Circuits Assembly - December 2007 - (Page 38)

Pb-Free Lessons Learned A Small Perk SAC 305 produces 10 times fewer mid-chip solder balls than does SnPb. I Chrys Shea is an R&D applications engineering manager at Cookson Electronics (cooksonelectronics. com); chrysshea@ cooksonelectronics. com. Her column appears monthly. n May, I reported Pb-free solder pastes create fewer SnPb era, we were able to limit solder ball production mid-chip solder balls and solder bridges than do by reducing the stencil aperture area relative to the pad. SnPb pastes. Why this phenomenon occurs was When Pb-free solders began gaining in popularity, we not well understood at the time. I suspect it may have found that the practice of aperture cropping often led something to do with superior hot slump resistance of to exposed copper along pad perimeters and corners. more thermally stable paste fluxes, but our chief paste We needed a better way to deposit solder paste to the formulator suggested it may be more likely a result of corners, while still limiting paste volumes and solder the oxide film that forms on the molten alloy; it’s more ball generation. The aperture design that emerged as tenacious in Pb-free alloys than in eutectic SnPb. the best in that 2004 experiment was what we now To get to the bottom of this, I tested three pastes call the Roundie, and it became part of our Pb-free head-to-head. The first was a standard SnPb paste; the aperture library. In this recent experiment, a total of second a standard Pb-free (SAC 305) paste, and the 1182 solder balls were created, but only 17 came from third a hybrid paste of SnPb solder powder combined the Roundie aperture (16 SnPb; one Pb-free). That’s with Pb-free flux. The logic behind the test was simple: an occurrence rate of 0.2%. Although we did not need If lower defect rates were due to the to fine-tune our aperture design to flux, then the hybrid should behave this degree in SnPb processing, it is more like Pb-free and produce fewer nice to know this is a robust design solder balls. If lower defect rates regardless of alloy used. were due to the alloy, the hybrid What’s this month’s Pb-free lesshould behave more like SnPb and son learned? The numbers speak produce more solder balls. for themselves. With the same A few details on the experiment: The Roundie aperture design permits boards, stencils, components and My test vehicle has 400 chip compo- scaling to 0402 and larger pads. (analogous) profiles, produced on nents total, 100 each of sizes 0402 the same assembly line on the same through 1206. Of these 100 each, day by the same technician, SAC 50 are mounted horizontally and 50 vertically. My test 305 produced 10 times fewer MCSBs than did SnPb. stencil was 0.005"-thick laser cut stainless steel, and for This is the first absolute benefit of Pb-free solder I’ve each block of chip components, half the apertures were been able to quantify. Although this is a small perk in a rectangular, while the other half were radius inverted huge challenge, it’s still a perk. home plate designs, also known as the “Roundie” (FigDoes the Pb-free alloy produce fewer solder bridges ure 1). Both were sized 1:1 with the pad. We ran 12 on fine-pitch devices? I did not even attempt to gather boards with each paste; six with a ramp profile and six data on this question. While my test vehicle has 400 with a soak profile, all in an air environment. This gave components, only four are QFPs, so generating a statisus 4800 opportunities for solder balls per paste type. tically significant sample size represents a considerable The results were crystal clear. Dr. Michael Liberaeffort, not a quick test. For now, I’ll continue to rely tore’s insights were right on the money! The SnPb alloy on the anecdotal evidence provided by my associates produced about 10 times more solder balls than the who have personally experienced the alloy transition Pb-free alloy, regardless of the flux vehicle. The Pb-free process. With respect to intrusive reflow, again I don’t paste produced 62 mid-chip solder balls, equivalent to have a statistically significant sample size, but this a 1.3% occurrence rate. The SnPb produced 504 solder recent test did generate good indications. The Pb-free balls, equivalent to a 10.5% occurrence rate. And the alloy appeared to perform comparably with SnPb on hybrid paste, which had SnPb alloy and Pb-free flux, smaller overprints and ramp profiles, but seemed to produced 616 solder balls, or a 12.8% occurrence rate. outperform SnPb in larger overprints and soak profiles (Note use of the term “occurrence rate” rather than – perhaps yet another perk as we continue down this “defect rate”; criteria under which MCSBs are considrocky road to Pb-free soldering. ered defects vary widely.) Au. note: Many thanks to Dr. Michael Liberatore Another interesting datum collected was the perforfor guidance on the physical mechanisms at work in mance of the Roundie versus the rectangular aperture. this phenomenon, and to Esse Leak for assembling the We first investigated and documented the Roundie boards, counting nearly 1200 solder balls, and screenabout three years ago. The investigation was prompted ing more than 1400 pin-in-paste locations. We would by the lack of spread of Pb-free solders on OSP. In the not have learned this lesson without their help. ■ Circuits Assembly DECEMBER 2007 circuitsassembly.com 38 http://cooksonelectronics.com http://cooksonelectronics.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - December 2007

Circuits Assembly - December 2007 Contents Caveat Lector Industry News Market Watch Talking Heads Global Sourcing Screen Printing Better Manufacturing Metalization Options for COB Assembly A Test Comparison of SAC and Non-SAC Pb-Free Solders An A-to-Z Guide to X-Ray Inspection BEST: A ‘Funky Chicken’ with an EMS Niche Tech Tips Wave Soldering Process Doctor Pb-Free Lessons Learned Getting Lean Eastern Approaches Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - December 2007

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