Behavior of Conformal Coatings in Sulfur VaporBearing Environments Barry Hindin, Ph.D., P.E. \\\\\\\ Michah Pledger Pledger Consulting Outline Why conformal coatings are important How conformal coatings were tested Results of Experiments Summary Acknowledgements 2 Introduction: Tarnishing of Silver Silver tarnishes when exposed to sulfur-bearing atmospheres The predominant (or only) reaction product is silver sulfide (Ag2S) when exposed to sulfur vapor 2Ag + S(g) → Ag2S (not a conductor) Terminations on chip resistors are often silver or silver alloys and can be susceptible to sulfidation. 3 Schematic of Susceptible Chip Resistor Susceptible Area Glass Protective Coat Inner Silver Electrode Resistor Layer Ceramic Substrate End Terminations Cross Section of Side View 4 Consequence of Silver Sulfidation in Chip Resistors Solder pad Glass 5 Ag2S Crystals on a Ag Surface Ag2S crystals on a Ag coupon at 60 ºC after 800 hours 6 Solubility of Gases in Silicone Rubber (or why not to use silicone as conformal coating) 7 Test Chamber All testing was conducted in a Flowers of Sulfur (FOS) chamber Oven was controlled to ± 1 ºC Air was circulated using a small fan 8 Testing Conformal Coatings to Prevent Sulfidation Chip resistors and Ag-coated glass slides were coated with different conformal coated to determine their effectiveness for preventing sulfidation Initial test temperature was 60 ºC for 600 hours but then raised to 71 ºC for an additional 700 hours to provide a more aggressive environment 9 Test Samples • • • • • 1 µm Ag on glass slide 4-Wire resistance measurements 25 type 0805 chip resistors with known susceptibility to sulfidation 1%, 1/8th W, 10kΩ Ω Wired in parallel 10 Conformal Coating Test Matrix Test Sample Type Chip Resistor Silver Coated Glass Slides Sample Code 223 323 423 Coating Control 221 222 321 322 421 422 Silver Control Coating Thickness (µm) low medium high 51 low low medium medium high high none 11 Comparison of Conformal Coating Effectiveness 12 Comparison of Conformal Coating Effectiveness 13 Comparison of Conformal Coating Effectiveness 14 A Separate Conformal Coating Test FOS at 60 ºC for 20 days (480 hours) Silver Dots This data taken from conformal coating tests on ceramic cards Epoxy (25 µm) Polyurethane (50 µm) Acrylic or Silicone (50 µm) 15 Summary FOS environment successfully used to compare performances of different proprietary conformal coatings at elevated temperatures. As expected, thicker coatings provided better protection. Threshold thickness may be required for some coatings. 16 Summary (cont.) For test results of conformal coatings tested on ceramic cards, polymers, epoxies performs best followed by urethanes, followed by acrylics Silicone should not be used for protecting against sulfidation. 17 Next Steps Additional conformal coatings will be compared including those with different fillers and chemistries. Higher temperatures (> 70 ºC) will be used to decrease testing time by accelerating time-to-failure of poor performing coatings. 18 Acknowledgements The authors would like to thank W.H. Abbott for valuable discussions and his review of data regarding silver sulfide thickness measurements and, Ernie Patterson for his assistance in assembling the flowers of sulfur chamber. 19 Thank You! Questions? Barry Hindin, Ph.D., P.E. Battelle (614) 424-4121; [email protected] 20
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