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76 The PCB Magazine • April 2014 materials and drill machine capabilities. But there are basic fundamentals that apply. So let's discuss the basics and illustrate what some of the undesired outcomes are when we do not follow the rules. Feeds and Speeds Feed refers to the in-feed rate of the drill bit into the drill stack—essentially this is the Z-axis penetration. The rate in-feed rate is im- portant for several reasons. If the in-feed rate is too great, there is risk to drill bit breakage and at the very least we get a very rough hole wall (Figure 1). One can easily discern the rough hole wall with gouges in the resin and glass fiber bun- dles protruding. A hole wall that looks like this makes it very difficult to ensure a plated via that is free of voids and has sufficient electroplated copper thickness to ensure that the deposit will withstand multiple thermal excursions. Of course, there are additional causes of the protruding glass fibers. In this situation, the cause can also be attributed to a slow drill tool retract rate. The retract rate is the rate at which the drill bit is withdrawn from the via. This is also known as the Z-axis return. Similar to the in-feed rate, the return rate is measured in inch- es per minute. I prefer a Z-axis return rate at 2x the rate of the in-feed penetration. Again, the reason behind the 2x retract rate is to minimize the amount of time the drill bit remains in the via. The longer the drill bit is in contact with the via, the greater the heat that is generated. This in turn increases the opportunity for drill smear (Figure 2). Another look at nail-heading in the cross section in Figure 3 brings to light another con- cern. This concern relates to some direct met- allization processes that require a micro-etch prior to the deposition of the conductive coat- ing. If there is excessive nailheading, there is a risk that the thinner portion of the flared cop- per interconnect will be etched away. This then creates an area of high resistance for subsequent electroplating of copper. Yes the cross-section is Figure 1: rough hole wall due to poor drilling practice. main cause—excessive in-feed rates of the drill tool. Figure 2: excessive drill smear and possible deformation of the innerlayer copper foil-nail heading. PTH DRILLING REVISITED: FUNDAMENTALS, PART 1 continues