TWI771197B - Welding structure of low temperature solder and its manufacturing method - Google Patents

Abstract

The present invention provides a welding structure for low-temperature soldering, which comprises at least sequentially from bottom to top: a circuit layer of a printed circuit board, an intermetallic compound layer, a first welding diffusion layer, a welding metal layer, and a second welding diffusion layer layer and an electrode layer of an electronic component. The surface treatment of the circuit layer of the printed circuit board uses lead-free solder alloy pads, so that the soldered structure has good reliability of cold and heat cycles. The present invention also provides a manufacturing method of a welding structure to manufacture the aforementioned welding structure of low temperature solder.

Description

Welding structure of low temperature solder and its manufacturing method

The present invention relates to an assembly welding structure of electronic components, in particular to a welding structure formed with electronic components by welding with low temperature solder paste, so as to form a welding structure of high reliability low temperature solder and a manufacturing method thereof.

In the conventional assembly and soldering process, the surface treatment of printed circuit board (Printed circuit board, PCB) circuit protection is mostly coated with an organic solder preservative (Organic Solderability Preservative, OSP) layer; the lower surface of electronic components is attached with tin or tin. Alloy electrodes, such as ball grid array (BGA) attached to the bottom surface of the wafer. The assembly sequence is after the solder paste is printed on the OSP layer, and then the electrodes of the electronic components are placed on the solder paste layer, and then the solder paste is melted and diffused through a tin furnace to form a solder structure and complete the assembly.

With the recent high density of integrated circuit (IC) chips, most of the package structures thereof are thin and large. When using SAC (tin-silver-copper) solder paste for soldering, the high temperature during soldering is about 245 ° C ~ 260 ° C, which will cause the IC package structure to deform during the heating process and lose its flatness, resulting in empty soldering, short circuit and other welding defects. problem occurs. Therefore, reducing the soldering temperature to reduce the material deformation of the IC package is one of the methods to improve the occurrence of soldering defects. Therefore, using a large amount of bismuth-containing low-temperature solder alloys, such as tin-bismuth alloy solder paste with a bismuth content greater than 10% (weight percent) (melting point of about 140 ° C ~ 180 ° C), to reduce the welding temperature has become the main solution.

The aforementioned tin-bismuth alloy solder paste can be soldered at low temperature during assembly due to its low melting point, which has the advantage of reducing component deformation. When using tin-bismuth alloy solder paste to solder PCBs with general OSP surface treatment, because the electrodes are mostly pure tin or tin-silver-copper alloys with higher melting points, in order to achieve a better welding effect, the temperature of the aforementioned tin-bismuth alloy solder paste (melting point less than 180°C) will increase by about 40°C or more than the original melting point (melting point is about 140°C) or increase the reflow time for a longer time. At this time, the fluidity of the tin-bismuth alloy solder paste will become better, and sometimes a large amount of bismuth will diffuse to the part carrier end of the BGA ball to form an inter-metallic compound (IMC) of gold and bismuth, causing the ball to interact with The problem of gold brittleness between the carriers; therefore, there is a requirement to limit the diffusion height of bismuth to only one-third of the height of the BGA ball. In addition, when the same tin-bismuth alloy solder paste is used at a lower reflow temperature, although a large amount of bismuth will not diffuse upward, it will cause a large area of tin-bismuth concentration in the upper part of the copper circuit. This results in a solder joint (called a tin-bismuth solder layer) forming a region with only a low-temperature solder layer and high brittleness. Once this area is formed, in addition to mechanical shock resistance (such as drop test), the most likely problem is that bismuth is easily diffused in repeated high and low temperature environments under cold and heat cycles, resulting in the growth of bismuth-rich phases. The formation of voids, coupled with the stress of thermal expansion and contraction, will cause the tin-bismuth zone to fracture, which is the so-called hot-tearing phenomenon, resulting in low-temperature soldering The reliability of the solder joint is not as good as the traditional one. The solder layer formed by the tin-silver-copper alloy SAC305 solder. Although there are many types of bismuth-containing solder alloys, it is difficult to avoid this phenomenon when soldering with OSP surface treatment.

Therefore, the purpose of the present invention is to provide a welding structure of low temperature solder, the welding structure of the low temperature solder is a printed circuit board using lead-free solder alloy pads, which makes the welding structure combined with electronic components have good resistance to cold and heat cycles Reliability.

