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PCBD-Aug2015

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26 The PCB Design Magazine • August 2015 This condition may result in a hole wall that looks like a stack of forward or backward "Ds" running the length of the hole where the top and the bottom of the "Ds" is at a internal in- terconnect (Figure 1). I have tested holes with HWPA that have survived for thousands of thermal cycles tested from ambient to 150°C. It appears that 99% of the time this HWPA stress relieves the copper plating in the PTH, making the hole more ro- bust to thermal cycling types of failures. Now, when I say that HWPA stress relieves the copper plating in the PTH, thus making the hole more robust to thermal cycling types of failures, what I mean is this: When the sam- ples are tested and the thermal cycles to failure are compared, coupons with HWPA typically outperform coupons without HWPA. This is with coupons that are from the same process that happen to have HWPA vs. those without HWPA. It appears that the coupons without HWPA have wear-out failures, while those with HWPA don't succumb to wear-out type of fail- ures. What I think happens is that the majority of the wear-out failures propagate from a glass fiber. The crack forms slowly, starting from a glass fiber, and works its way across the copper of the barrel, with each successive thermal cy- cle. With HWPA we do not have the glass fibers constrained in the copper and the hole wall is free to expand and contract without encum- brances, and because of this no wear-out type of failures ensue. Stress-Inducing HWPA We rarely see HWPA cause an electrical fail- ure, but on occasion this condition may cause a catastrophic failure. I call this condition stress- inducing HWPA. If it is severe enough the HWPA may put enough stress on the interconnect to cause it to pull away from the copper plating in the hole. The result is a break between the in- ternal interconnection and plating in the hole. A crack develops between the internal intercon- nection and the copper plating that widens in to an open. On this rare occasion, the bond is weak between the internal interconnection and the copper plating of the hole wall, or there is extreme out-gassing that causes the internal in- terface to crack. When HWPA is severe, it usu- ally goes open within 100 thermal cycles. Note that in the graphic of stress-inducing HWPA the copper in the hole wall is not de- formed uniformly. In hole No. 1, the pullaway is from the wall on the left side of the hole and bridges the gap so that it is pressed against the right side of the hole. In hole No. 2, the defor- mation is equal from both sides and in hole No. 3, the right side of the hole is deformed to touch the left side of the hole. The question becomes, what is going on in this section? To understand what was happening, we decided to do a hori- zontal cross-section. What we found was surprising. The hole walls were deformed into three or four cusps. It appeared that the three or four cusps roughly aligned with the wrap or weft of the glass bun- FAILURE MODE: HOLE WALL PULLAWAy continues Figure 1: Graphic of stress-relieving HWPA. Figure 2: Graphic of stress-inducing HWPA. reid on reliability

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