Circuits Assembly - December 2007 - (Page 25)

Solder Materials Table 1. Shear Test Results Summary Device Location Maximum SAC 305 R19 R21 R34 C13 C14 C15 C22 C34 Average resistor Deviation resistor Average capacitor Deviation capacitor Overall average Overall deviation 76.13 82.27 68.83 103.60 103.65 99.16 74.65 68.32 75.74 6.73 89.88 17.03 84.58 15.24 Shear Force (N) Cobalt 64.93 85.01 78.62 90.43 92.69 77.28 69.03 72.42 76.19 10.26 80.37 10.66 78.80 9.98 Table 2. Pull Test Results Summary Device Location Maximum SAC 305 U2 U7 U11 Average Deviation 305.2 368.7 383.6 352.5 41.6 Shear Force (N) Cobalt 305.7 315.8 328.1 316.5 11.2 shear test is that the greater the force required to shear the component, in theory at least, the better and stronger the solder joint. Much discussion regarding solder voiding and joint reliability has recently occurred. Solder voiding tests might be performed. The sole IPC reference found regarding solder voiding is in IPC-A-610D and regards BGAs, for which a defect is classified as “more than 25% voiding in the ball x-ray image area.”2 The IPC Solder Value Products Council recently addressed this issue, stating, “Nine separate methods of statistical analysis comparing cycles to failure looking at both voids greater than 25% of the interconnection area and total voids have been done. Absolutely no correlation between voids and failures under thermal cycling has been demonstrated … there is no evidence that the type of solder joint voiding observed in the SAC alloy solder joints has any significant impact on solder joint reliability.”3 Yet voiding remains an oft-discussed issue. A red dye test may indicate the circuitsassembly.com potential for joint failure, and may be performed to visibly show micro cracking in solder joint surfaces before and after thermal cycling. It is believed that micro cracks are the point source for additional cracking, which may lead to joint failure. Temperature/humidity tests can be performed to determine whether an alloy is susceptible to tin whisker formation. Tin whisker formation and dendrytic growth may cause short circuits at an undetermined time after the PCB has been placed in use. iNEMI recently published recommendations to mitigate whisker formation potential. The organization stated that “unalloyed tin electroplating has a long history of whisker formation and growth that has resulted in reliability problems for various types of electronic equipment. … It is generally accepted that the driving force for whisker formation is compressive strength on the tin films.” 4 Various solutions are promulgated by iNEMI to reduce the possibility of failure. The authors contracted with an independent laboratory to conduct an exhaustive test comparison between SAC 305 and Cobalt995 (a cobalt enhanced binary alloy consisting of Sn99.5Cu0.5Co). As the industry standard, SAC 305 was the control. Thermal cycling (including subsequent shock and vibration), solder voiding quantification, and pull and shear testing were performed for both alloys. The intermetallic thickness and alloy diffusion into the copper was examined both before and after thermal cycling/ aging for both alloys. In addition, the cobalt alloy was subjected to a temperature/humidity test. Boards hot air leveled with the Sn99.5Cu0.5Co alloy were manufactured. Half the boards used SAC 305 no-clean solder paste for the SMT portion and the same alloy for through-hole. The remaining half used Sn99.5Cu0.5Co alloy in no-clean solder paste for SMT and bar solder for wave soldering. Both pastes were manufactured with the same flux chemistry to reduce potential variables. These Circuits Assembly DECEMBER 2007 25 http://www.seikausa.com http://www.seikausa.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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