Silver sulfide contamination is a latent failure mode that is undetectable at the time the resistor is manufactured and when the resistor is mounted into its electrical circuit.įactors such as incomplete or misaligned passivation, incomplete plating, and using low palladium materials for the inner termination, can accelerate the formation of silver sulfide on the chip and lead to failure much faster than would be expected. Sulphur can be found in various types of oils and lubricants, rubber gaskets, hoses, belts, and grommets, connectors, and in some applications in the very air that the electronic device operates in. Sulphur contamination of thick film resistors source: Stackpole Electronics, Inc. This process can be initiated or inhibited already by heat-stress while mounting.įig.3. The reaction velocity in this case is influenced by sulphur gas density, temperature and humidity greatly. Silver sulfide unfortunately makes the termination material non-conductive and effectively raises the resistance value until it is essentially open circuit. Silver is so susceptible to combination with sulphur that the sulphur diffuses through the outer termination layers to the inner termination forming silver sulfide. The silver in the inner termination is very susceptible to contamination via sulphur which produces silver sulfide in chip resistors. Sulphur contamination is mainly associated with use and reliability of thick-film chip resistor with Ag-system as inner termination. ![]() The prevention and recommendation especially for highly populated PCBs would be to use dry packed components or use appropriate pre-dry process to remove maximum moisture out of the components packaging. As an example 7343 SMD chip tantalum capacitors can blow out small 0201 MLCC capacitors placed very closed to the capacitor body. Outgassing from large components can blow out and displace nearby small parts in this case. However, the issue arises for high density mounted PCBs, where small & light components are placed very close to large components. Outgassing itself may not present a danger for the components itself, if the reflow conditions such as preheat and temperature gradients are within the manufacturer specifications. While, the components itself are designed and constructed to operate reliably in accordance to their specifications, some outgassing and fast moisture evaporation can occur during the high temperature board mounting process. Non-hermetic, epoxy based packages used by number of component designs are featuring a micro porous structure that may act as a moisture residuals traps. The wetting forces vary with chip dimensions the downward forces vary with the square of the chip dimensions. Tombstoning occurs more frequent with short, wide terminations, and with thin & light devices such as miniature capacitors and resistors. ![]()
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