Authored By:
Tony Lentz
FCT Solder
CO, USA
Mike Bixenman
Magnalytix and Kyzen
TN, USA
Summary
Voiding in bottom terminated component (BTC) solder joints and cleaning under low standoff components are common problems in the printed circuit board assembly (PCBA) process. Water-soluble solder pastes are particularly susceptible to voiding for a couple of reasons. Water-soluble solder pastes typically contain a higher volatile content than no clean solder pastes. Water-soluble solder pastes are made with hygroscopic materials that absorb moisture out of the air, which leads to an increased potential for voiding, especially in humid environments.
The prevalence of BTC usage presents a challenge not only for voiding in solder joints but also with respect to the washability of flux residues. BTCs have low standoff gaps, which make it difficult to completely remove flux residues. The cleaning process must be tuned to remove highly active water-soluble fluxes. Corrosive and electrically conductive residues may be left behind. This can lead to electro-chemical failures on the PCBA. There is a need for water-soluble solder pastes that have low voiding potential and ease of washability without compromising solder paste performance.
This paper details a new water-soluble lead-free solder paste that has dramatically improved voiding performance and washability over previous generations of solder paste. Voiding performance is tested with BTCs and multiple reflow profiles in high humidity conditions. Washability is tested with DI water vs. engineered cleaning chemistry, and the reliability of flux residues is measured using surface insulation resistance (SIR) testing. The results for the new water-soluble solder paste are summarized and compared to an older generation of solder paste. This data is presented along with recommendations for improving the reliability of PCBAs through reduced voiding and the use of an optimized wash process.
Conclusions
Advances in Electronic Assembly are accomplished from material advances and assembly equipment. Many advances require smaller components and high density.
This research paper introduced a new Water-Soluble Solder Paste. Beneficial properties include tack time, open storage, and ultra-low voiding. A key limitation of water-soluble solder pastes is the activity level and the need to clean (remove) the flux residue post soldering. Partially cleaned water soluble flux residue under low profile leadless and bottom terminated components exhibit the risk of electrochemical failures. When using water-soluble materials, cleaning is a critical factor. The flux residue using the new water-soluble solder paste was less voluminous but drier than the current water-soluble paste. The flux residues were effectively cleaned when using DI-Water.
Engineered cleaning agents have been used to drop surface tension, provide a lower wetting angle, and assist in removing burnt flux residues. The required concentration is typically 5% in DI Water. The results from this study using the engineered cleaning were comparable to DI-Water.
Initially Published in the SMTA Proceedings
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