Circuits Assembly - December 2008 - (Page 37) Head-In-Pillow and Hybrid Solder Pastes The HIP epidemic proliferates, but the cure must not create new problems. arlier this year I reported on the head-inpillow epidemic affecting many assembly lines running Pb-free reflow processes (“HOP-ping Mad,” July 2008). I postulated that a combination of circumstances was occurring simultaneously to create this plague: The higher surface-area-to-volume ratios of progressively smaller paste deposits and BGA balls create a higher proportion of exposed (readily oxidizable) exterior solder to (protected) interior solder, which accelerates flux consumption; reflow processes with extended pre-liquidus thermal exposures that also help to exhaust the flux; and Pb-free oxide films that are more difficult for fluxes to break through than SnPb ones. Since those initial observations, reports of HIP defects continue to increase, and not just in Pb-free soldering. My completely unscientific, statistically insignificant polls indicate the incidence of HIP in SnPb processes is also rising. When we remove Pb-free alloy from the HIP equation, we’re basically left with flux exhaustion as the root cause. No-clean fluxes are designed to activate and deactivate under certain time-temperature conditions. If their thermal exposure limits are exceeded, their effectiveness wanes. Whether it’s from running too hot in the soak zone, taking too long to reach liquidus temperatures, or exposing too much surface area to heat and airflow – either from package warpage or questionable stencil design – spent flux is spent flux, and there’s no reviving it. The first line of defense in SnPb HIP should be profile adjustment to limit the thermal energy input, and in the majority of cases, it works. But what if it doesn’t? Luckily, solder paste manufacturers have a solution at the ready – what I like to refer to as hybrid solder pastes. These products are a blend of flux designed for Pb-free solder paste combined with SnPb solder powder. When we bring together the more thermally tolerant (Pb-free) flux with the SnPb powder, voilà! A SnPb solder paste that really can take the heat, so to speak. These hybrid pastes were introduced in 2007 to address mixed metals soldering, primarily for SAC 305/405 BGA balls on SnPb assemblies. The logic behind the blend is straightforward and with obvious advantages: Fluxes devised for Pb-free soldering would better facilitate wetting to Pb-free surfaces than those designed for SnPb alone, and these fluxes circuitsassembly.com Pb-Free Lessons Learned E are sufficiently thermally robust to perform in the top quartile of the SnPb peak temperature range typically associated with mixed metals assembly. Henkel was one of the first to introduce this hybrid solder paste, and according to Dr. Brian Toleno, global director of technical services, the blends were devised specifically to provide increased activity over a larger thermal window. He explains that “the increased activity over a wider profile range permits assemblers to push SnPb’s traditional top-end temperature and time above liquidus limitations, thereby extending the conventional process of solder joint formation to accommodate the differences in the alloys.” Since those first few formulations hit the market early last year, many suppliers have followed suit. Nearly every major player now offers a paste that blends SnPb solder powder with a flux designed for Pb-free. Although the original intent of these products was to facilitate mixed metals soldering, they do provide a seemingly handy solution for problems related to flux exhaustion. Toleno adds, “In the case of head-in-pillow, the increased activity is more effective at penetrating the oxide films, and can substantially increase the probability of forming good, reliable solder joints.” With many of last year’s mixed metals dilemmas now resolved, some suppliers are specifically marketing hybrid pastes as tools to help mitigate HIP defects. When the solder paste supply base introduced these products, I cautioned the higher temperature flux might not be a foolproof drop-in lower temperature SnPb flux. Three performance attributes concerned me, and since then a fourth point of interest has emerged. The initial concerns centered on electrochemical reliability, voiding and intrusive reflow. The latest concern is the inclusion of halogenated materials in flux products. As I described in June 2007, the two most notable risks of hybrid pastes include lower electrical reliability and increased voiding rates (“Lead Spread,” June 2007). A Pb-free flux designed to run in a hotter reflow process may not provide the desired electrical reliability when run in a SnPb process. Recall noclean pastes are designed to activate and deactivate under certain ranges of thermal energy. Running a higher temperature paste may prevent it from burning out prematurely in a hot process, but if it runs Circuits Assembly DECEMBER 2008 Chrys Shea has 20 years’ experience in electronics manufacturing and is founder of Shea Engineering; chrys@sheaengineering.com. 37 http://www.circuitsassembly.com
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