PCB007 Magazine

PCB-Nov2016

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46 The PCB Magazine • November 2016 Introduction In the extensive process of printed circuit board fabrication, one of the steps involves mechanically drilling through-hole vias. Via formation is then followed by desmear and metallization. The quality of the through-hole drilling process (and by inference the quality of the drilled through-hole) or lack thereof will also impact the desmear and metalization processes. While drilling equipment, tools and methods have been continually improved over the years, the fabricator must adhere to strict guidelines with respect to maintenance of the equipment, quality of the drill bits, the drill - ing operation parameters as well as the proper selection and use of entry and back-up materi- als. When simple maintenance of drill equip- ment and drill bits is lacking, drilled hole wall quality deteriorates. This then leads to defects in the PTH that are not attributable to the plating processes such as electroless copper . However, for this month's edition of "Trouble in Your Tank," I will present some of the basics of drilling. Drilling Basics Through-holes are formed by the cutting ac- tion of drill bits in numerically-controlled drill- ing machines. Figure 1 shows a five-head drill- ing machine. There are multiple factors to consider when mechanically drilling plated through-holes as well as blind and buried vias (assuming me- chanical drilling will be used on a few applica- tions for blind/buried vias). These include the drill machine, drill bits, entry and back-up ma- terials, drilling parameters, stack heights, etc. In addition, the type of resin that makes up the circuit board, along with glass weave style and content, will play a role in via drilling quality. This will be the subject of a future column of "Trouble in Your Tank." Figure 2 depicts what can happen when the drilling operation is not in control. Note the gouges in the resin and the torn out glass bun- dles. When a drilled hole such as this makes its way to the PTH (plated through-hole) process, the ability of the electroless copper or direct metallization chemistries to properly deposit in by Michael Carano RBP CHEMICAL TECHNOLOGY Via Formation and Drilling Mechanics, Part 1 FEATURE COLUMN: TROUBLE IN YOUR TANK Figure 1: Five-head drilling machine. Figure 2: Rough hole walls due to less than opti- mum drilling (Source: IPC photo archive).

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