Welding structure of low-temperature solder and manufacturing method thereof

Abstract

The present invention provides a solder structure of low-temperature soldering, which comprises, in sequence from bottom to top, a circuit layer of a printed circuit board, an intermetallic compound layer, a first solder diffusion layer, a solder metal layer, a second solder diffusion layer, and an electrode layer of an electronic component. The surface treatment of the circuit layer of the printed circuit board is performed by using a lead-free solder alloy pad, so that the solder structure after soldering has good cold and hot cycle reliability. The present invention also provides a manufacturing method of a solder structure for manufacturing the aforementioned solder structure of low-temperature soldering.

Description

Technical Field

[0001] The present invention relates to an assembly welding structure of electronic components, and more particularly to a welding structure formed by welding with electronic components using low-temperature solder paste to form a welding structure with high reliability and a manufacturing method thereof. Background Art

[0002] In the current soldering process, printed circuit boards (PCBs) are typically protected by a coating of organic solderability preservative (OSP). Tin or tin alloy electrodes are attached to the bottom surface of electronic components, such as the ball grid array (BGA) on the bottom surface of a chip. The assembly process involves printing solder paste on the OSP layer, placing the component electrodes on the solder paste layer, and then passing the solder paste through a soldering furnace to melt and diffuse it, forming the solder structure and completing the assembly.

[0003] The recent increase in the density of integrated circuit (IC) chips has led to thinner and larger packaging structures. When soldering with SAC (tin-silver-copper) solder paste, the high temperatures of approximately 245°C to 260°C can cause the IC package structure to deform and lose its flatness during the heating process, leading to defects such as empty solder joints and short circuits. Therefore, lowering the soldering temperature to minimize deformation of the IC package is one method to alleviate soldering defects. Consequently, using a low-temperature solder alloy containing large amounts of bismuth, such as a tin-bismuth alloy paste with a bismuth content greater than 10% by weight (melting point approximately 140°C to 180°C), to lower the soldering temperature has become a primary solution.

[0004] Because of its low melting point, the aforementioned tin-bismuth alloy solder paste allows for low-temperature soldering during assembly, minimizing component deformation. When soldering tin-bismuth alloy solder paste to PCBs with conventional OSP finishes, the electrodes are often made of pure tin or higher-melting-point alloys such as tin-silver-copper. Therefore, to achieve optimal soldering results, the temperature of the tin-bismuth alloy solder paste (melting point less than 180°C) must be increased by approximately 40°C above its original melting point (melting point approximately 140°C), or the reflow time must be increased. This improves the fluidity of the tin-bismuth alloy solder paste, sometimes causing significant bismuth diffusion to the substrate end of components such as BGA balls, forming an intermetallic compound (IMC) of gold and bismuth, leading to gold brittleness between the ball and the substrate. Therefore, the bismuth diffusion height is limited to only one-third of the height of the BGA ball. Furthermore, using the same tin-bismuth alloy solder paste at lower reflow temperatures, while not causing significant bismuth diffusion upward, can result in large areas of tin-bismuth concentration above the copper trace. This results in the soldered portion (called the tin-bismuth solder layer) being a low-temperature solder layer with high brittleness. Once this zone forms, the solder joint is not only vulnerable to mechanical shock (such as drop tests), but also prone to bismuth diffusion during repeated hot and cold cycles, causing the growth of bismuth-rich phases and the formation of voids. Combined with the stress of thermal expansion and contraction, this can cause the tin-bismuth zone to fracture, a phenomenon known as hot-tearing. Consequently, low-temperature solder joints are less reliable than those formed with traditional tin-silver-copper alloy SAC305 solder. Although there are many types of bismuth-containing solder alloys, this phenomenon is difficult to avoid when soldering to OSP surface treatments. Summary of the Invention

[0005] Therefore, the purpose of the present invention is to provide a low-temperature soldering structure, which is a printed circuit board using lead-free solder alloy pads, so that the soldering structure after combining electronic components has good resistance to cold and hot cycle reliability.

[0006] Based on the above-mentioned objectives, a low-temperature soldering structure is provided, comprising at least: a circuit layer; an intermetallic compound (IMC) layer formed on the circuit layer; a first solder diffusion layer formed on the IMC layer; a solder metal layer formed on top of the first solder diffusion layer; a second solder diffusion layer formed on top of the solder metal layer; and an electrode layer of an electronic component, the electrode layer being located on top of the second solder diffusion layer.

