Avoiding Electrolytic and Strain‐induced Shorting Mechanisms—A Review

Abstract

The paper describes two well‐known and occasionally confused mechanisms for degradation of electronic circuitry. Intended as a tutorial for individuals working in electronic packaging who have limited background in materials and little experience with these mechanisms, the paper defines and describes the two latent shorting phenomena. Major papers and conferences dealing with the phenomena are cited. Electrolytic or electrochemical shorting is an electrical field‐induced mechanism that can destroy the integrity of modern, densely packed circuits operated in the presence of moisture and ionic contaminants. Examples of copper migration to form electroplated shorts in both thick film hybrid multilayer and printed circuit multilayer boards are discussed, and common features to both systems are outlined. Related mechanisms that may occur with the simple electrochemical (metal plating) mechanisms to produce a broad array of electrical isolation breakdowns are also described. The closing of this part of the paper is a brief review of the Sarnoff‐developed RCA/GE multilayer copper materials system. By design this system solves the problems raised regarding thick film copper multilayer latent failure mechanisms. The discussion of whisker growth is limited to proper whiskers, including those that grow without the application of external stress, squeeze whiskers, and whiskers that result from classic electromigration. All of these grow from solid sources in contact with the whisker. The whisker growth direction is not electrical field related. Identification is made of Sn, Cd, Sb and Zn as the materials classically found to grow whiskers at room temperature. Avoiding the use of electroplated films of Cd, Sb and Zn in close proximity to electronic circuitry is encouraged, and the modern requirements that Sn films be used only after melting, or be alloyed with lead, and not on brass substrates are discussed. In more recent literature indium alloys have been identified as room temperature whisker growth systems. Finally, mechanical design to eliminate squeeze whisker shorting that can result from fasteners in contact with the above and other metals is briefly treated.

Citation

Hitch, T.T. (1990), "Avoiding Electrolytic and Strain‐induced Shorting Mechanisms—A Review", Circuit World, Vol. 16 No. 2, pp. 41-44. https://doi.org/10.1108/eb044022