Based on the above purpose of the invention, a welding structure of low temperature solder is provided, which at least comprises: a circuit layer; an Inter-metallic Compound (IMC) layer, the IMC layer is formed on the circuit layer; a first welding diffusion layer , the first welding diffusion layer is formed on the top of the IMC layer; a welding metal layer is formed on the upper part of the first welding diffusion layer; a second welding diffusion layer is formed on the The upper part of the welding metal layer; an electrode layer of an electronic component, the electrode layer is located on the upper part of the second welding diffusion layer.

According to the above-mentioned low-temperature soldering structure, the circuit layer is a copper-containing copper circuit, and the IMC layer is mainly a metal compound formed by the reaction of copper and tin, which is formed when a pad structure is fabricated during low-temperature soldering. The first solder diffusion layer is a lower bismuth diffusion layer formed by a lead-free solder alloy pad layer due to diffusion of bismuth in a lead-free tin-bismuth alloy solder paste during soldering. The soldering metal layer is a lead-free tin-bismuth alloy layer, which is a part of the lead-free tin-bismuth alloy solder paste in which the bismuth is not diffused after soldering. The second solder diffusion layer is an upper bismuth diffusion layer caused by the diffusion of bismuth in the lead-free tin-bismuth alloy solder paste into the electrode layer during soldering. The electrode layer is a tin-containing or tin alloy electrode layer, or tin alloy balls (eg BGA balls) attached to the lower surface of an electronic component or input/output (I/O) under the package structure.

The present invention is based on the welding structure of the aforementioned low-temperature solder, and further provides a manufacturing method of the welding structure, which includes the following steps in sequence: 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.

First, please refer to FIG. 1 , a pad structure 1 of the present invention includes at least sequentially 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. The circuit layer 11 is located on the upper surface of the printed circuit board 10 and is usually a conductive circuit pattern. The circuit layer 11 may be made of 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 , and 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 , and 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 alloys containing tin. Lead-free solder alloys containing less than 1wt% bismuth. In practice, it can be lead-free solder SAC305 (Sn 96.5wt%, Ag 3.0wt%, Cu 0.5wt%) or SAC0307 (Sn 99.0wt%, Ag 0.3wt%, Cu 0.7wt%). 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 1 wt % bismuth. The lead-free solder alloy layer (the lead-free solder alloy pad layer A) can be formed by electroplating, tin spraying or solder paste heating and other methods, and has a thickness of about 10 μm to 100 μm (microns), but does not contain high concentrations of bismuth ( greater than 1 wt%). It should be noted that the "lead-free" referred to in the present invention is essentially a lead-free standard definition that complies with RoHS (Restriction of Hazardous Substances, the directive on restricting the use of certain harmful components in electrical and electronic equipment). The aforementioned substantially free of lead means that in principle, as long as lead is not intentionally added to the alloy (eg, unintentional but unavoidable impurities or contact during the manufacturing process), it can be regarded as substantially free of lead or can be regarded as lead-free. "wt %" refers to weight percent, and further, definitions of numerical ranges stated herein are always inclusive.

Please refer to FIG. 2 , a low-temperature solder soldering structure 100 of the present invention sequentially includes the circuit layer 11 , the IMC layer 12 , a first solder diffusion layer 13 , a solder metal layer 14 , and a second solder layer from bottom to top. Solder the diffusion layer 15 and an electrode layer 16 of an electronic component 17 . Wherein, the IMC layer 12 is formed on the top of the circuit layer 11; the first solder diffusion layer 13 is formed on the top of the IMC layer 12; the solder metal layer 14 is formed on the upper part of the first solder diffusion layer 13; Two solder diffusion layers 15 are formed on the upper part of the solder metal layer 14 ; the electrode layer 16 is located on the upper part of the second solder diffusion layer 15 , and the electrode layer 16 is also attached to the lower surface of the electronic element 17 .

The first welding diffusion layer 13 is a lower bismuth diffusion layer containing tin and bismuth, the welding metal layer 14 is a tin-bismuth alloy layer containing tin and bismuth, and the second welding diffusion layer 15 is a On the bismuth diffusion layer, the electrode layer 16 is a tin-containing solder alloy electrode layer.