[0007] According to the aforementioned low-temperature soldering structure, the circuit layer is a copper-containing copper circuit, and the IMC layer is primarily a metal compound formed by the reaction of copper and tin, formed during the fabrication of a pad structure during low-temperature soldering. The first solder diffusion layer is a lower bismuth diffusion layer formed by the diffusion of bismuth from a lead-free tin-bismuth alloy solder paste into a lead-free solder alloy pad layer during soldering. The solder metal layer is a lead-free tin-bismuth alloy layer, representing the undiffused portion of the lead-free tin-bismuth alloy solder paste after soldering. The second solder diffusion layer is an upper bismuth diffusion layer formed by the diffusion of bismuth from the lead-free tin-bismuth alloy solder paste into the electrode layer during soldering. The electrode layer is a tin or tin alloy electrode layer, or a tin alloy ball (e.g., a BGA ball) attached to the bottom surface of an electronic component or the input / output (I / O) of a package structure.

[0008] Based on the aforementioned low-temperature solder welding structure, the present invention further provides a method for manufacturing the welding structure, which sequentially includes the following steps: a step of providing a printed circuit board with lead-free solder alloy pads, a step of printing a low-temperature tin-bismuth alloy solder paste, a step of placing electronic components, and a step of reflow heating. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 It is a structural schematic diagram of the welding pad structure of the present invention.

[0010] Figure 2 It is a structural schematic diagram of the welding structure of the present invention.

[0011] Description of the figure number:

[0012] 1: Pad structure

[0013] 10: Printed Circuit Board

[0014] 11: Circuit layer

[0015] 12: IMC layer

[0016] 13: First welding diffusion layer

[0017] 14: Welding metal layer

[0018] 15: Second welding diffusion layer

[0019] 16: Electrode layer

[0020] 17: Electronic components

[0021] 100: Welded structure

[0022] A: Lead-free solder alloy pad layer. DETAILED DESCRIPTION

[0023] First see Figure 1A solder pad structure 1 of the present invention includes, from bottom to top, at least: a printed circuit board 10, a circuit layer 11, an IMC layer 12, and a lead-free solder alloy pad layer A. The circuit layer 11 is located on the upper surface of the printed circuit board 10 and is generally a conductive circuit pattern. The material of the circuit layer 11 can be copper. In other words, the circuit layer 11 is a copper circuit. The IMC layer 12 is formed on the upper surface of the circuit layer 11. The IMC layer 12 is mainly a metal compound formed by copper and tin or other elements. The lead-free solder alloy pad layer A is located on the upper surface of the IMC layer 12. The material of the lead-free solder alloy pad layer A includes tin, such as a tin-containing alloy, preferably a tin-silver-copper alloy or a tin-copper alloy or other lead-free solder alloy containing less than 1wt% of bismuth. In practical applications, the lead-free solder material may be SAC305 (96.5wt% Sn, 3.0wt% Ag, 0.5wt% Cu) or SAC0307 (99.0wt% Sn, 0.3wt% Ag, 0.7wt% Cu). Preferably, the material of the lead-free solder alloy pad layer A does not contain intentionally added bismuth. Alternatively, the lead-free solder alloy pad layer A is a tin-containing alloy layer or a lead-free solder alloy layer containing less than 1wt% bismuth. This lead-free solder alloy layer (the lead-free solder alloy pad layer A) can be formed using electroplating, tin spraying, solder paste heating, or other methods, with a thickness of approximately 10μm to 100μm (micrometers), but does not contain alloys with a high concentration of bismuth (greater than 1wt%). It should be noted that the term "lead-free" as used herein refers to compliance with the RoHS (Restriction of Hazardous Substances) lead-free standard. The aforementioned "substantially lead-free" designation means that, in principle, as long as lead is not intentionally added to the alloy (e.g., as an unintentional but unavoidable impurity or contact during the manufacturing process), the alloy can be considered substantially lead-free or lead-free. "wt%" refers to percentage by weight. Furthermore, the numerical ranges described herein are always inclusive.

