Why is the brightness of lead-free solder joint inferior to that of tin lead solder joint?

The solder joints of Sn Pb solder are very bright. For example, the solder joints of sn63pb37 alloy are as bright as a mirror. This is mainly due to the fact that Sn Pb solder is eutectic solder and there is no IMC (interfacial metallic components). When the temperature is higher than 183 ℃, the solder is in liquid state, but lower than 183 ℃, it is in solid state. The binary alloy of sn63pb37 is like single crystal, So the solder joint is bright.
But lead-free solder is not the same, usually lead-free solder alloy is not eutectic solder, and there are different crystal components in solder. Taking sn96.5ag3.0cu0.5 alloy as an example, the alloy contains three different elements. When the alloy solidifies, the three elements eutectic with each other and form their respective melting point and solidification state. Different eutectic compositions may be formed in sn96.5ag3.0cu0.5 alloy: Cu6Sn5, its melting point is 227 ℃; The melting point of ag3sn is 221 ℃. The melting point of the alloy is 217 ℃.
When the solder joint is cooled to 232 ℃, the tin in the tin rich alloy will solidify into tin crystals (usually dendritic crystals) on the surface of the alloy layer until the rest of the liquid mixture reaches the ideal ternary eutectic composition. With the decrease of temperature, the crystals with different melting points separate out and adhere to the surface of the tin crystal according to the temperature. If the component pins are plated with Sn Pb alloy, the melted lead will also form eutectic grains. For Sn Pb eutectic, the melting point of some parts of solder in solder joint will decrease to 183 ℃; For snpbag, the melting point of some parts of the solder in the solder joint will decrease to 178 ℃.
Therefore, the lead-free solder joint is a multicomponent eutectic. According to the principle of light diffusion, the lead-free solder joint will not reflect light and present "orange peel shape". In addition, the volume shrinkage of the solder is about 4% when it solidifies, and most of the volume shrinkage occurs at the last solidification place of the solder. The eutectic grains with the lowest melting temperature usually appear in these places. By the way, volume reduction is often the cause of microcracks in solder joints.
So, for lead-free solder, how to increase the brightness of solder joint? We can improve from the following aspects:
1. In reflow soldering, properly speeding up the cooling rate (provided that it will not cause solder joint wrinkles) can reduce various grain sizes, which is beneficial to increase the brightness of solder joints
2. Increase the peak temperature in reflow soldering
3. Enhance the activity of solder paste flux, weak activity, easy to appear false welding and cold welding
4. Increase the flux content