The welding structure 100 of the low temperature solder is fabricated by a manufacturing method of a welding structure, and the manufacturing method of the welding structure includes the following steps in sequence.

A step of providing a printed circuit board with lead-free solder alloy pads: providing the pad structure 1, the pad structure 1 at least sequentially includes: the printed circuit board 10, the circuit layer 11, the IMC layer 12 and the The lead-free solder alloy pad layer A.

A step of printing low-temperature tin-bismuth alloy solder paste: the solder pad structure 1 is printed on the lead-free solder alloy solder pad layer A of the solder pad structure 1 with a low-temperature solder paste containing bismuth (lead-free tin-bismuth alloy solder paste) For example, the lead-free tin-bismuth alloy solder paste is a tin-bismuth solder paste (such as 42wt% tin and 58wt% bismuth), or a tin-bismuth-silver paste (such as 42wt% tin, 57wt% bismuth and 1wt% bismuth) silver). Alternatively, a low-temperature solder paste containing tin and bismuth as the main component and adding several ppm to several wt% of other one or more metal elements.

A step of placing electronic components: the electrode layer 16 of the electronic component 17 is stacked on the top of the lead-free tin-bismuth alloy solder paste, that is to say, the electronic component 17 or the BGA ball is stacked on the lead-free tin-bismuth in the intermediate process. Alloy solder paste on top to form a semi-finished solder structure.

One reflow heating step: the semi-finished welded structure is heated at 160°C to 190°C for 4.5 minutes to 6.5 minutes (eg, heated in a reflow oven), and then cooled, eg, to a room temperature of 25°C. During this reflow heating step, part of the bismuth in the lead-free tin-bismuth alloy solder paste will migrate down (also referred to as diffusion) to the lead-free solder alloy pad layer A to make the lead-free solder alloy pad layer A It becomes the first solder diffusion layer 13, in other words, the first solder diffusion layer 13 is the lower bismuth diffusion layer formed by the diffusion of bismuth in the lead-free tin-bismuth alloy solder paste when the lead-free solder alloy pad layer A is heated; the Part of the bismuth in the lead-free tin-bismuth alloy solder paste will also migrate up to the lower part of the electrode layer 16 so that the lower part of the electrode layer 16 becomes the second solder diffusion layer 15 . In other words, the second solder diffusion layer 15 is used for heating When the bismuth in the lead-free tin-bismuth alloy tin paste diffuses into the upper bismuth diffusion layer caused by the lower part of the electrode layer 16; the lead-free tin-bismuth alloy tin paste forms the welding metal layer 14 when heated, and the welding metal layer 14 It is a tin-bismuth alloy layer, which is the undiffused part of bismuth in the lead-free tin-bismuth alloy solder paste after soldering. Accordingly, the welding structure 100 of the low-temperature solder is obtained. The concentration of bismuth in the first welding diffusion layer 13 is lower than the concentration of bismuth in the welding metal layer 14 , and the concentration of bismuth in the second welding diffusion layer 15 is lower than the concentration of bismuth in the welding metal layer 14 .

If the lead-free solder alloy pad layer is not used, the lower bismuth diffusion layer (ie, the first solder diffusion layer) cannot be formed during the fabrication of the low-temperature solder solder structure, in other words, the general OSP surface treatment is used as in the prior art When the PCB is soldered with tin-bismuth alloy solder paste, only the bismuth diffusion layer (upper bismuth diffusion layer) above the welding metal layer is formed without the formation of the lower bismuth diffusion layer (lower diffusion layer). However, the structure and method of the present invention can have the following advantages over the prior art: (1) reduce the thickness of the tin-bismuth alloy layer (that is, the welding metal layer 14 ) to avoid hard brittleness and excessive low temperature regions; (2) can The upper and lower diffusion layers are formed at the same time, and the bismuth content of the upper and lower diffusion layers is less than that of the welding metal layer 14, so the toughness is higher than that of the tin-bismuth welding layer of the prior art and the melting point is higher, which can avoid hot tearing (Hot tearing) (3) During the reflow heating step, since the lead-free tin-bismuth alloy solder paste does not contact the copper of the circuit layer 11, the bismuth in the lead-free tin-bismuth alloy solder paste is not Due to the thermodynamic effect caused by the poor compatibility of bismuth and copper, a large amount of bismuth unilaterally diffuses to the end of the electrode layer to the end of the part such as the BGA ball. Therefore, the present invention can easily achieve a low height of the second welding diffusion layer. less than one-third the height of a BGA ball. Therefore, compared with the prior art, the low-temperature soldering structure 100 of the present invention has better reliability in cooling and heating cycles, and relevant embodiments thereof will be described later.

