Head in Pillow An Easily Missed and Costly Soldering Defect
Posted 10/24/2012 by Mitch Holtzer
The Ball Grid Array (BGA) package is a widely-used surface mount technology component and today's lead-free solder pastes are designed for use with most BGAs. But often the mounting suffers from problems like incomplete sphere to paste coalescence, otherwise known as a Head-in-Pillow defect. Upon closer inspection, the issue appears like a small head at rest on a pillow with little or no intermetallic formed between the BGA sphere and the solder paste deposit. At a functional level, Head-in-Pillow leads to faulty electrical contacts, which require rework at best, and field failures at worst and forces electronics manufacturers to lose valuable time, resources, and money in the search for a solution. Complex assemblies with long reflow profiles can make this defect more likely.
Imperfections in the manufacturing process often lead to Head-in-Pillow problems. The surest method to correct incomplete solder joint wetting starts with examining the operation steps for warped BGAs or deformed PCBs, alignment problems in the PCB printer, excessive time and/or temperature in reflow, and z-axis location in the device mounter. Yet solving these problems, is not an exact science because issues always arise and force process engineers to readjust the steps. This is where advanced surface mount solder material makers, like Alpha Cookson Performance Materials with their ALPHA® solder paste, have taken a leadership role.
In the last decade, the Restriction of Hazardous Materials (RoHS) standard, limited the use of Tin-Lead alloys. Meanwhile, groups like the Japan Electronics and Information Technology Industries Association (JEITA) set guidelines on package quality. Lead-free alloys require a longer, hotter reflow profile versus tin lead alloys. This puts more stress on the solvent life and activation of solder paste, as well as increasing the tendency for BGA packages to warp. If the sphere from a BGA lifts out of the solder paste deposit, and the solder paste looses its activity before the package relaxes and rejoins the sphere to the paste, a head in pillow defect is very likely.
After years of focused R&D time and resources, Alpha Cookson Performance Materials developed its ALPHA® solder paste, which combines Lead-free alloys with advanced Zero Halogen paste flux. In most surface mount applications, with or without BGAs, their solder paste produces taller and more consistent volume deposition, has a greater resistance to longer soak profiles, and has faster wetting speed all while maintaining long term electrical reliability. By increasing the height of the solder paste deposit, it is less likely that the sphere will leave the paste deposit. ALPHA® solder pastes are designed to give consistently high paste deposit height at the <240 micron feature size required by many popular BGA packages. This results in a measurable decrease in head in pillow defects., but Alpha doesn't stop there. Their tireless field professionals are experts in resolving print process and reflow profile problems. In test after test, Alpha has shown that using ALPHA® solder paste produces more first time right solder joints and helps manufacturers meet JEITA standard requirements for BGA assembly. All said, new problems arise with each successive improvement in the solder process and Alpha Cookson Performance Materials is a dedicated partner in helping manufacturers meet the demands of a constantly evolving business environment.