DEVELOPMENT OF A LOW VOIDING LEAD-FREE SOLDER PASTE FOR HIGH RELIABILITY APPLICATIONS

摘要

Voiding in solder joint is a concern in high reliability application electronics like automotive, power electronics, LED lighting, etc, as electrical conductivity and thermal dissipation performance are lowered if the percentage and the size of voids is too high: large area components such as bottom termination components (BTC) and components with bottom thermal plane (DPAK) are particularly critical. So the reduction of voids level and voids size is very important to achieve the reliability in such assemblies. Many factors are responsible of voiding. Among those, the solder paste is a main contributor: voids are partly caused by flux outgassing which remains entrapped in the solder joint. This paper will first highlight the expected flux medium characteristics then describe the method used to develop a novel no-clean lead-free solder paste capable to significantly reduce voiding. The influence of raw materials will be studied through a screening plan. Afterwards, an optimized combination of solvents will be defined to reduce voids level. Finally, the role of activators and additives will be underlined in both wettability and voids performance. The "low voids" paste developed will be compared to a reference paste with similar specifications: voiding performance in large area components will be assessed on several substrates with different finishes under air convection reflow. The comparison between the reference paste and the low voids paste will be completed by other process tests including printability, slumping, solder-balling, wetting ability or chemical residue reliability.