[0024] See also Figure 2 The low-temperature soldering structure 100 of the present invention includes, from bottom to top, a circuit layer 11, an IMC layer 12, a first solder diffusion layer 13, a solder metal layer 14, a second solder diffusion layer 15, and an electrode layer 16 of an electronic component 17. The IMC layer 12 is formed on the circuit layer 11; the first solder diffusion layer 13 is formed on the IMC layer 12; the solder metal layer 14 is formed on the first solder diffusion layer 13; the second solder diffusion layer 15 is formed on the solder metal layer 14; and the electrode layer 16 is located on the second solder diffusion layer 15 and is also attached to the bottom surface of the electronic component 17.

[0025] The first solder diffusion layer 13 is a lower bismuth diffusion layer containing tin and bismuth. The solder metal layer 14 is a tin-bismuth alloy layer containing tin and bismuth. The second solder diffusion layer 15 is an upper bismuth diffusion layer containing tin and bismuth. The electrode layer 16 is a solder alloy electrode layer containing tin.

[0026] The low-temperature soldering structure 100 is manufactured using a method for manufacturing a soldering structure. The method for manufacturing a soldering structure includes the following steps in sequence.

[0027] A step of providing a printed circuit board with a lead-free solder alloy pad is as follows: providing the pad structure 1, which comprises at least: the printed circuit board 10, the circuit layer 11, the IMC layer 12 and the lead-free solder alloy pad layer A in order from bottom to top.

[0028] A step of printing a low-temperature tin-bismuth alloy solder paste is performed by printing a bismuth-containing low-temperature solder paste (lead-free tin-bismuth alloy solder paste) on the upper surface of the lead-free solder alloy solder pad layer A of the solder pad structure 1. For example, the lead-free tin-bismuth alloy solder paste can be a tin-bismuth solder paste (e.g., 42 wt% tin and 58 wt% bismuth), or a tin-bismuth-silver solder paste (e.g., 42 wt% tin, 57 wt% bismuth, and 1 wt% silver). Alternatively, a low-temperature solder paste can be used that is primarily tin-bismuth and contains a few ppm to several wt% of one or more other metal elements.

[0029] An electronic component placement step: stacking the electrode layer 16 of the electronic component 17 on the lead-free tin-bismuth alloy solder paste. In other words, the intermediate process is to stack the electronic component 17 or the BGA ball on the lead-free tin-bismuth alloy solder paste to form a semi-finished solder structure.

[0030] A reflow heating step: heating the solder structure semi-product at 160-190°C for 4.5-6.5 minutes (for example, using a reflow oven), and then cooling, for example, to room temperature 25°C. During the reflow heating step, the bismuth in the lead-free Sn-Bi alloy paste partially migrates (also referred to as diffusion) downward to the lead-free solder alloy pad layer A, causing the lead-free solder alloy pad layer A to become the first solder diffusion layer 13, in other words, the first solder diffusion layer 13 is a lower bismuth diffusion layer formed from the lead-free solder alloy pad layer A upon heating due to the diffusion of bismuth from the lead-free Sn-Bi alloy paste; the bismuth in the lead-free Sn-Bi alloy paste also partially migrates upward to the lower portion of the electrode layer 16, causing the lower portion of the electrode layer 16 to become the second solder diffusion layer 15, in other words, the second solder diffusion layer 15 is an upper bismuth diffusion layer caused by the diffusion of bismuth from the lead-free Sn-Bi alloy paste into the lower portion of the electrode layer 16 upon heating; and the lead-free Sn-Bi alloy paste forms the solder metal layer 14 upon heating, which is a Sn-Bi alloy layer, and is the portion of the lead-free Sn-Bi alloy paste that does not diffuse after soldering. Thus, the low-temperature soldering structure 100 is obtained. The concentration of bismuth in the first solder diffusion layer 13 is less than the concentration of bismuth in the solder metal layer 14, and the concentration of bismuth in the second solder diffusion layer 15 is less than the concentration of bismuth in the solder metal layer 14.

