Issue link: https://iconnect007.uberflip.com/i/1221561
MARCH 2020 I PCB007 MAGAZINE 103 Drillability Drillability is an important consideration in the selection of a laminate for a backplane, particularly with high PTH aspect ratios and the increase in backdrilling. Constant diame- ter and clean finish are essential. The grade of glass fiber, fillers, and dielectric material hard- ness all contribute to the hole quality. Ceramic-filled materials can also introduce extra cost in fabrications. The ceramic can re- duce the lifetime of a drill bit from 5000 to 500 hits. There are concerns with hole wall quality as fabricators move up the material technology curve—most notably rough drilled holes along with deep gouges (Figure 1). Drill quality—or lack thereof, as shown in Figure 1—affects several aspects, including solderability, plating adhesion, and potentially component insertion. Regardless, higher-per- formance materials are more difficult to drill. Hole wall roughness must be reckoned with. CAF Formation CAF is caused by the separation or lack of bonding between the woven glass and the res- in material. Essentially, the separation causes a hollow tube, so to speak, that allows moisture to provide sufficient conductivity under bias to form an actual copper filament. This filament can lead to an electrical short. The hollow fiber shown in Figure 2 is the typical medium leading the conductive fila- ment. The resin can separate from the glass due to the glass-coupling chemistry, resin moisture absorption, drill fractures, or the de- gree of effectiveness of the prepreg resin coat- ing (no bare fiber ends should be exposed). Then, the pathway provides the ideal situation for the growth of the filament (Figure 3). Recently, resin suppliers and laminators have made great strides in the understanding of CAF and even better methods to detect the issue. In addition, IPC-4101B helps to address the concerns with resin materials and the pro- pensity to lead to CAF. Most datasheets now state whether materials are CAF-resistant. Even with the development of CAF-resistant materials, higher-density circuit board de- signs, higher aspect ratio vias, as well as al- ternative glass styles, one should not assume that CAF will not occur. Always perform due diligence. PCB007 Michael Carano is VP of technology and business development for RBP Chemical Technology. To read past columns or contact Carano, click here. Figure 2: Example of the formation of hollow fiber. (Source: IPC TM-2.6.25A User Guide). Figure 3: Arrow pointing to copper filament working its way to the internal layer to cause a short.