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The Interfacial Reaction between SnPb Solder and Cu Substrate during Soldering

2022-04-18

The Interfacial Reaction between SnPb Solder and Cu Substrate during Soldering

 

The interface is formed when the Cu substrate is soldering with an Sn-based solder

There are two layers of intermetallic compounds (IMC) of ε-Cu3Sn and η-Cu6Sn5 formed sequentially from the Cu side. As shown in Table 1, the IMCs grow in the solder depending on the composition of the solder alloy.

Table 1. The IMC between Sn-based solder and Cu substrate

Solder Alloy

The IMC at interface

Large IMC in solder

Sn37Pb

Cu6Sn5、Cu3Sn

N.A

Sn0.7Cu

Cu6Sn5、Cu3Sn

Cu6Sn5

Sn3.5Ag

Cu6Sn5、Cu3Sn

Ag3Sn

Sn3.8Ag0.7Cu

Cu6Sn5、Cu3Sn

Ag3Sn、Cu6Sn5

Sn-9Zn

β'-CuZn、γ-Cu5Zn8

-


The difference is that there are Cu6Sn5 and Ag3Sn formation for Sn3.8Ag0.7Cu while only Ag3Sn appears in Sn3.5Ag solder. The reason is that the Cu source for Sn3.8Ag0.7Cu solder is more abundant during reflow soldering.


The characteristics and influences of interfacial reaction between SnPb solder and Cu substrate during soldering

When soldering Cu substrate with SnPb solder, ε-Cu3Sn (substrate side) and η-Cu6Sn5 (solder side) grow between Cu and SnPb solder at ordinary soldering temperatures. The temperatures above 300 ℃ cause the generation of Cu31Sn8 (γ phase) and other alloys with unknown structures. This result is related to temperature. The thickness of the ε-Cu3Sn alloy layer is extremely thin under reasonable reflow temperature, so it is hard to distinguish. η-Cu6Sn5 alloy is the major layer.

Cu6Sn5 has good adhesion to Cu in all solders. The shape of the interface layer has a great influence on the reliability of the connection. Cracks are likely to occur because of the IMC brittleness, the differences in thermal expansion coefficients, etc.

The melting of the solder (usually soldering) is the growth process of the reactive layer. The curing of the liquid solder promotes the precipitation and growth of the reaction layer on the surface, but its influence only accounts for a small part of the overall reactive later growth.


The Interfacial Reaction between SnPb Solder and Cu Substrate during Soldering

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