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April 2014 • The PCB Magazine 77 aptly dubbed the "wall of shame." But nonethe- less, this condition is a possibility. Spindle Speeds This parameter refers to the rotational rate of the drill spindle measured in revolutions per minute (RPM). The rotational rate of the drill spindle impacts hole-wall quality in a number of ways. First, excessive speeds will generate more heat, resulting in excessive smear forma- tion. In addition, exceeding the recommended spindle speeds will cause the cutting edge of the drill bit to wear prematurely. Cutting edge wear or rounding as it is often called, results in the drill bit punching its way through the via. Cer- tainly, the punching condition results in rough hole-walls, torn-out glass bundles and broken drill tools. Surface Feet per Minute, or per Meter The surface feet per minute, or meter param- eter (SFM), is directly related to spindle speeds. Basically, SFM is the distance a point on the drill bit traverses over a period of time. To calculate SFM, use the following equations: Cutting Speed (SFM) = spindle speed (RPM) × π × diameter (in)/12. Cutting Speed (m/min) = spindle speed (RPM) × π × diameter (mm)/1000. PTH DRILLING REVISITED: FUNDAMENTALS, PART 1 continues What Material are we Drilling Through? Why is this significant? Well, the longer the drill bit stays in the hole, the greater the oppor- tunity to generate more heat. And with more heat comes resin smear. Obviously, minimizing smear is a top priority. It is also important to re- alize that one SFM parameter will not apply to all material sets. As an example and from work- ing with numerous PCB fabricators in trouble- shooting situations, I have set up a few SFM rules (to serve as a starting point guideline). Again, these are only starting point suggestions and may require adjustments based on layer counts, copper thicknesses, etc. In all cases un- less otherwise noted, chip load varies with drill bit diameter. So as a starting point: • Standard FR-4 epoxy (135–140 T g ) 470–505 SFM • Epoxy-higher T g (170–180 T g ) 460–500 SFM • Polyimide 390–495 SFM • Epoxy PPO/IPN 375–400 SFM Ideally, these are guidelines only, but should serve as a good starting point when optimizing the drilling operation. Conclusion Good quality plated through-holes must first start with a high-quality drilled hole. Un- derstanding that drill feeds and speeds, surface feet per minute and the actual material drilled through will influence the overall drilled hole quality. Being able to recognize drill defects early will enable the engineer to properly adjust the key parameters in the operation and ensure a high quality drilled via. PCB michael carano is with omg electronic chemicals, a devel- oper and provider of processes and materials for the electronics industry supply chain. to read past columns, or to contact the author, click here. Figure 3: excessive nailheading (a.k.a., the wall of shame).

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