Issue link: https://iconnect007.uberflip.com/i/1536435
JUNE 2025 I PCB007 MAGAZINE 35 assume that's not the case here. Note the areas of thin copper plating, small voids, and rough hole walls. All these issues will con- tribute to the potential for the occurrence of blow holes. While it is possible that thin cop- per-plated deposits and voids can be attrib- uted to the plating processes, these issues can also be directly related to poor hole drill- ing conditions. at is why the adage "Gar- bage in, garbage out" applies here. One can- not expect plating chemistries to defy phys- ics and Faraday's Law and always make up for poor hole wall conditions (Figure 2). Drilling and Basic Definitions Drilling conditions dep-end 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 poten- tial for 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: e speed of a drill bit toward and through the circuit board stack to be drilled. e feed rate is measured in meters per minute (m/minutes) or inches per minute (IPM). It's one of the key parame- ters of chip load. • Spindle speed: e rotational rate (RPM) of the drill spindle. It is the other key parameter of chip load. • Chip load: e depth or distance through a stack that a drill bit travels for each revo- lution it turns. Chip load is typically mea- sured in mils or inches per revolution (IPR), or micrometer per revolution (μm/ rev). To calculate chip load, use the following 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 accuracy. On the negative side, low feed rates will cause excessive heat, resulting in resin smear and nail heading (of the inner layer copper). Low feed rates may also cause glass fiber damage, resulting in wicking at plating or fiber removal. On the positive side, high feed rates will reduce resin smear. On the negative side, high feed rates create burrs, hole wall roughness, and debris in the holes. According to standard drilling guidelines, smaller-diameter holes and thicker boards require lower chip loads. In a future column, I will present more in-depth information on drilling and its ramifications for PTH quality and reliability. Materials to be Drilled One must also consider the materials used to construct the circuit board. e drill bits eventually wear out, leading to poorly drilled vias. Some higher-performance materials, such as ceramic-filled, lead to faster drill bit Figure 3: Large air pockets show where gas or moisture was expelled during the assembly process.