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Conformal Coatings in Preventing Resistor Silver Sulfide Corrosion
Conformal Coatings in Preventing Resistor Silver Sulfide Corrosion
The evaluation of conformal coatings to mitigate silver sulfide corrosion of thick film resistors is discussed.
Materials Tech

Materials Tech programs cover topics including:
Adhesives, Chemicals, Cleaning Solutions, Coatings, Components, Design, Embedded Technology, Fasteners, Finishes, Flex Circuits, Flip Chip, Fluxes, PC Fab, Solders, Solder Masks, Solder Paste and more.
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Authored By:
Marie Cole, Jacob Porter, Jason Wertz, Marc Coq
IBM Corporation
Poughkeepsie, NY, USA

Jim Wilcox and Mike Meilunas
Universal Instruments Corporation
Conklin, NY, USA

Summary
Thick film resistors are used extensively in a variety of electronics applications. The silver in a conventional thick film resistor is prone to the attack of sulfur-bearing gaseous contamination. This problem has been documented for servers that are found in data centers, due to the environmental pollution of sulfur in certain industrial locations and more typically, in growth market countries where the use of coal to produce electricity is prevalent.

The growth of silver sulfide, resulting from silver corrosion, can cause an increase in resistance and eventually, an electrical open of the resistor. The best method to increase the robustness of resistors in high sulfur environments is to employ Anti-Sulfur Resistors (ASR). These resistors either have a structure alteration to seal the ingress path from sulfur bearing gases or use a noble metal for the resistor contacts. Occasionally, unique resistor part numbers have limited availability in ASR construction. Thus, it is beneficial to have alternate techniques to mitigate sulfurinduced corrosion.

This paper will discuss the evaluation of conformal coatings to mitigate silver sulfide corrosion of thick film resistors. Two-part epoxy has been demonstrated to prevent resistor corrosion, but has manufacturability concerns in high volume production. Conversely, coatings that contain silicone are known to increase silver sulfide corrosion due to their inherent nature in readily absorbing sulfur. Other conformal coating chemistries are available for various applications, but have not been tested for their ability to mitigate silver sulfide corrosion of resistors. A Flowers of Sulfur (FoS) test procedure can evaluate the tendency for silver sulfide corrosion of resistors to occur as a predictor of field performance. This technique was used to evaluate polyurethane and acrylic materials, in addition to a nanocoating. Uncoated, epoxy-coated and silicone-coated samples were used as controls for comparison to the coatings evaluated. Results and observed corrosion trends for a variety of resistor body sizes will also be discussed

Conclusions
The inconsistent results and lack of protection provided by many of the coatings reinforces the need for accelerated corrosion testing prior to the selection of any conformal coating that will be used in a high sulfur environment.

The search continues for a conformal coating material that can be applied in a highly manufactuable process and that consistently provides a high degree of protection from corrosion. A relatively new UV curable, single part epoxy with properties similar to the successful two part epoxy could be an attractive alternative that should be evaluated.

Initially Published in the SMTA Proceedings

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