<Examples 1 to 3 and Comparative Examples 1 to 3>

Examples 1-3 and Comparative Examples 1-3 use BGA balls with 25*25 array tin balls (that is, the aforementioned electrode layer 16, and the SAC305 composition is 96.5wt% tin, 3.0wt% silver and 0.5wt%). % copper, electronic components with a ball diameter of 0.45mm). The corresponding bismuth-containing solder pastes in Table 1 were printed by stencil printing. The solder pads on the PCBs of Examples 1 to 3 are lead-free solder alloy solder pad layers of about 25 μm, while the printing thickness of the bismuth-containing solder paste is 50 μm, and the peak value of reflow is about 190°C. The pads on the PCBs of Comparative Examples 1 to 3 were OSP, and the printing thickness of the bismuth-containing solder paste was 100 μm, and the soldering was carried out with the same reflow curve as the Example. Wherein, the pad structures (OSP pads) of Comparative Examples 1 to 3 are composed of printed circuit boards, circuits and OSP layers in sequence from bottom to top, which are generally known and will not be repeated here; 3. The pad structure (lead-free solder alloy pad) is sequentially composed of the printed circuit board 10, the circuit layer 11, the IMC layer 12 and the lead-free solder alloy pad layer A from bottom to top, which is the present invention Structure. Then, the welded structures of Comparative Examples 1 to 3 and Examples 1 to 3 were put into a cold-heat cycle tester, respectively, to conduct a reliability test of the cold-heat cycle. In the test of the cold and heat cycle tester, after keeping the low temperature at -40°C for 7 minutes, the heating rate is 15°C/min until it reaches a high temperature of 100°C, and then the temperature is kept at 100°C for 7 minutes, and then the cooling rate is 15°C/min. °C/min up to -40 °C, which is regarded as a cycle. The welded structure was taken out at the 500th cycle, the 1000th cycle and the 2000th cycle, and crack observation and measurement were performed on the stacked structure in the welded structure after taking out. Judgment criteria are: when the length of the crack is more than 25 μm or more, it is judged that the defect has occurred, which is the evidence of hot-tearing, and the number of defects in the 500th cycle, the 1000th cycle and the 2000th cycle is regarded as the occurrence of the defect. Comparison of Examples and Comparative Examples.

Table 1 Example 1 Comparative Example 1 Example 2 Comparative Example 2 Example 3 Comparative Example 3 Lead-free solder alloy pads or OSP pads SAC305 OSP SAC305 OSP SAC0307 OSP Bismuth-containing solder paste SnBi SnBi SnBiAg SnBiAg SnBi SnBi Bad number after 500 cycles 0 2 0 1 0 2 Bad number after 1000 cycles 0 10 0 8 1 12 Bad number after 2000 cycles 1 16 0 12 2 17 SAC305: 96.5wt% tin, 3.0wt% silver, 0.5wt% copper; SAC0307: 99.0wt% tin, 0.3wt% silver, 0.7wt% copper; SnBi: 42wt% tin and 58wt% Bismuth; SnBiAg: 42wt% tin, 57wt% bismuth and 1wt% silver.

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 (that is, hot tearing) in Example 1 is much smaller than that in Comparative Example 1. Obviously, Therefore, the welding structure of Example 1 of the present invention improves the reliability of the conventional welding structure of Comparative Example 1 in cooling and heating cycles. The maximum number of defects in Example 3 is 2, and the maximum number of defects in Comparative Example 3 is 17. The number 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 comparative example. 3. The reliability of the traditional welded structure in the cooling and heating cycle. 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 comparison. The reliability of the conventional welded structure of Example 2 in the cooling and heating cycle.

To sum up, compared with the traditional structure, the welding structure of the low-temperature solder of the present invention has excellent reliability of the cooling and heating cycle, so it can indeed achieve the purpose of the present invention.

