Circuits Assembly - November 2007 - (Page 54)

Process Doctor Diagnosis of an Intermittent High Impedance Circuit Is your selective solder process causing leakage failures? or years clients have brought us a variety of illassembly was presented for investigation from a noperforming electronics hardware. We see a lot of clean process using double-pass reflow and selective interesting stuff like tin whiskers, open resistors, solder pallets over the wave. The performance probdendrite shorts under conformal coating, and a variety lems were in moderate humidity at 65% RH; the high of burned assemblies (even with just a 3.3V battery impedance areas of the circuit would cause the system power supply). These failure mechanisms have residue to reset or shut down amid normal operations. This from the process/fabrication or outside source that, in was seen in the field and repeated in the environmenthe right location, is a critical variable and causes failtal chamber on the failed assembly. If the assembly ures. Such failures leave behind a visible element where was baked and kept in low humidity conditions, the the damage can be observed; the element is seen in problem did not present itself. Only after exposure to just one or two locations on an assembly, not over the 65% RH did the problem reoccur. This was also found entire surface or on all components. Localized extracto be true on most of the current production samples tion techniques have been used for years to isolate and (nine of 10) evaluated under environmental exposure compare problem areas vs. areas that did not show testing. Only one of 10 assemblies did not exhibit poor problems in order to understand chemical differences performance in high humidity conditions. in ionic and organic residues. Using ion chromatography and localized extracThese techniques are used in a growing number of tion, we looked at five assemblies in three locations cases where no visible degradation has occurred, nor each. Production cleanliness data records indicated innerlayer issues seen, but there is measurable stray that all tested assemblies for the current production voltage (electrical leakage) around, across and below and field return date codes passed the 10 µg/in2 of components or trace-to-trace over the soldermask. This stray voltage phenomenon has a greater effect Continued on pg. 58 on the surface of the assemblies on high Table 1. Flux Residue Results impedance circuits. Ion Chromatography In historical work, Foresite ID# Sample Description ClBrNO3Na+ WOA we have seen issues Foresite recommended limits for no-clean wave 3.0 12.0 3.0 3.0 150.0 stemming from com- Foresite recommended limits for no-clean SMT 3.0 12.0 3.0 3.0 25.0 ponent cleanliness, Returned Field Failure Assembly A wave-solder fluxing, Sample 1 Bottomside area 1 (Selective Solder) 0.40 0 0.18 1.12 78.14 HASL bare board Sample 2 Bottomside area 2 (Crystal area) 0.31 0 0.11 6.65 201.73 flux, and overspray Sample 3 Topside area 3 (Via area) 0.35 0 0.15 4.51 168.09 from a no-clean Sample 4 Bottomside area 4 (SMT reference) 0.29 0 0.15 0.79 12.33 fluxing system – all responsible for leakCurrent Production (Failed humidty testing) age and stray voltage Sample 5 Bottomside area 1 (Selective Solder) 0.65 0 0.10 1.05 77.74 poor performance. Sample 6 Bottomside area 2 (Crystal area) 0.75 0 0.14 8.14 248.95 Now we see a greater Sample 7 Topside area 3 (Via area) 0.21 0 0.15 6.19 209.23 occurrence of leak- Sample 8 Bottomside area 4 (SMT reference) 0.49 0 0.18 0.86 7.69 age failures due to selective solder palCurrent Production (Passed humidity testing) lets over a standard Sample 9 Bottomside area 1 (Selective Solder) 1.37 0 0.20 1.11 57.95 wave solder system Sample 10 Bottomside area 2 (Crystal area) 1.51 0 0.20 1.48 24.55 or a robotic selec- Sample 11 Topside area 3 (Via area) 0.54 0 0.12 1.36 9.64 tive solder system, as Sample 12 Bottomside area 4 (SMT reference) 0.64 0 0.11 0.84 8.41 outlined in this case Ionic residues analyzed for these ions: fluoride, chloride, acetate, formate, MSA, nitrite, nitrate, bromide, phosphate, study. sulfate, WOA, sodium, potassium, ammonia, calcium, magnesium. An electronics All values are in µg/in2 F Terry Munson is with Foresite Inc. (residues.com); tm_foresite@ residues.com. His column appears monthly. 54 Circuits Assembly NOVEMBER 2007 circuitsassembly.com http://residues.com http://circuitsassembly.com

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Circuits Assembly - November 2007 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Better Manufacturing Pb-Free Manufacturing from a Tier III EMS Perspective ‘Living Documents, Subject to Change’ Creating Ideal Solder Joints An Alternative Drying Process for MSDs ‘Customer Satisfaction is More than a Score’ Optoelectronic Substrates: Will They Happen? Tech Tips Wave Soldering Process Doctor Test and Inspection Getting Lean Materials World Equipment Advances Product Spotlight Assembly Insider Technical Abstracts Ad Index

Circuits Assembly - November 2007

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