Circuits Assembly - July 2008 - (Page 74)

Process Doctor How Clean Are Plugged Microvias? Not very. And the residues can affect circuit performance. icrovia contamination from the fabrication process is not new, but is leading to a new appearance in immersion tin, silver and OSP boards. Besides microvias (holes with a 0.013" or smaller opening) with visible attacks on the metalization surface, we are seeing open or intermittent open cracked vias. These small openings are plugged on one side and function as a turnedup cup in a dishwasher. Wash and rinse water get inside and do not flush out easily, leaving corrosive residue. Figure 1 shows an OSP process with good plugged vias below a BGA, which held sulfuric acid (H 2SO 4) residues from the etch process and tap water rinsing. The bare boards had plugged vias. As part of the fabrication process, the boards were subjected to an OSP process, which included sulfuric acid (as a microetch) and tap water cleaning. The boards were processed such that the plugs were always down (relative to gravity). We believe the sulfuric acid gets into the vias as part of the OSP coating process, and the subsequent tap water cleaning does not adequately remove the sulfate residues, especially for tall/narrow holes. The sulfates dry in place and, when later combined with either heat (preheat and wave solder) or water (from aqueous cleaning), the acid reforms at a significant concentration. The lower crack (Figure 1) was formed by liquid acid etching away copper. The upper crack was formed by acid vapors etching the copper. Circuits Assembly JULY 2008 M Terry Munson is with Foresite Inc. (residues.com); tm_foresite@ residues.com. This column appears monthly. Because of the surface tension of water (especially tap water), and the closed nature of the capped v ia, there was no effective cleaning in those vias. Using a localized extraction system to assess Figure 1. In this OSP PWB, good plugged vias below a BGA ! just a couple caught H2SO4 residues from the etch process. Rinsing didn’t vias revealed help. 18 to 27 µg/in 2 of sulfate in these areas. The residue is trapped during the wet chemical fabrication processes, and when the boards are reflow-soldered, the chemicals (H 2SO 4 ) do as designed: attack and dissolve metal. The second investigation showed the residue will outgas during reflow, leaving conduc! Figure 2. Smaller holes are tive and corrosive residue in and prone to surface metalization around the vias. Where sensidegradation … tive circuits are present, we see stray voltage and attack of the via metalization. These areas of the vias (Figure 2) show visible attack of the surface metalization and affected circuit performance. The level of sulfate on these Ag-plated assemblies is 24 to 30 µg/in 2 in a 0.1 in 2 area of testing. The amount of ionic ! igure 3. While larger holes F residue found in larger holes is show more resistance. much less; for example, a 0.030 2 of in. hole shows 0.81 µg/in volume of solution or surface sulfate residue (Figure 3). area calculation. We need to see When testing the total bare the cleanliness in a few vias to board or even assembly with understand the pocket of cona bag or bulk amount of ionic tamination effect and determine residue, these small via cups control points needed. of residue get diluted by the n circuitsassembly.com 74 http://residues.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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