However, the above are only examples of the present invention, and should not limit the scope of the present invention. All simple equivalent changes and modifications made according to the scope of the application for patent of the present invention and the contents of the patent specification are still within the scope of the present invention. within the scope of the invention patent.

1: Pad structure 10: Printed Circuit Board 11: Circuit layer 12: IMC layer 13: The first solder diffusion layer 14: Solder the metal layer 15: Second Solder Diffusion Layer 16: Electrode layer 17: Electronic Components 100: Welded structure A: Lead-free solder alloy pad layer

Other features and effects of the present invention will be clearly presented in the embodiments with reference to the drawings, wherein: FIG. 1 is a schematic structural diagram of the pad structure of the present invention. Fig. 2 is a schematic structural diagram of the welding structure of the present invention.

10: Printed Circuit Board

11: Circuit layer

12: IMC layer

13: The first solder diffusion layer

14: Solder the metal layer

15: Second Solder Diffusion Layer

16: Electrode layer

17: Electronic Components

100: Welded structure

Claims (9)

A welding structure of low-temperature solder, comprising at least: a circuit layer (11); an Inter-metallic Compound (IMC) layer (12), the IMC layer (12) is formed on the circuit layer (11); a first solder diffusion layer (13), the first solder diffusion layer (13) is formed on the IMC layer (12); a welding metal layer (14), the welding metal layer (14) is formed on the upper part of the first welding diffusion layer (13); a second solder diffusion layer (15) formed on the upper portion of the solder metal layer (14); and an electrode layer (16), the electrode layer (16) is located on the upper part of the second solder diffusion layer (15). The welding structure of low-temperature solder according to claim 1, wherein the circuit layer (11) is a copper circuit containing copper. The soldering structure of low-temperature solder according to claim 2, wherein the IMC layer (12) is a metal compound of copper and tin. The welding structure of low-temperature solder according to claim 3, wherein the first welding diffusion layer (13) is a lower bismuth diffusion layer containing tin and bismuth. The welding structure of low-temperature solder according to claim 4, wherein the welding metal layer (14) is a tin-bismuth alloy layer containing tin and bismuth, and the second welding diffusion layer (15) is a layer containing tin and bismuth. upper bismuth diffusion layer, the concentration of bismuth in the first welding diffusion layer (13) is less than the concentration of bismuth in the welding metal layer (14), and the concentration of bismuth in the second welding diffusion layer (15) is less than the concentration of bismuth in the welding metal layer (14). 14) in the concentration of bismuth. The welding structure of low-temperature solder according to claim 5, wherein the electrode layer (16) is a tin-containing tin alloy electrode layer. A method for manufacturing a welded structure comprises the following steps in sequence: A step of providing a printed circuit board with lead-free solder alloy pads: providing a pad structure (1), the pad structure (1) from bottom to top at least sequentially comprises: 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 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); A step of placing electronic components: an electrode layer (16) of an electronic component (17) is stacked on the top of the lead-free tin-bismuth alloy solder paste to form a semi-finished product of a welding structure; A re-soldering heating step: heating the semi-finished product of the welded structure, the bismuth part in the lead-free tin-bismuth alloy solder paste migrates down to the lead-free solder alloy pad layer (A) to make the lead-free solder alloy pad layer (A) ) becomes a first solder diffusion layer (13), and the bismuth part in the lead-free tin-bismuth alloy solder paste migrates up to the lower part of the electrode layer (16) so that the lower part of the electrode layer (16) becomes a second solder The diffusion layer (15), the lead-free tin-bismuth alloy solder paste forms a soldering metal layer (14), thereby obtaining a soldering structure (100) of low-temperature soldering. The method for manufacturing a welding structure according to claim 7, wherein the first welding diffusion layer (13) is a lower bismuth diffusion layer containing tin and bismuth. The method for manufacturing a welding structure according to claim 8, wherein the welding metal layer (14) is a tin-bismuth alloy layer containing tin and bismuth, and the second welding diffusion layer (15) is a layer containing tin and bismuth. upper bismuth diffusion layer, the concentration of bismuth in the first welding diffusion layer (13) is less than the concentration of bismuth in the welding metal layer (14), and the concentration of bismuth in the second welding diffusion layer (15) is less than the concentration of bismuth in the welding metal layer (14). 14) in the concentration of bismuth.

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