Circuits Assembly - February 2008 - (Page 40)

Process Doctor PWB Cleanliness, Part 2 Localized extraction clearly shows residue volumes at isolated spots. Ed: This continues a discussion begun last month. e can define cleanliness from an observational or theoretical stance, but what do the data say? This is not to say a general understanding of the overall board is wrong, but rather that we need to understand the cleanliness of the critical areas. Using results from a large investigation of process variables, let’s look at two process variables on Umpire Test coupons. Group 1 used dirty (high chloride and bromide residues from HASL) incoming boards processed with water-soluble solder paste and wave flux, then cleaned with DI water. Group 2 used dirty (high chloride and bromide residues from HASL) incoming boards processed with water-soluble paste and wave flux, then cleaned with DI water with 3% saponifier added to the wash solution. The testing plan included localized extraction of the 80-pin TQFP using the C3 extraction system; then the 68-pin LCC breakaway coupon was ROSE-tested and the total board was bag extracted for ion chromatography and modified ROSE testing. Group 1 (No. 26) Bag extraction (total) ROSE testing (LCC) Modified ROSE 80-pin TQFP leads Cl Br WOA OM600 5.04 SMD W Figure 1. Umpire test coupon (above) with localized extraction area (above: circled in red; below: magnified). 5.1 7.24 - - 3.7 µg/in 2 NaCl equiv - 9.3 µg/in 2 NaCl equiv - 9.51 14.57 11.43 (C3 results 41 sec. = dirty) Group 2 (No. 72) Ch Br WOA OM600 1.53 - SMD Bag Extraction (total) 1.05 3.22 ROSE testing (LCC) Modified ROSE - - 3.0 µg/in2 NaCl equiv - 8.1 µg/in2 NaCl equiv 4.40 - Terry Munson is with Foresite Inc. (residues.com); tm_foresite@ residues.com. His column appears monthly. 80-pin TQFP leads 2.65 0.57 (C3 results: 151 sec. = clean) The tables showing the Umpire Assembly SIR results for both groups are online at circuitsassembly.com/cms/content/view/6019. The results reflect only a single board from each group of a much larger experiment. These individual boards are a good representation of the results of each group. Saponified cleaning (using low pressure) of the water-soluble flux on a dirty incoming bare board does a good job of removing ionic residues from the assembly. Conclusions The difference between bag and localized extraction of the boards showed a clear difference between the samples. The sample from Group 1 bag extraction shows acceptable chloride levels as defined by Foresite’s recommended limits (less than 6.0 µg/in 2 ) after cleaning, but the localized level of contamination is high for chloride and bromide, with poor C3 electrical response (which correlates to assembly SIR performance). The comparison between Group 1 and Group 2 reveals the same issues: Bag extraction of the total board shows a lower ionic level, while localized extraction shows a pocket of ionic residue that, in this case, is acceptable and shows good electrical performance under C3 and SIR testing. But comparing the differences between Groups 1 and 2 for the localized residue pockets gives a better residue understanding than do the total board in a bag or ROSE and modified ROSE when compared to SIR performance. A localized extraction gives a clear view of the residue volume between pads on a critical circuit, while the general assembly cleanliness is a different data point, revealing gross surface residues over the entire board, a reflection of process conditions. ■ 40 Circuits Assembly FEBRUARY 2008 circuitsassembly.com http://residues.com http://circuitsassembly.com/cms/content/view/6019 http://circuitsassembly.com/cms/content/view/6019 http://circuitsassembly.com

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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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