Laser Soldering of Sn-based Solders with Different Melting Points

With the miniaturization and multi-functionalization of electronic devices, a large number of electronic components with various requirements on the interconnections and packaging need to be integrated on one substrate. There are specific cases when it is necessary to use a series of solders with the melting points in a wide range. The repetitive and lengthy heating in the sequential reflow soldering leads to the overgrowth of the brittle intermetallic compound at the interface and inevitable thermal damage to the substrates. The high density in energy input and the small heat affected area enable laser soldering to make reliable interconnections using solders with different melting points on the same substrate. And the laser with a wavelength in the visible range can further improve the energy absorption of the solders. In this paper, experiments were carried out by both the laser soldering and the infrared reflow soldering on the tinned copper pads using Sn-based solders with the melting point ranging from 96 to 309 °C. A continuous wave solid-state laser with a wavelength of 532 nm and a relatively low power of 1.8 W was applied to all the samples. The wettability of solders, the mechanical properties and the microstructures of the joints were investigated. With better wettability induced by high superheating, reliable joints using solders with significantly different melting points on one substrate have been successfully achieved by the laser. When using Sn16-Pb32-Bi52, Sn42-Bi58 and Sn10-Pb88-Ag2, refined microstructure in the solder bumps resulted in the improved mechanical properties of the laser soldered joints. The fast cooling rate in the laser soldering lead to the non-equilibrium solidification in the eutectic Sn63-Pb37. The increase in the precipitation of Pb-rich phases gave rise to a slight decrease in the shear strength of laser soldered joints.