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PCB-Aug2016

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August 2016 • The PCB Magazine 79 the significance of the defect and the genesis of a plating void. One often overlooked cause of wedge voids is the drilling operation. During the drilling op- eration, vibration of the drill bit can create mi- crofractures in the copper/dielectric interface. Of course, dull drill bits, excessive chip loads, extremely high heat generation during drill- ing and poor quality entry and back-up mate- rial will increase the likelihood of a wedge void. Figure 3 shows an extreme example of a wedge void caused mainly by poor drilling practice. (Note evidence of gouging in the hole wall and turn-out glass bundles.) This type of drilling will generate significant heat leading to the open- ing of the interface between the B-stage and the copper. Copper plating subsequently is unable to "level" out the wedge leading to copper plat- ing folds. These folds typically are very thin in the plated copper and thus more susceptible to cracking from thermal stress. Keep in mind, thickness of the plated copper in the PTH does matter (Figure 4). Summary The integrated nature of printed circuit fab- rication requires that engineers recognize the influence of up and down stream processes. BUILDING RELIABILITY INTO THE PCB, PART 2 Information presented in this month's column underscores the fact. When poor drilling or lack of control of plating processes causes low cop- per plating thickness, long-term reliability is compromised. PCB References 1. Lalanne, C. (2009). Fatigue Damage. ISTE—Wiley (ISBN 978-1-84821-125-4). Michael Carano is VP of technol- ogy and business development for RBP Chemical Technology. He can be reached by clicking here. Figure 3: Wedge void (Photo courtesy of Dr. Karl Dietz, formerly with DuPont). Figure 4: Effect of PTH plating thickness on number of thermal cycles to failure for thermal cycles with the indicated peak temperatures. For acid-sulfate copper and FR-4 170 Tg boards. (Source: Printed Circuits Handbook, Clyde Coombs, 2006.)

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