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Automated Drill Bit Resharpening
February 5, 2013 |Estimated reading time: 4 minutes
Editor's Note: This article originally appeared in the November 2012 issue of The PCB Magazine.
When it comes to drill bits for the PCB industry, the drill point and its sharpness have a dramatic effect on the final yield of the completed board. It has been estimated that about 25% of the production cost of the PCB is dependent on the performance of the drilling process.
We know that PCBs are a composite material consisting of glass fiber, resins and ductile material such as copper foil. All of these elements can work against each other, complicating the cutting process. In any case, we know that hole-wall quality is vital to the success of plating, and optimal plating transfers the electrical properties throughout the entire board.
The fact is that the plated hole begins with the drilled hole and there is no tolerance for a poorly drilled hole.
The Repointing Process
Ultrasonic Cleaning, Rinse, and Dry
After drill bits have been used by the PCB manufacturer, they are sent out for resharpening. Typically, drill bits arrive in drill boxes, mixed together in varying sizes and stages of previous repoints. It is therefore important to sort the bits to achieve the best uniformity. Bits also arrive covered with various types of debris, including dirt, grime, metal turnings, melted resin and other unwanted contaminates.
Figure 1: Before ultrasonic cleaning.
Figure 2: After ultrasonic cleaning.
Figure 3: Further example of impregnated resins.
Drill bits are first sent to the ultrasonic cleaning station, the first of three cleaning processes to occur during the entire sharpening exercise. The specific time in the ultrasonic cleaning bath is carefully controlled, then rinsed and dried. Further cleaning occurs to remove carbide dust, which accumulates during sharpening. Drill bits with impregnated debris along the margin can cause clogged holes, smear, nailheading, burrs, drill breakage, and misregistration, among other problems. During the re-sharpening, ringsetting and AOI processes, every drill bit is cleaned two more times.
Understanding the Technical Challenge
Throughout the years, drill bit engineers have developed comprehensive ways to identify and document defects while refining procedures to achieve the “perfect point,” defined as drill bit dimensions meeting the minimum criteria of measurements for its class and size as defined by the parameters in Figures 4 and 5.
Figure 4: Geometry illustration, top view.
Figure 5: Geometry illustration, side view.
There are a host of specifications and tolerances for industry acceptance of drill bits.
Here is one example of specifications for “GAP” tolerances.
Figure 6: Specification allowances.
As well as our GAP example, a total of eight specific industry classifications are used to define drill defects: Chips, flair, negative, overlap, gap, offset, hook, and layback/taper.
When we see actual microscopic pictures of bits ready for resharpening, the damages are complex and varied. Note some of the wear and tear in Figures 7, 8, and 9.
Figure 7: Primary cutting edge chips.
Figure 8: Excessive gap.
Figure 9: Excessive wear.
With our goal to create a perfect point, the automatic equipment is designed to address all of these defects during repointing.
Figure 10 shows the automatic system with each of its component stations.
Figure 10: Fully automated sharpening stations.
The Setup
The sharpening process begins with the selection of bits relative to their size. Drill bits smaller than 0.0118” are sorted for the fully automatic equipment. Typically the smallest (most fragile) bits are sized down to 0.006”. To avoid breakage and to obtain a perfect point, the use of the automatic process, including resharpening, ringsetting and AOI, makes this ideal for small drills. Drill bits greater than 1/8” are sorted for the semi-automated sharpening machines.
Automated Sharpening Process
A fully automatic robotic machine combines all operations (sharpening, ringsetting and AOI) in one piece of equipment. These hands-off procedures are designed to address the industry’s previously defined eight classifications for the perfect point.
AOI Reporting
Today’s sub-miniature PCB drilling bits have become impossible to inspect by eye, and even using a microscope is a tedious task to review every tool specification. A computer, with its AOI capability, is the ideal device to perform analysis of the completed task. The AOI operation is particularly extensive, with checks against 13 potential defect areas at drill point, including the key eight defined industry classifications: diameter, overlap, gap, flair, taper, thickness, width, length, offset, chamfer, layback, hook, and chip. Figures 11 and 12 show the reporting format and each is accompanied with a clear monitor display of the area. Hard copy reports/pictures are available for each occurrence.
Figure 11: Thirteen-point AOI inspection with picture of each bit.
Figure 12: Thirteen-point AOI inspection with sigma and CPK.
Figure 13: Finished drills.
Similarly, Sigma and CPK values are made available in graphic form, fully describing the incidence of defect--an excellent way to determine corrective action required in key areas.
Edson Bosetti is the general manager of the Perfect Point division of Matrix Electronics. Perfect Point is one of the leading drill bit resharpening services in the U.S. and Canada, as well as a distributor of new drills and routers for the PCB industry.