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48 The PCB Magazine • November 2016 these drill gouges is compromised. Even if the PTH metallization is successful, subsequent elec- trodeposition of copper will yield areas where the copper thickness is either too thin (thus violating copper thickness requirements) or potentially having a situation where the plated copper folds over. In the latter case, the copper cannot level due to the drill gouge. This leaves a very thin area and leads to the potential for a blowhole in assembly or an open in the PTH. This is precisely why many finished bare boards successfully pass in-circuit test—only to exhibit some interrelated non-conforming defect after assembly (Figure 3). Certainly, it is possible that there may be issues with the wave solder flux. But for pur- poses of this discussion, assume flux is not contributing to the issue. Note the areas of thin copper plating, small voids and rough hole walls. All of these issues will contribute to the potential for the occurrence of blow holes. While it is possible that thin copper plated de- posits and voids can be attributed to the plat- ing processes, these issues can also be directly related to poor hole drilling conditions. That is why the old adage "Garbage in, garbage out" applies here. One cannot expect plating chem - istries to defy physics and Faraday's Law and always make up for poor hole wall conditions as shown in Figure 2. Drilling and Basic Definitions Drilling conditions depend on the drilling machine, brand of drill bit, drill bit design, type of spindle, stack height, PCB material, amount of copper to be drilled, drill aspect ratio, entry and backup materials, etc. Hole wall quality, drilling accuracy, and the potential of drill bit breakage are results of the combined effect of all drilling parameters. Let's review a few very key drilling param- eters and their definitions: Feed Rate: The speed of a drill bit toward and through the circuit board stack to be drilled. Feed rate is measured in meters per minute (m/ minutes) or inches per minute (IPM). One of the key parameters of chip load. Spindle Speed: RPM or rotational rate of the drill spindle. The other key parameter of chip load. Chip Load: The depth or distance through a stack that a drill bit travels for each revolution it turns. Chip load is typically measured in mils or inches per revolution (IPR), or micrometer per revolution (µm/rev). In order to calculate chip load, use the fol- lowing equation: Chip Load (IPR) = Feed Rate (IPM) Spindle Speed (RPM) So, chip load in inches per minute is deter- mined by feed rate of the drill spindle divided by the rotational rate of the spindle. One may also calculate chip load in mils of feed divided by the RPM of the spindle. Variation in the feed rate and chip loads affects hole wall quality. On the positive side, low feed rates will improve hole positional ac- curacy. On the negative side, low feed rates will cause excessive heat, resulting in smear and nail heading. Low feed rates may also cause glass fi- ber damage resulting in wicking of copper plat- ing chemistries. On the positive side, high feed VIA FORMATION AND DRILLING MECHANICS, PART 1 Figure 3: Large air pockets show where gas or moisture was expelled during the assembly process.

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