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66 The PCB Magazine • December 2016 fold is evident due to the wedge creation. Thus, even if this finished circuit board passes in cir- cuit test (ICT), there exists the real possibil- ity that the thin plated copper will fail during thermal cycling or while in service. Secondly, the thin copper in the fold area will often rup- ture during wave soldering, allowing gas to es- cape. This in turn will lead to blowholes. Another defect that is drilling induced is "plowing." This de- fect is characterized by furrows or grooves in the hole wall (Figure 2). Plowing is mainly caused by worn-out drill bit cutting edges, undercured laminate and exces- sive spindle speeds. Spindle speed is measured in RPMs. Therefore, check the spindle speed for that particular hole size and board thickness and adjust accordingly. The higher the RPM rate and the longer the drill bit stays in the hole, the more heat is generated. So, resin smear will also be an is- sue. If undercured resin material is the root cause, this issue resides within the lamination cycle, age of the pre-preg, moisture content and final cure temperatures. Some Things to be Aware of While engineers mostly focus the attention on drilling param- eters such as feeds and speeds, there are other less obvious criteria that when not understood or maintained, will lead to poor hole wall drill quality. One such issue is spindle run-out. Run- out or runout is an inaccuracy of the drill spin- dle. In this case, the drill tool or shaft does not rotate exactly in line with the main axis. For example, when drilling, run-out will result in VIA FORMATION AND MECHANICAL DRILLING, PART 2 Table 1. Figure 2: Close-up of plowing. (Source: IPC 9121.)

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