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Surface Finish Effect on Creep Corrosion in PCBs
Materials Tech |
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Authored By:Cherie Chen, Jeffrey ChangBing Lee, Graver Chang, Jandel Lin IST-Integrated Service Technology, Inc. Casa Hsieh, Jesse Liao, Jerry Huang Tripod Technology Corporation TranscriptIn this study, the major focus was the investigation of creep corrosion on PCBs with 3 different types of common surface finishes. The study investigated the effect of the mixed flowing gas with various hydrogen sulfide concentrations in order to understand how hydrogen sulfide drives corrosion acceleration. The result shows the corrosion rate of copper coupons rapidly increases with hydrogen sulfide concentration above 1000 parts per billion. Silver coupons have more active corrosion in low hydrogen sulfide concentration than high hydrogen sulfide concentration. Flaking corrosion also happens on copper coupons in a high hydrogen sulfide concentration test condition. Creep corrosion can be driven by multiple factors. In addition to environmental factors, such as pollution, temperature and humidity, the complicated PCB manufacturing process is also a concern. SummaryCreep corrosion normally happens in the end system, PCB, connectors and components are widely noted due to the exposure of high sulfur environments under elevated humidity. In this study, the major focus is the investigation of PCBs with 3 different types of surface finish (ImAg, Post-Treatment ImAg, HT-OSP), SMD vs NSMD and non clean organic acid flux residue from simulating wave soldering process under MFG Test (Mixed Flowing Gas Test). The realistic mixed flowing gas (H2S, SO2, NO2,Cl2) at certain concentration of each and relative humidity are designed to accelerate creep corrosion happening. One of the purposes in this study is to investigate the effect of the mixed flowing gas with various H2S concentration (500 ppb,1000 ppb, 1700 ppb) at 5 days duration on the corrosion rate (nm/day) in the Cu coupon and Ag coupon in order to understand how H2S drives the corrosion acceleration. The data are also verified by the methods of Weight Gain Analysis and X-Section with SEM/EDX. The result shows much higher corrosion rates are observed on Cu coupon in both Individual and Mixed Flowing Gas Tests. The corrosion rate of Cu coupon rapidly increases with H2S concentration above 1000 ppb. Ag coupon have more active corrosion in low H2S concentration than high H2S concentration. Flaking corrosion also happens on the Cu coupon with heavy corrosion product in the high H2S concentration test condition. And more visible creep corrosion is observed on HT-OSP finished circuit boards and SMD, as the residue of organic acid flux residue is not able to prevent corrosion occurrence ConclusionsCreep corrosion can be driven by multiple factors. In addition to environmental factors, such as pollution, temperature, humidity the complicated PCB manufacturing process is also another concern. There might be many potential influences on creep corrosion during the process and not only surface finish, flux, and board design. The corrosion occurrence on the PCB is very sensitive to surface chemical properties. Ionic cleanliness and the roughness and surface chemistry of the soldermask might be other factors that influence the rate of creep corrosion growth. That will be the topic for further study Initially Published in the IPC Proceedings |
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