[0031] If the lead-free solder alloy pad layer is not used, the lower bismuth diffusion layer (i.e., the first solder diffusion layer) cannot be formed when making the low-temperature soldering structure, in other words, as in the prior art, when using a Sn-Bi alloy paste for soldering a PCB with a general OSP surface treatment, only an upper bismuth diffusion layer (upper bismuth diffusion layer) is formed, and a lower bismuth diffusion layer (lower diffusion layer) is not formed. However, the organization structure and method of the present application have the following advantages over the prior art: (1) the thickness of the Sn-Bi alloy layer (i.e., the solder metal layer 14) is reduced, avoiding hard and brittle and excessive low-temperature regions; (2) the upper and lower diffusion layers can be formed at the same time, and the bismuth content of the upper and lower diffusion layers is less than that of the solder metal layer 14, so the toughness is greater than that of the prior art Sn-Bi soldering layer, and the melting point is also higher, avoiding the effects of hot tearing and high brittleness; (3) during the reflow heating step, since the lead-free Sn-Bi alloy paste does not come into contact with the copper of the circuit layer 11, the bismuth in the lead-free Sn-Bi alloy paste is not subject to the thermodynamic effects caused by the poor compatibility of bismuth and copper, which would cause a large amount of bismuth to unilaterally diffuse to the component end, for example, the BGA ball end, of the electrode layer, so the present application easily meets the requirement that the height of the second solder diffusion layer be less than one-third of the height of the BGA ball. Thus, compared to the prior art, the low-temperature soldering structure 100 of the present application has better cold and hot cycle reliability, and relevant examples will be described below.

[0032] <Examples 1 to 3 and Comparative Examples 1 to 3>

[0033] Examples 1 to 3 and Comparative Examples 1 to 3 are electronic components using BGA balls with a 25*25 array of solder balls (i.e., the aforementioned electrode layer 16, SAC305 composition of 96.5wt% tin, 3.0wt% silver, and 0.5wt% copper, with a ball diameter of 0.45mm). The corresponding bismuth-containing solder paste in Table 1 is printed using a steel plate printing method. The solder pads on the PCBs of Examples 1 to 3 are lead-free solder alloy solder pad layers of approximately 25μm, and the printed thickness of the bismuth-containing solder paste is 50μm, with a reflow peak of approximately 190°C. The solder pads on the PCBs of Comparative Examples 1 to 3 are OSP, and the printed thickness of the bismuth-containing solder paste is 100μm, and soldering is performed using the same reflow curve as the examples. Among them, the solder pad structure (OSP solder pad) of Comparative Examples 1 to 3 is composed of a printed circuit board, a circuit and an OSP layer from bottom to top, which is generally known and will not be described here in detail; while the solder pad structure (lead-free solder alloy solder pad) of Examples 1 to 3 is composed of the printed circuit board 10, the circuit layer 11, the IMC layer 12 and the lead-free solder alloy solder pad layer A from bottom to top, which is the structure of the present invention. Then, the soldering structures of Comparative Examples 1 to 3 and Examples 1 to 3 are placed in a hot and cold cycle tester to conduct hot and cold cycle reliability tests. The test in the hot and cold cycle tester is to maintain a low temperature of -40°C for 7 minutes, then increase the temperature at a rate of 15°C / min until it reaches a high temperature of 100°C and maintain it at 100°C for 7 minutes, and then cool it down at a rate of 15°C / min until it reaches -40°C. This is considered as one cycle. The welded structures were removed at the 500th, 1000th, and 2000th cycles, and cracks in the stacked structures were observed and measured. The criterion for judging failure was that a crack length greater than 25 μm was considered a failure, indicating hot-tearing. The number of failures at the 500th, 1000th, and 2000th cycles was used to compare the Example with the Comparative Example.

[0034] Table 1

[0035]

[0036]

[0037] SAC305: 96.5wt% tin, 3.0wt% silver, 0.5wt% copper;

[0038] SAC0307: 99.0wt% tin, 0.3wt% silver, 0.7wt% copper;

[0039] SnBi: 42 wt% tin and 58 wt% bismuth;

[0040] SnBiAg: 42 wt% tin, 57 wt% bismuth and 1 wt% silver.

