SMT007 Magazine

SMT-Apr2014

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86 SMT Magazine • April 2014 PCB cleanliness on the assembly floor is now getting more attention, due to tiny resi- dues and contaminants being left on assemblies after new, advanced assembly processes. Clean- ing methodologies, testing, analysis, and spe- cial chemistries are being taken to a new level to assure customers of ultraclean boards to avoid costly latent issues. Board cleanliness is especially critical for mil/ aero and medical applications. The right types (and amounts) of cleaning agents must remove flux residues to assure the integrity of processes such as bonding and conformal coating. Dur- ing the coating process, if residues result in poor wetting or delamination, they can cause assem- bly failures and ultimately lead to field failures. In today's assemblies, there are growing numbers of advanced packages like land grid arrays (LGAs), fine pitch BGAs, micro BGAs, micro CSPs, and the list goes on. Packages such as LGAs are flush to the board, and they have bumps, not balls or spheres, like BGAs. If a small amount of flux remains trapped under the board's surface, it can change the characteristics of impedance, resistivity. This could also change what could have been a per- fect eye diagram, and in general, change the optimal level of operation. Ultra cleanliness is even more critical if underfill or conformal coating is applied to a device. It's difficult to clean such a device after the board has been processed. Therefore, assembly personnel have to be absolutely sure they've taken every possible measure to assure devices and boards are ultraclean. If residues are left in the conformal coating or underfill, then these contaminants are trapped inside the capsule and hinder device performance. Plus, a vast array of other residues and con- taminants can be left on a PCB, as shown in Fig- ure 1. The contaminants may be either ionic or non-ionic contaminants. Ionic contaminants contain molecules that are conductive when the cleaning solution is applied. Ionic residues by Zulki Khan nexlogic TechnologieS inc. ZUlKi'S pCB NUGGETS COluMn Tighter Scrutiny Needed for pCB Cleaning agents

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