We propose a mechanism of electrochemical migration (ECM) to explain the dendrite growth phenomenon in FR-4 type printed circuit boards based on our investigation using highly accelerated temperature and humidity stress test (HAST). Using ToF-SIMS, we found that copper was present primarily in the upper solder resist (SR) area between the electrodes. From the observed TG-GCMS data, we inferred that the amount of outgassing from the SR was consistent with the volume of the micro-cavities generated in the SR. This observed data enhances our proposed ECM mechanism. That is, a dendrite is initiated in a micro-cavity in the SR. Then the micro-cavity by aggregation with other micro-cavities in the SR and the dendrite grows micro-cavity. We also found that the migration time of copper ions and chlorine ions in SR was about a hundred thousand times slower than that in the aqueous solution.

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