From Electronics Manufacturing: With Lead-Free, Halogen-Free, and Conductive-Adhesive Materials
20.1 INTRODUCTION
20.1.1 FACTORS AFFECTING GALVANIC CORROSION
Experimental results in Chap. 19 strongly indicate that metal oxide formation resulting from galvanic corrosion of a nonnoble metal at the interface between an electrically conductive adhesive (ECA) and the metal was the main mechanism for the unstable contact resistance of conductive adhesives. In this chapter, various approaches will be used to stabilize the contact resistance.
Several factors, such as moisture absorption and concentration of electrolyte, can affect the rate of galvanic corrosion. One of the critical requirements for galvanic corrosion is the presence of electrolyte solution. Without aqueous solutions, galvanic corrosion does not happen. An ECA formulation with lower moisture pickup should have less condensed water at the interface between the ECA and the nonnoble metal. Such an ECA is expected to show less serious galvanic corrosion at the interface and less increase in contact resistance.
Electrolytes provide electrical conductivity, which is also required for a galvanic corrosion process. Without electrolytes, galvanic corrosion is very slow. The electrolytes in this case mainly come from impurities in the resin. Therefore, purer resins should provide ECAs with better resistance to galvanic corrosion.
20.1.2 ADDITIVES TO PREVENT GALVANIC CORROSION
Oxygen Scavengers.
Oxygen scavengers are chemicals that are added to water solutions to inhibit oxygen corrosion. As their name implies, they react with dissolved oxygen in aqueous solution. However, to assume that they act as inhibitors of oxygen corrosion by scavenging the oxygen from the water is too simple a view. Their reactions with oxygen are...
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