Drilling Copperclad Substrates
The techniques for drilling copperclad for double-sided and multilayer PCBs with automated equipment are identical, with the exception that multiple drilling steps will be needed if your multilayer design includes buried or blind vias. Refer to the documentation that came with your drilling machine for more information (standard boilerplate cop-out).
Items to remember include:
- set the STACK HEIGHT parameter to clear all dowel pins during traverse
- set the SPINDLE FEED (inches per minute) and SPINDLE SPEED (RPM) for each drill size to values consistent with drilling standard 0.062" (1.6mm) FR-4 copperclad.
- set the SPINDLE PLUNGE DEPTH so that the tip of the largest diameter drill bit fully enters the backing material. Otherwise, these large diameter holes will not totally penetrate to bottom laminate and exit foil.
- DO NOT contour route the board immediately after drilling the stack. This should only be done after all other processing is complete.
With the laminate stack formatted as detailed above, manual drilling is a straightforward, if somewhat mind-numbing process. Items to consider include:
- ALWAYS WEAR SAFETY GLASSES WHEN OPERATING A DRILL PRESS, ESPECIALLY IF YOU ARE DRILLING WITH CARBIDE PCB DRILL BITS.
- IF AVAILABLE, ALWAYS USE A VACUUM CLEANER TO REMOVE DEBRIS AND COLLECT AIRBORNE DUST DURING THE DRILLING OPERATION. THE DUST GENERATED DURING PCB DRILLING CAN POSE A VERY SERIOUS HEALTH HAZARD AND SHOULD NOT BE INHALED OR INGESTED UNDER ANY CIRCUMSTANCES.
- When using a conventional drill press, hole placement accuracy can be improved and drill breakage minimized through the use of a "sensitive drilling" or "finger" chuck. Small format, precision high-speed drill presses, ideal for PCB fabrication, are also available from a number of sources.
- Regardless of the type of drill press being used, a pressure foot should be employed if available.
- If available, position a work lamp on a flexible mount as close to the work surface as possible.
- Although more brittle than conventional high speed steel (HSS) drills, tungsten carbide bits designed specifically for PCB drilling will yield far superior hole wall quality. minimize burr formation, and outlast HSS bits almost 10 to 1. The downside is that, with smaller diameters [0.018"(0.46mm) and less], the carbide drills are easier to break and must be handled carefully.
- Always use drill bits that have been fitted with depth setting rings. This will allow you to set the plunge depth stop on your drill press to a single value that will work for all bit diameters.
- Prepare a chart that links the various diameter bits with the symbols used in the drillmaster.
- Load the largest diameter bit to be used into the drill chuck, making sure that the depth ring is pressed firmly against the ends of the chuck jaws when they are fully tightened.
- Using a piece of scrap backing material as a gauge, adjust the spindle travel stop on your drill press to a depth that insures that the entire tip of the drill bit penetrates at least half of the material's thickness. You can also use two pieces of entry foil as a "feeler gauge" to set the depth. Under no circumstances allow a PCB drill bit to drill into the table of your drill press. PCB bits are specifically designed to drill copperclad and will shatter if plunged into cast iron, steel, or Aluminium.
- Starting with the largest diameter drill bit, drill all of the through holes, stopping periodically to insure that the drill bit has not snapped off and that the spindle travel stop has not slipped.
- As you drill each hole size (from the largest to the smallest) check off that diameter on the drilling chart. This is a good bookkeeping technique that will help you keep track of your progress and insure that no hole size is missed.
- After all of the holes have been drilled, remove the backing material from the stack and reptape the remaining sheets with the dowel pins in place.
- Hold the stack up to the light for visual inspection. Ascertain that all of the holes have been drilled through and that none are blocked by drill debris. If some debris is seen, remove by carefully pushing a smaller diameter drill bit through the hole.
- If all of the holes in your circuit design go all the way through the board, you are now ready to activate the hole walls to prepare for through-hole plating.
blind or buried vias
Designs that use blind or buried vias (vias that do not penetrate through the PCB) need supplementary drilling operations before proceeding. Unfortunately, they are also quite a bit more difficult to activate and through plate since each must be processed singly.
- Fully disassemble the drilled stack.
- Reassemble a substack consisting of the backing sheet, one of the copperclad substrates that need additional drilling, and the entry foil that carries the drillmaster.
- Re-pin with the dowels and tape as before.
- Playing close attention to the drillmaster symbols representing the holes needed by the included substrate, drill the substack.
- Disassemble the substack and repeat steps 2 through 4 for each layer that needs further drilling.
- Inspect each layer after it is drilled and remove any debris that might be blocking the holes.
- If all of the holes are drilled to your satisfaction, the individual layers are now ready for activation .
included by permission of Think & Tinker, Ltd copyright 1986 to 2012