Whilst it is important to select a modern Lead Free Solder Paste with sustained activity with a superior thermally stable activator package to withstand the harsher reflow profiles required for Lead Free soldering. It is also vital that the reflow profile is designed to suit the component mix on the assembly.
This project looked at the effect of preheat conditions on the resultant lead free solder joints using the latest KokiEco+Plus S3X58-M406 series solder paste for a variety of component types.
Three typical types of thermal profile were used as described below. In each case only the preheat zones were different whilst the reflow zones were the same in each case
For each profile the following results were obtained showing clearly the effect preheat has on the final joint appearance and possible defect.
The results show that if excessive preheat is used then the activation of the flux medium will struggle to fully coalesce the joints especially in fine pitch components such as micro BGAs and CSP outlines. This is due in part to the actual volume of flux deposited on these small footprints combined with the higher thermal exposures.
Also compared with tin lead solders, lead free alloys are based on very high tin contents and the rate of oxidization of the tin content is very much faster than tin lead. Thus the flux system has to work much harder to ensure oxide free powder, and if over exposed to temperature will have insufficient activity left in reflow to ensure good solder joints.
It is not true that dull grainy joints are the norm for lead free soldering. Bright shiny fully coalesced joints can be obtained in air reflow, provided the paste is carefully selected with a modern activator package and attention is paid to the design of the reflow profile, especially where very fine pitch components are concerned.
By Gordon Clark, Director Global Support
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