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Ball-Attach Flux WS-446-NRD for Poor Quality OSP-wetting

While on a recent trip to Malaysia, I interviewed two colleagues regarding trends in semiconductor assembly. My previously-published interview with Sze-Pei Lim appears here.

This time, while on a visit to a logic device manufacturer in the North West, I [ACM] talked briefly to Sehar Samiappan [SS], Indium Corporation's Area Technical Manager, about our recently-developed water-soluble pin-transfer Ball-Attach Flux, WS446-NRD, which is designed for BGA applications of 0.5mm pitch, and greater than 1500  I/Os.

[ACM] What is the origin of WS446-NRD?

[SS] The development was driven by a customer need for a guaranteed good quality BGA (ball-grid array) solder joint, but with reduced environmental impact. Our very quality-focused customer uses several different suppliers of organic FC-BGA substrates with copper OSP pads. The customer had serious concerns about occasional poor solderability of SAC305 solder spheres onto substrates. The key defect seen was poor wetting onto the OSP-coated copper pad, which would give rise to variability in both joint strength and bump coplanarity,  and even (in worse cases) missing-ball / “big ball” effects. Some of the pad finishes were seen to be highly oxidized, severely restricting solder wetting during reflow. Variability in the surface finish was found to be not just from supplier to supplier, but also showed up as lot-to-lot variability from lower cost suppliers.

Some of the differences seen could be attributed to the method of mask desmear from the C4 “cage” of the flip-chip (top side) area, which was either a plasma-based desmear or an oxidizing inorganic acid dip, that was clearly having effects on solderability of the opposite (bottom) BGA side of the substrate.

[ACM] What steps have customers previously taken to get around this issue?

[SS] This is a serious issue for many ball-attach flux users, and some customers have gone to the lengths of using a special fluxing step to remove contaminants such as oxide and OSP (organic solderability protectant) coatings. These liquid fluxes are very reactive, but require  separate spraying, reflow, and cleaning stages that add cost and time. The halogenated ball-attach fluxes of the WS446 series have an established good chemistry that allows wetting of SAC105, 305, and 405 onto a variety of metallizations. In the semiconductor assembly industry, the WS446 fluxes are well-known in Taiwan, and throughout South East Asia, for their good solderability and long pot-life in a variety of FC-BGA applications.

[ACM] What was different about WS446-NRD, and why was it developed?

[SS] WS446 fluxes are all colored, using a bright red dye, so the flux can be seen by eye and automatically detected by vision systems. Red coloration also allows automated ball-attach flux dipping replenishment systems to detect flux levels. Normally, colored fluxes are not a problem, but the customer had some environmental concerns with the red color contaminating the water-wash equipment, and building up in their water-recycling system. WS446-NRD was developed from the basic WS446 flux series chemistry, but  without the red dye. The solderability performance of WS446-NRD was excellent, eliminating the variations in OSP solderability without requiring any additional processing steps. WS446-NRD also passed internal process and product requirements, such as cleanability, and the customer was very pleased with Indium’s ability to rapidly tailor a chemistry to their specific requirements.

[ACM] Sehar: thank you. I look forward to sharing a durian with you again when they are back in season.