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Fluxes Effective in Suppressing Non-Wet-Open at BGA Assembly
Keywords: Non-wet-open, Flux, BGA
With the advancement in miniaturization, the die is getting thinner and the solder bump is getting smaller for BGAs. Consequently, the thermal warpage is getting more severe due to coefficient of thermal expansion mismatch between die and molding compound, and often resulted in non-wet-open (NWO) at BGA assembly. NWO has been ailing the industry for a long time, and costly rework is required to remove the problem. Mechanisms of NWO has been investigated, and the defect has been attributed to solder paste being stick to the solder bump during development of component warpage. When the temperature reaches melting temperature of solder, the solder paste body coalesced with solder bump body, and the final solder bump becomes a fat bump. Upon cooling, the thermal warpage lessened, but this fat and short bump failed to reach solder pad, and eventually resulted in NWO. Attempts have been made to develop specialty solder pastes to address this issue, with only very limited success. This challenge actually drives the industry look into low temperature soldering in order to reduce the thermal warpage. Unfortunately this path is accompanied with highly compromised solder joint reliability. In this work, a specialty flux family has been developed as either a creamy flux or a solid flux coating. When assemble BGA with severe thermal warpage, after printing paste onto BGA pads, the BGA is dipped into this creamy flux, then placed onto the paste printed and followed by reflow. Alternatively, this flux can be applied to the solder bump surface as a solid coating. Then this coated BGA is placed onto paste printed, and followed by reflow. In both practices, BGA can be assembled with negligible occurrence of NWO. The flux can be no-clean or water-washable as a creamy flux, depending on the solder paste type to be used, but need to be no-clean as solid flux coating onto BGA bumps.
Ning-Cheng Lee, Vice President, Technology
Indium Corporation
Clinton, NY

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