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122 PCB007 MAGAZINE I APRIL 2020 per—under a bias, there will be migration of copper along this pathway. Essentially, under these conditions, an electrochemical cell is cre- ated. Corrosion products move from anode to cathode thanks in part to the pathway created by the hollow fiber. There are several known causes attributed to separation of the fibers from the resin. Typi- cally, the majority of these have the root cause within the laminate manufacturing process. The glass sizing used to enhance the resin's bond to the glass cloth may have been defec- tive. In addition, resin moisture absorption and overall less than adequate resin encapsulation of the glass fabric are often to blame. From the bare board fabrication side, fracturing of the glass bundles during drilling will leave ex- posed fiber ends and the dreaded pathway for ions to creep along. Figure 2 shows an example of CAF bridging. In recent years, there has been a better awareness of CAF, and the IPC-4101B standard was recently released driven by concerns of some OEMs. Individual companies, however, continue to use their own test conditions and vehicles. A number of organizations, including the HDP User Group, have published various studies on this subject. There is recognition that not all resin systems or laminate suppli- ers are created equal when it comes to CAF. These studies have supported the theory that lead-free assembly and HUE (harsh use environment) con- tribute to CAF. How to Prevent CAF Formation There are several suggestions below for mitigating CAF formation: • Better control of ionic impurities in the rinse processes during PCB fabrication (high levels of ionic impurities contribute to CAF • Improved glass-to-resin bond strength • Low moisture (<0.3%) pick-up resin properties • Improved hole formation methods and care with drilling. • With the introduction of lead-free processing, it is necessary to select thermally robust materials, such as phenolic-cured resins. However, phenolic-cured resins are more brittle, leading to fractures at drilling, so care is needed with drill-bit selection and drill speed As circuit densities increase and harsh use environment (HUE), as well as lead-free as- sembly, is used, fabricators and circuit board designers must cooperate to ensure CAF resis- tant materials are carefully selected. PCB007 References 1. IPC TM-650 2.6.25 Conductive Anodic Filament (CAF) Resistance Test: X-Y Axis. 2. C. Hillman, "A Novel Approach to Identifying and Vali- dating Electrical Leakage in Printed Circuit Boards Through Magnetic Current Imaging," Proceedings from the 30 th In- ternational Symposium for Testing and Failure Analysis, November 14–18, 2004, Worcester, Massachusetts. Michael Carano is VP of technology and business development for RBP Chemical Technology. To read past columns or contact Carano, click here. Figure 2: CAF bridging from one side of the via to an adjacent via.

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