Opens at BGA Component Corner Balls

At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.

In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

Richard Boyle is a Global Product Champion at Henkel Electronics. He has over 25 years experience in the electronics assembly industry and is responsible for the global technical service of all of Henkel's solder materials.

Subrat has 10 year of extensive experience in PCB assembly process optimizing for quality, process includes screen printing, wave, reflow. He has a copyright in stencil design published in Apex Expo2010 at Las Vegas US.

In order to mitigate the effects of warp and twist of theboard and BGA causing Opens or Head In Pillow defects ample past needs to bedeposited.For the more difficult air atmosphere reflow additionalpaste volume is needed than in a nitrogen atmosphere, no less than 1:1 paste topad or over print may solve the problem. Nitrogen atmosphere reflow is moreforgiving and if employed it may also solve the problem providing ample pastevolume is present, generally 1:1 will provide the required volume in this atmosphere.

A head-in-pillow open solder joint is often caused by a "perfect storm" of three contributing factors. The primary cause is, of course, warping of either board or component near the high end of the profile. However in many cases, as the part cools, the tension relaxes, bringing the sphere and paste into contact while still molten. So long as there is sufficient flux activity remaining, and sufficient volume of solder to make contact, the solder will still wet and a good joint will form. Solutions to head-in-pillow opens, then, usually take three approaches: 1) attempting to minimize warping of component (or board, in some cases); 2) increasing the volume of solder in the problem area(s); or 3) increasing flux activity at the end of the profile. Each of these process tweaks comes with some cost, risk, or trade-off.

1. Component warpage may be mitigated by slowing the ramp rate of the thermal profile, but keep in mind that adding extra ramp time or soak zones increases the overall profile time, which might reduce the activity of your flux when you need it most. Deflection (warping) of the assembly itself can sometimes be corrected by special supports or custom fixtures.
2. Solder volume variation is one possible issue, and indicates a review of the printing process is needed. It's also possible that your stencil is thin, such that the paste deposit is too shallow to overcome the coplanarity deltas of the warped part, resulting in no contact during reflow. Besides increasing the stencil thickness, you can try increasing the aperture size over the problem pads, but be careful not to exceed the pad diameter too much, or you risk creating "stray" solder that can result in shorts.
3. You might have success with a more aggressive flux. You can work with your paste vendor to see what they have available. You'll have to decide whether it's worth all the re-qualifying, profile development, validation, etc. that might accompany such a change. Another possibility is doing a flux dip of the component, prior to placement. A good, no-clean gel flux would provide plenty flux activity during a long profile. However, this is often a manual process, one which might not scale well enough to cost-effective at higher volumes.