Mechanically drilled through holes in PWBs form interconnects between copper layers through the use of copper pads. These pads are larger than the drill diameter to account for such factors as layer-to-layer misregistration during lamination, drill misregistration, and drill wander. The requirements for the pad's remaining annular ring after drilling are specified in IPC-6012, Qualification and Performance Specification for Rigid Printed Boards. For Performance Class 3 (High Reliability Electronic Products), these requirements are 0.001" minimum annular ring for internal layers, and 0.002" for external layers. Performance Classes 2 and 1 allow 90° and 180° breakout of the hole from the pad, respectively. This leads to the conclusion that breakout or tangency of the hole to the pad are less reliable than complete annular rings. Testing was performed to determine the reliability of plated through holes that met Class 2 requirements compared to holes that met Class 3 requirements. Interconnect Stress Testing (IST) coupons were used as test vehicles, with drill location offsets applied to some coupons to force tangency or breakout (Class 2). In addition, two pad sizes were used, one representing standard density designs (larger pads) and the other representing higher density designs (smaller pads). Results from 1ST showed that the higher density design practice had equivalent or higher reliability, even with tangency/breakout. Finite Element Analysis (FEA) is used to validate these results.
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