Composition of lead-free solder paste

- Aug 18, 2018-

In the composition of the lead-free solder paste, the composition is mainly composed of tin/silver/copper, and the original lead is replaced by silver and copper.Fundamental characteristics and phenomenaIn tin/silver/copper systems, metallurgical reactions between tin and secondary elements (silver and copper) are the main factors determining application temperature, curing mechanism and mechanical properties.According to the binary phase diagram, there are three possible binary eutectic reactions between the three elements.A reaction between silver and tin tin at 221 ° C to form the matrix phase eutectic structure and combination between epsilon metal phase (Ag3Sn).Copper and tin tin reaction at 227 ° C to form the matrix phase of eutectic structure and eta intermetallic compound phase (Cu6Sn5).Silver also can react with copper at 779 ° C to form rich silver alpha and rich copper alpha phase eutectic alloy.1 in the current study, however, of tin/silver/copper triple compound curing temperature measurement, at 779 ° C, found no phase shift.This suggests that it is likely that silver and copper react directly in a triple compound.The temperature kinetics is more suitable for the reaction of silver or copper with tin to form the compound between Ag3Sn or Cu6Sn5 metals.Therefore, the tin/silver/copper triad reaction can be expected to include the tin matrix phase, the chemical combination phase (Ag3Sn) between the metals of import and export (Cu6Sn5).

As confirmed by the two-phase tin/silver and tin/copper systems, the relatively hard Ag3Sn and Cu6Sn5 particles in the tin/silver/copper triple alloy of the tin matrix can effectively strengthen the alloy by establishing a long-term internal stress.These hard particles can also effectively block the spread of fatigue cracks.The formation of Ag3Sn and Cu6Sn5 particles can separate the smaller tin matrix particles.The smaller the Ag3Sn and Cu6Sn5 particles, the more effectively they can separate the tin matrix particles. As a result, the overall smaller microstructure can be obtained.This contributes to the sliding mechanism of particle boundaries and thus extends the fatigue life at elevated temperatures.Although the specific formula of silver and copper in the alloy design is critical to the mechanical properties of the alloy, it is found that the melting temperature is not sensitive to changes in the content of 0.5~3.0% copper and 3.0~4.7% silver.The mechanical properties of the relationship between silver and copper content are summarized as follows. 2. When the silver content is about 3.0~3.1%, the yield strength and tensile strength both increase to about 1.5% with the copper content, which increases almost linearly.Over 1.5% of copper, the yield strength decreases, but the tensile strength of the alloy remains stable.The overall alloy plasticity is high for copper of 0.5-1.5%, and then decreases with further increase of copper.For the content of silver (copper in the range of 0.5-1.7%), both yield strength and tensile strength increased to 4.1% with the content of silver, which increased almost linearly, but decreased plasticity.

Fatigue life reached its maximum at copper with 1.5% of silver between 3.0 % and 3.1%.It was found that the content of silver increased from 3.0 per cent to a higher level (up to 4.7 per cent) without any improvement in mechanical properties.When both copper and silver are higher, plasticity is damaged, such as 96.3Sn/4.7Ag/1.7Cu.Optimum alloy compositionAlloy 95.4Sn/3.1Ag/1.5Cu is considered the best.Its good performance is the result of the formation of fine microstructure which gives high fatigue life and plasticity.For a 0.5-0.7% copper solder alloy, any silver content above about 3% will increase Ag3Sn's particle volume fraction, resulting in higher strength.However, it will no longer increase fatigue life, possibly due to the formation of larger Ag3Sn particles.At higher copper content (1~1.7%Cu), the larger Ag3Sn particles may exceed the influence of the higher Ag3Sn particle volume fraction, resulting in lower fatigue life.The Cu6Sn5 particle volume fraction also increases when copper exceeds 1.5%(3~3.1%Ag).However, strength and fatigue life will not increase further with copper.In the tin/silver/copper triple system, 1.5 percent copper (3~3.1 percent Ag) is most effective in producing the appropriate number of Cu6Sn5 particles with the smallest microstructure size, thus achieving the highest fatigue life, strength and plasticity.According to the report, 93.6 Sn alloy/Ag/Cu is 217 ° C temperature 1.7 4.7 3 triple eutectic alloy.However, the precise melting temperature was not observed in the cooling curve measurements.And get a small range of temperature: 216 ~ 217 ° C.This alloy component increases the highest tensile strength of the triple alloy component currently studied, but its plasticity is much lower than 63Sn/37Pb.The yield strength of alloy 95.4Sn/4.1Ag/0.5Cu is lower than that of 95.4Sn/3.1Ag/1.5Cu.The fatigue life of 93.6Sn/4.7Ag/1.7Cu is lower than 95.4Sn/3.1Ag/1.5Cu.If particle boundary sliding mechanism primarily determines eutectic solder alloys, then 95.4Sn/3.1Ag/1.5Cu, instead of 93.6Sn/4.7Ag/1.7Cu, should be closer to true eutectic properties.