obtain a formate solder paste for chip packaging, wherein, based on 100% of the total weight of the formate solder paste for chip packaging, the weight proportion of the flux is 5% to 15%, and the weight proportion of the lead-free solder powder is 85% to 95%.

4. A use of the formic acid solder paste for chip packaging according to any one of claims 1 to 2 in chip packaging, characterized in that: include: a coating step of coating the chip encapsulation formate solder paste on the pads of the substrate to form a solder layer; In the welding process, the chip is mounted on the solder layer and formic acid gas is introduced to perform reflow welding so as to weld the chip on the welding pad.

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