[0041] From Table 1, the maximum number of defects in Example 1 is 1, and the maximum number of defects in Comparative Example 1 is 16. The amount of defects (i.e., hot tearing) in Example 1 is much smaller than that in Comparative Example 1. Obviously, the welding structure of Example 1 of the present invention improves the reliability of the traditional welding structure of Comparative Example 1 in hot and cold cycles. The maximum number of defects in Example 3 is 2, and the maximum number of defects in Comparative Example 3 is 17. The amount of defects in Example 3 is much smaller than that in Comparative Example 3. Obviously, the welding structure of Example 3 of the present invention improves the reliability of the traditional welding structure of Comparative Example 3 in hot and cold cycles. The maximum number of defects in Example 2 is 0, and the maximum number of defects in Comparative Example 2 is 12. The amount of defects in Example 2 is much smaller than that in Comparative Example 2. Obviously, the welding structure of Example 2 of the present invention improves the reliability of the traditional welding structure of Comparative Example 2 in hot and cold cycles.

[0042] In summary, the low-temperature soldering structure of the present invention has excellent reliability in thermal cycling compared to the conventional structure, and thus can truly achieve the purpose of the present invention.

Claims

1. A low-temperature soldering structure, characterized in that: At least: a circuit layer (11); an IMC layer (12), the IMC layer (12) being formed on the circuit layer (11); a first solder diffusion layer (13), the first solder diffusion layer (13) being formed on the IMC layer (12); a solder metal layer (14) formed on an upper portion of the first solder diffusion layer (13); a second solder diffusion layer (15) formed on an upper portion of the solder metal layer (14); and an electrode layer (16), the electrode layer (16) being located on an upper portion of the second solder diffusion layer (15); The first solder diffusion layer (13) is a lower bismuth diffusion layer containing tin and bismuth, the solder metal layer (14) is a tin-bismuth alloy layer containing tin and bismuth, and the second solder diffusion layer (15) is an upper bismuth diffusion layer containing tin and bismuth. The concentration of bismuth in the first solder diffusion layer (13) is lower than the concentration of bismuth in the solder metal layer (14), and the concentration of bismuth in the second solder diffusion layer (15) is lower than the concentration of bismuth in the solder metal layer (14).

2. The low-temperature soldering structure according to claim 1, wherein: The circuit layer (11) is a copper circuit containing copper.

3. The low-temperature soldering structure according to claim 2, wherein: The IMC layer (12) is a metal compound of copper and tin.

4. The low-temperature soldering structure according to claim 3, wherein: The electrode layer (16) is a tin alloy electrode layer containing tin.

5. A method for manufacturing a welded structure, characterized in that: The following steps are included in order: A step of providing a printed circuit board with a lead-free solder alloy pad: providing a pad structure (1), the pad structure (1) comprising at least, in order from bottom to top: a printed circuit board (10), a circuit layer (11), an IMC layer (12), and a lead-free solder alloy pad layer (A); a step of printing a low-temperature tin-bismuth alloy solder paste: printing a lead-free tin-bismuth alloy solder paste on the upper surface of the lead-free solder alloy pad layer (A); An electronic component placement step: stacking an electrode layer (16) of an electronic component (17) on the lead-free tin-bismuth alloy solder paste to form a semi-finished solder structure; A reflow heating step: heating the semi-finished product of the soldering structure, causing the bismuth portion of the lead-free tin-bismuth alloy solder paste to migrate downward to the lead-free solder alloy pad layer (A), thereby converting the lead-free solder alloy pad layer (A) into a first solder diffusion layer (13); the bismuth portion of the lead-free tin-bismuth alloy solder paste to migrate upward to the lower portion of the electrode layer (16), thereby converting the lower portion of the electrode layer (16) into a second solder diffusion layer (15); and the lead-free tin-bismuth alloy solder paste to form a solder metal layer (14), thereby obtaining a low-temperature soldering structure (100).

6. The method for manufacturing a welded structure according to claim 5, wherein: The first solder diffusion layer (13) is a lower bismuth diffusion layer containing tin and bismuth.

7. The method for manufacturing a welded structure according to claim 6, wherein: The soldering metal layer (14) is a tin-bismuth alloy layer containing tin and bismuth, the second soldering diffusion layer (15) is an upper bismuth diffusion layer containing tin and bismuth, the concentration of bismuth in the first soldering diffusion layer (13) is lower than the concentration of bismuth in the soldering metal layer (14), and the concentration of bismuth in the second soldering diffusion layer (15) is lower than the concentration of bismuth in the soldering metal layer (14).

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