What is Circuit Design

3. Process Overview

3.2. Drilling and Routing

Panels are placed in a very accurate drill/mill machine where the through-holes are drilled and the basic board shape is routed.  These machines are accurate to approximately one ten-thousandth of an inch.  A ten-thousandth of an inch is roughly 30 times smaller than a human hair.  The precision is not necessarily needed for modern printed circuit boards, but without the rigidity and precision that these machines offer, shear stresses caused by lateral vibration would cause the tiny drills to break. 

Here's an important note -- even though the machines are accurate to within a ten-thousandths of an inch, the boards will not be located inside the machines to that level of precision.  It's not uncommon to see manufacturers guarantee that holes are drilled within 3 mils of the target location for standard processes and 1 mil for extended processes. 

Hard Way Hughes"If you've ever worked in a machine shop, you'll likely know why the manufacturers only guarantee 3 mils.  Dialing something in with 3 mils over 24 inches isn't impossible, it just takes extra time.  For non-machinists, imagine dropping a thin string twenty-feet straight down from the highest point of your roof to the ground below.  A deviation of 3 mils over 2 feet becomes 30 mils over 20 feet -- in short, any deviation would likely be confined within the diameter of your test string -- you wouldn't be able to tell."

Lenz Mill Drill machine

This LENZ mill/drill machine can handle two boards at once.  The drilling and routing takes place in a single machine, saving operator time and customer money.

The smallest hole that can be drilled with these machines is around 5.9 mils. The limitation is due to the fact that these miniature drills lack the rigidity and strength to punch through copper foil layers -- they tend to bend and deflect (wander) as they encounter copper on various layers of the board.  Additionally, to reach proper cutting speed with such a small diameter, the spindle reaches around 140,000 RPM.  Smaller diameter drills would require even higher spindle speeds.  

5.9 Mil twist drills

These are Royal Circuit Solutions' smallest twist drills ~ 5.9 mils in diameter.

Drill life is calculated, tallied with each job, and each drill is monitored with machine-vision systems.  When a drill has reached the end of its useful life, it is unceremoniously discarded into a five-gallon bucket for recycling.

Twist Drills in Bucket

Used drills are not resharpened -- they are collected and recycled.

It is possible to drill smaller holes in PCBs with laser drill machines, but that will increase the cost of your PCB.