Circuits Assembly - February 2008 - (Page 42)

Pb-Free Lessons Learned Time to Decommoditize PWB Fabrication Pennies saved on the front end can cost big money in rework. ver since I began assembling PWBs nearly 20 years ago, I’ve been addressing problems whose root causes lie not within assembly but PWB fabrication. I did always appreciate that fact. Then, as luck would have it, I transferred under a boss whose background was in fabrication. He opened my eyes to the influence of the fabrication process on the assembly process, and how to investigate, prevent or compensate for the bare board’s effects. Just like every other major factor we’ve studied during this transition, influential considerations in SnPb processes are just as influential in Pb-free, which always has a few extra-special concerns. I have yet to encounter a SnPb process consideration whose Pb-free corollary doesn’t have some distinctive twist. Some of the general fabrication issues we have learned to anticipate regardless of alloy include: • PWB feature location accuracy. Sometimes, locations of the centroids of the devices on the PWB do not match their CAD file locations. The lamination process often causes PWBs to shrink from their nominal locations. To compensate, fabricators typically factor the expected shrinkage into their imaging processes, but variations in raw materials and processes can push the “fudge factor” a little off the mark. The result for the assembler? Misaligned paste prints and misplaced devices, despite good process control on the stencil fabrication, stencil alignment and pick-and-place. The paste deposit and placement might be exactly as designed from an X-Y perspective, but if the landing pads are off-target, they can undermine many of the benefits running good process controls on the assembly line. • PWB feature size accuracy. Copper overetch and underetch on the PWB surface can cause problems on the assembly line. Overetch seems to be the greater menace, as it renders pads slightly (say 0.0005" to 0.001") smaller than they should be. Smaller-than-expected pads can cause gasketing problems during printing, resulting in bridges and random solder balls. A slight bit of overetch can cause wet bridges in the paste print, and the root cause can be very difficult to pick up with an unaided eye. Over the years, I’ve witnessed some great engineers stymied by undiagnosed overetch issues, spinning their wheels investigating paste viscosities, print parameters, support tooling – all to no avail. E Chrys Shea is an R&D applications engineering manager at Cookson Electronics (cooksonelectronics. com); chrysshea@ cooksonelectronics. com. Her column appears monthly. • PTH drilling and plating. Poor drilling or desmear can leave glass fibers protruding into the PTHs that breach the plating coverage. The tiny breaches provide paths for internal moisture to outgas during soldering, resulting in blowholes. Typical SnPb blowholes look like mini volcanoes, with a small pinhole at the top, and can usually be addressed by baking moisture out of the PWB prior to soldering. Pb-free blowholes are one of those special considerations – those distinctive twists – and are not so easily resolved. • Final finish. I learned the hard way that not all final finishes are created equal, and if you don’t specify exactly what you want, you get what the fabricator chooses. That can mean the lowest cost option for the fabricator, which is not necessarily the best performance option for the assembler. It’s not unreasonable to anticipate a final finish’s solderability and shelf life, and thermal degradation resistance will take a backseat to the bottom line if specific product or performance criteria are not identified upfront. • Solder mask selection and application. Just like final finishes, not all solder masks are created equal, and if you are not specific in your selection, you get “whatever.” If the mask du jour is not the least expensive material available, it is likely whatever was set up on the line from the prior run, which means it may vary with each lot of boards. The importance of mask selection on soldering performance should not be underestimated. Glossy masks can hinder wave soldering by limiting a liquid flux’s ability to spread evenly across the PWB surface, and by creating micro solder balls (MSB) that stick to the board. Undercured masks of any variety can also contribute to MSB adhesion, and even microdross adhesion. I recall one particular batch of PWBs that passed visual inspection after soldering, but came up full of shorts at ICT. Closer inspection revealed microscopic pieces of dross were clinging to the mask, shorting out vias to pads and to other vias. If the ICT failure reports hadn’t told us the exact locations to examine, it may have taken a long time to find them visually. They were fine, dull, brownish-grey webs that clung to the mask. But they were conductive, and formed electrical shorts that could not be seen under the ring light given typical inspection conditions. We'll pick up here next month. ■ circuitsassembly.com 42 Circuits Assembly FEBRUARY 2008 http://cooksonelectronics.com http://cooksonelectronics.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - February 2008

Circuits Assembly - February 2008 Contents Caveat Lector Letters Industry News Market Watch Global Sourcing Better Manufacturing Maximizing Lean Copper As a Viable Solution for IC Packaging Embedded Active Components for High-Rel Products Cover Story: XRF Equipment As a RoHS Screening Tool Tech Tips Selective Soldering Test and Inspection Process Doctor Pb-Free Lessons Learned Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - February 2008

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