Effects of Curing Agents and Drilling Methods on CAF Formation in Halogen-Free Laminates

Abstract

Increasing demands for more reliability and functionalities in electronic devices have pushed the electronics industry to adopt newly developed materials and reduce interconnect sizes and spacing. These adaptations have led to concerns of reliability failures caused by conductive anodic filament formation (CAF). CAF is a conductive copper-containing salt that forms via an electrochemical process. It is initiated at the anode and grows along the epoxy/glass interface to the cathode, and once CAF reaches the cathode a short circuit will occur.

The objective of this research is to evaluate and compare the effects of curing agents (DICY vs. phenolic-cured epoxy) and drilling methods (laser vs. mechanical drilling) on CAF formation using an insulation resistance test at 85 ºC, relative humidity of 85%, and a voltage gradient of 0.4V/µm.

Time-to-failure for DICY-cured and phenolic-cured epoxy with laser drilled microvias and mechanically drilled vias were determined using the insulation resistance test. The failed coupons were cross-sectioned and examined using a Scanning Electron Microscope equipped with Energy-dispersive X-ray spectroscopy to verify the existence of CAF. Weibull analysis was used to compare the reliability and identify the failure modes of the failed coupons.

Test results show that DICY-cured epoxy is a better CAF resistant material than phenolic-cured epoxy. It is believed that the brittleness of phenolic-cured material might enhance the damage to the epoxy/glass fiber interface during drilling; and hence, facilitate subsequent CAF formation.

The study also shows that laser drilled microvias are less prone to CAF formation than mechanically drilled vias, because there is less mechanical damage and lower glass fiber content. Finally, using Weibull analysis, it is determined that laser drilled microvias experienced infant-mortality failure, whereas mechanically drilled vias exhibited a wear-out type failure.