Circuits Assembly - July 2008 - (Page 72)

Pb-Free Lessons Learned HOP-ping Mad Reprofiling may be the best containment method for head-on-pillow. H Chrys Shea is an R&D applications engineering manager at Cookson Electronics (cooksonelectronics. com); chrysshea@ cooksonelectronics. com. Her column appears monthly. ead-on-pillow, ball-in-socket, ball-in-cup – whatever you call it, a defect by any other name still requires rework. In the past few months, reports of head-on-pillow (HOP) defects have increased dramatically. For those who have not yet experienced the HOP phenomenon, here’s a simple description: · The solder paste deposit on the board melts and fuses in the reflow oven. · The solder sphere on the BGA also melts in the reflow oven. · The sphere material and solder deposit do not fuse to become one contiguous mass. Typically, the reason they do not fuse is because one or both of the molten masses are covered with a thin but tough oxide film that prevents them from mingling. The oxide may be present on the surface of the spheres in their as-received condition, or it may be created in the oven. As is typical with soldering issues, there are a number of contributing factors, so there’s no simple fix. But before investigating the root causes of the HOP defect, the savvy process engineer will make sure it really is a HOP defect. If HOP problems suddenly appear on an assembly line, the first step should be to check the sphere material. Many BGA component providers are switching their sphere material to alloys with lower silver content. This raises the melting temperature of the spheres to approximately 227°C. If the assembly process is running on the cool side – i.e., spiking in the 230°-235°C maximum temperature range – it is possible a change in sphere alloy is causing the strange shape of the BGA joints. From the outside, joints with low-Ag SAC spheres that do not collapse during reflow look strikingly similar to joints with the HOP defect: There is often a “waist” or elongation in the joint where the sphere meets the solder deposit above the pad. The best way to differentiate between the HOP and low-Ag sphere alloy defect mode is by cross-section, but since that ruins the PWB, a more economical option is to check the components themselves. If they do turn out to be low-Ag alloy, a great deal of troubleshooting time has been saved, and the corrective action is clear: Increase the peak temperature and/or time above liquidus. If the sphere material is SAC 305 or 405 as expected, and the surfaces are solderable, then material-related root causes are ruled out, and reflow process investigations can begin. A number of factors contribute to HOP defects, including: · Higher surface area:volume ratios of both the solder deposits and the spheres. This ratio is an important factor in fine feature soldering because solder mateCircuits Assembly JULY 2008 rial surfaces exposed to air atmospheres oxidize readily, while the solder masses’ interior matters are more protected. As the size of a solder sphere decreases, the surface area:volume ratio increases. The smaller the solder sphere or paste deposit, the higher the surface area:volume ratio, and the greater the ratio of oxidizable-to-protected solder. As the solder oxidizes under reflow, it consumes flux, flux that will be needed to help aid in wetting when the metals reach liquidus. Although the assembler cannot change the BGA sphere size, they can change the solder deposit size. Increasing the stencil aperture diameter by as little as 0.001" will result in more solder on the board and a lower surface area:volume ratio for the paste deposit. At first glance, increasing aperture size may appear to be a risky endeavor that invites poor print quality and increased solder bridge defects, but it’s not as scary as it seems. Experience with SAC 305 solder pastes indicates they are less likely than SnPb pastes to form bridges and mid-chip balls. Our investigations into this phenomenon indicate it is a function of the alloy; the oxide film that forms on the surfaces of molten SAC 305 solder is more tenacious than its SnPb counterpart, which, unfortunately, works against us in the formation of HOP defects. · The oxide film that forms on Pb-free solders during reflow is tougher to break through than similar films on SnPb solders. We have to take the good with the bad in Pb-free; that same property that gives us fewer solder bridges and mid-chip solder balls also gives us a higher propensity for HOP. There are two ways to systematically address the oxide film, but neither is a quick fix to an immediate assembly problem. Approach no. 1 is to turn on the nitrogen. If one wants to prevent oxides from forming in the reflow process, it’s a no-brainer to remove the oxygen from the process. That’s not an inexpensive option, however, and for some assemblers, it’s not an option at all. Approach no. 2 is to use a stronger flux – one that has higher activity or better thermal endurance. For most assemblers, this means qualifying a new material, which can be time-consuming and expensive. Couple that with the increased flux activity, which usually means decreased electrochemical reliability, and that thermal endurance is sometimes achieved by using halogenated materials in the flux, and both system-level fixes may rapidly become complicated or even totally impractical. · Pb-free reflow processes provide more opportunity for package warpage. Thermal excursions run longer and hotter than they do in SnPb. There’s no way around it: circuitsassembly.com 72 http://cooksonelectronics.com http://cooksonelectronics.com http://circuitsassembly.com

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Circuits Assembly - July 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Tech Tips Will Electronics Follow the Sun? ‘Warm’ Manufacturing Heats Up Reputation Trumps Recession, China ‘Web Circuits’ A Novel Non-VOC Conformal Coating Krypton: Benchmarking Customer Satisfaction 2008-09 CIRCUITS ASSEMBLY Buyers Guide Wave Soldering Pb-Free Lessons Learned Process Doctor Materials World SMTA Int'l Product Preview Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - July 2008

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