Authored By:
Ed Briggs
Indium Corporation, Clinton, NY, USA
Ron Lasky, Ph.D., PE
Indium Corporation and Dartmouth College
Summary
The combination of higher lead-free process temperatures, smaller print deposits, and temperature restraints on electrical components has created difficult challenges in optimizing the reflow process. Not only are the electronic components and the PWB at risk, but the ability to achieve a robust solder joint becomes difficult, especially if the PCB is thermally massive. In addition, the constant miniaturization of electronic components, hence smaller solder paste deposits, may require the use of smaller particle-sized powders.
Both the small solder paste deposits and small particle size result in a large surface area-to-volume ratio that challenges the solder paste's flux to effectively perform its fluxing action. The possible resulting surface oxidation can lead to voiding, graping, head-in-pillow, and other defects. Smaller components are also more susceptible to tombstoning and defects related to solder paste slump.
This paper is a summary of best practices in optimizing the reflow process to meet these challenges of higher reflow temperatures, smaller print deposits, decreased powder particle size, and their affect on the reflow process. It also discusses trouble-shooting of the most common defects in lead-free reflow, such as tombstoning, solder beading/balling, residue discoloration, voiding, graping, and head in pillow.
Initially Published in the SMTA Proceedings
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