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24 The PCB Design Magazine • August 2015 Hole wall pullaway (HWPA) is an insidious defect that is not usually a cause of electrical failure. What happens with HWPA is that the copper plating in a plated through-hole (PTH) is pulled away from the dielectric of the drilled hole wall. The hole must not be filled with any sort of a hole fill in order to see HWPA. There are two distinct types of HWPA: stress-relieving and stress-inducing. In stress- relieving HWPA, the condition appears to dis- tress the PTH, allowing it to survive hundreds or thousands of thermal cycles without failure. In stress-inducing HWPA, the stress appears to greatly increase, causing the PTH to fail in just a few thermal cycles. What we consider a failure is an increase greater than 10% in the overall resistance in the circuit. A crack that partially bridges the copper at the internal interface is enough to cause a failure. This column is based on my experience in test reliability of interconnect stress test (IST) coupons. I am addressing HWPA that has moderate to severe outgassing. There may be HWPA due to thermal stressing of the board without any significant outgassing, but this type of HWPA is subtle, and it presents as a dark line between the plating and the dielec - tric of the hole wall. This type of HWPA is rare- ly detected. Stress-Relieving HWPA Stress-relieving is the most common type of HWPA. It appears that the adhesion of copper plating to the dielectric is reduced most likely due to problems with the application of electro- less copper plating adhering to the dielectric of the hole wall. At the same time, the adhesion is strong at the copper's internal interconnection. In fact, experience suggests that the adhesion of the electroless copper is stronger than the cop- per plating. This process frequently produces strong interconnections to copper inner layers. feature column by Paul Reid ConsulTAnT REID ON RELIABILITy Failure Mode: Hole Wall Pullaway

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