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HIGH-ASPECT-RATIO DRILLING
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As circuits have gotten denser, not only have the traces gotten smaller, but the number of layers in a board has increased. The result is smaller holes penetrating thicker boards. To accommodate this increase in hole aspect ratio, two processes have been developed:
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Peck drilling Pulse drilling
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PRECISION INTERCONNECT DRILLING
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Peck Drilling Peck drilling is accomplished by dividing the total z-stroke into separate increments rather than completing the drill stroke in one action (see the example shown in Fig. 25.5). 25.6.1.1 Advantages. The advantages of peck drilling are as follows: Decreased drill bit breakage Lower aspect-ratio values Improved positional accuracy Decreased bottom-panel burring
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Peck Drilling 1 Peck Aspect Ratio: 15.3 4 Peck Aspect Ratio: 3.8/Peck Top of Stack Zero Reference
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.012" Entry Peck Retract .020" 1 .052" .060" Product Product Thickness
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3 .154" .060" Product 4 .014" .206"
.285"
.093" Backup
FIGURE 25.5 An example of through-hole peck drilling, showing a stack of three boards with entry and backup material creating a stack height of .285 in. and a total z-axis drill stroke of .206 in. The effective aspect ratio of 15.3 is reduced to 3.8 per peck when four pecks are made to complete the hole.
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25.6.1.2 Effective Aspect-Ratio Reduction. The primary accomplishment of peck drilling is the decrease in effective aspect ratio value. Whereas aspect ratio is defined as the thickness of the board divided by the hole diameter, effective aspect ratio is defined as the total z-stroke divided by the smallest drill bit diameter. For example, if the total z-stroke equals 0.209 in. (the total thickness of the board) and the smallest diameter is 0.025 in., the aspect ratio and the effective aspect ratio would have the same value of 8.36. By taking a total z-stroke of 0.209 in. and dividing it into four pecks, the aspect ratio is decreased from 8.36 to 2.08. In general terms, an aspect ratio scale of 1 through 15 is commonly used, 1 being conservative and 15 being on the aggressive side of the scale. 25.6.1.3 Decision to Use Peck Drilling. The decision as to when to use peck drilling is very subjective. Drill bit breakage and bottom-panel burring are ordinarily the main factors considered when electing to peck drill:
Decreased drill bit breakage Decreased drill bit breakage is the primary benefit of peck drilling. If the z-stroke is reduced, the drill bit has less debris to extract per revolution, decreasing the chances of clogged flutes, which can cause the drill bit to seize in the hole and break. Setting pecking parameters is a very subjective science; at best, it is an approximation. Hole wall quality Analysis of the hole wall quality results should be completed before implementing any pecking procedures. Hole location improvement Another benefit of peck drilling is an improvement in hole location. Once a drill bit is deflected, it will continue at that deflection angle as it drills through the stack. Essentially, hole location is worse at the exit point than at the entrance point, particularly when drilling through thick panels with small-diameter tools (highaspect-ratio drilling). Peck drilling can assist in achieving better hole location by lessening the effects of deflection by dividing up the total drill stroke, therefore decreasing the effective aspect ratio value. Bottom-panel burring Peck drilling can assist in decreasing the amount of bottom-panel burring by reducing the drilling temperatures resulting from drilling thick panels with small drill bits. Plowing, voids, and debris pack are reduced by peck drilling due to the decrease in the amount of debris the tool must extract per z-axis stroke. Drill bit cooling When peck drilling, each individual stroke increment returns to the upper limit value. For the example in Fig. 25.5, a value of 0.015 in. above the stack was used.
Figure 25.5 is an example of through-hole peck drilling, showing a stack of three boards with entry and exit material creating a total stack height (or z-axis stroke) of 0.285 in. The effective aspect ratio is reduced when four pecks of the drill are made to complete the hole. Increasing the distance above the stack (sometimes referred to as the second upper limit value) assists in cooling the drill bit before completing the next z-stroke. By cooling the drill bit, the manufacturer can substantially reduce nail-heading values and smearing. 25.6.1.4 Disadvantages. following:
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