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Trouble in Your Tank: Conductive Anode Filament (CAF) Formation

Editor's note: Mike Carano has since published a follow-up column titled "CAF Formation: Correction of Misrepresentation of Origins and Causes."

Introduction

There are two additional concerns fabricators must understand and reconcile as the circuit technology continues on the high-density curve along with the plethora of new materials to meet the technological demands: conductive anode filament (CAF) formation and wicking.

CAF Versus Wicking

While both issues may lead to electrical failure in a PCB, there are some subtle differences worth mentioning. In addition, CAF is typically related to the resin and glass material while wicking is more process related. More about wicking in a future column.

CAF commonly occurs between adjacent vias (i.e., plated through-holes) inside a PCB, as the copper migrates along the glass/ resin interface from anode to cathode. CAF failures can manifest as current leakage, intermittent electrical shorts, and even dielectric breakdown between conductors in PCBs. This often makes CAF very difficult to detect, especially when it occurs as an intermittent issue.

There are a few things that can be done to isolate the fault location and confirm CAF as a root cause of a failure. If the issue is intermittent then putting the sample of interest under combined temperature-humidity-bias (THB) may help re-create the failure mode. In addition, techniques such as cross-sectioning can be used to identify the failure.

CAF is caused by the glass fiber and the dielectric resin separating from each other within the laminate material itself. This is sometimes seen as a hollow glass fiber (Figure 1) that acts as a conduit, allowing process chemistry and moisture to travel along the opening. Moisture and ionic residues can access the void and enable conductive copper filament growth along the glass fiber reinforcement, leading to shorts.

To read this entire column, which appeared in the April 2020 issue of PCB007 Magazine, click here.