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PCB007-Apr2020

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120 PCB007 MAGAZINE I APRIL 2020 Introduction There are two additional concerns fabrica- tors must understand and reconcile as the cir- cuit 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 fail- ure in a PCB, there are some subtle differences worth mentioning. In addition, CAF is typical- ly related to the resin and glass material while wicking is more process related. More about wicking in a future column. CAF commonly occurs between adja- cent 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 break- down between conductors in PCBs [1] . This often makes CAF very dif- ficult 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 inter- mittent then putting the sample of interest under combined tempera- ture-humidity-bias (THB) may help re-create the failure mode. In addi- tion, techniques such as cross-sec- tioning can be used to identify the failure [2] . Conductive Anode Filament (CAF) Formation CAF is caused by the glass fiber and the di- electric 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 en- able conductive copper filament growth along the glass fiber reinforcement, leading to shorts. At the end of the day, there are three things required to cause CAF: 1. A pathway, such as hollow glass fibers or separation of the resin from the glass 2. Bias (applied electric field) 3. Moisture and other ions With a pathway, as shown in Figure 1, along with moisture and ionic species from various chemical processes—such as electroless cop- Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY Figure 1: Hollow glass fiber.

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