Formatting Copperclad Substrates
Formatting refers to the process of assembling the copperclad substrates with backing and entry materials into a drilling stack and marking the entire stack in such a way that the orientation or each substrate is uniquely determined. In the case of a multilayer board, the layers must be stacked in the same orientation and order that they will occupy in the finished board. You must also determine beforehand whether your design will require blind or hidden vias, and establish a plan to drill these added holes in the proper layers. If you plan to make a number of multilayer PCBs, building a formatting template will save you a lot of work and help insure consistent results.
To some degree, the steps that you follow when stacking the substrates will depend on whether you will be drilling manually or have the use of an automated drilling machine.
For the sake of simplifying this discussion, the component side of the finished PCB will be referred to as the top and the solder side will be referred to as the bottom. The process for formatting and drilling a double-sided board is identical to the treatment of the inner layer.
To get your layers ready for drilling:
- Assemble the copperclad substrates into a stack wherein each substrate is in the same position and orientation (copper side up or copper side down) as it will be in the final PCB.
- On the bottom of the stack, add a sheet of exit foil and piece of backing material (min. thickness = 0.08"). To the top, add a sheet of entry foil. Starting from the top, the stack should consist of: entry foil / top laminate / inner laminate / bottom laminate / exit foil / backing material
- Backing material and entry foil, when used in conjunction with a pressure foot during drilling will virtually eliminate the formation of burrs on the top and bottom of the substrate. This is no minor consideration since copper is very ductile and tends to form long, difficult to remove cylindrical burrs around each hole. Entry foil (which is the same thing as exit foil) is also used on both sides of the stack to minimize cleanup after hole-wall activation. Tape the stack together by using four short pieces of masking tape angled across each corner.
- In the middle of one side, about 0.25" (6.4mm) in from the edge, drill a 0.125" (3mm) dowell hole through the entire stack and insert a dowell pin until it is flush with the bottom of the backing material.
- In the opposing side, offest from the middle of the side by at least 1" (3cm) and inset about 0.25" (6.4mm) in from the edge, drill another 0.125" (3mm) dowell hole through the stack and insert a dowell pin as before.
- Make sure that both dowell holes are outside of the area to be occupied by your circuit pattern.
- The reason for offsetting the second hole is to establish enough left to right asymmetry to insure that the stack can only be reassembled with the various layers in their original orientation. You could also do this visually by notching the same corner on each layer, but it is a lot easier to drill a hole off center than it is to cut or file FR-4.
- If you will be using an automated drilling machine, the the stack is now ready for positioning on the drill table.
Mounting a drillmaster
If you intend to manually drill the stack, there is a further step that can significantly simplify the entire operation and give you a reliable visual check to make sure that all of the needed holes have been drilled.
- Using adhesive label stock (e.g. "laser" labels or reprographic transfer film comprised of a paper or plastic printing surface, a layer of pressure sensitive adhesive,and a siliconized release liner), instruct your ECAD system to print or plot a 1 to 1 (100% scale) drillmaster.
- It is very important that you determine the scales of both axes of your printer or plotter before printing the drillmaster (or subsequent artwork). These numbers should be used during the configuration of your ECAD software printer driver to insure that all printed / plotted output conforms to real world measurements; i.e. that a plotted inch is really equal to an inch plus-or-minus an acceptable tolerance (0.005" or 0.1mm max.)
- Properly configured, a drill master will plot out the location of every hole in your board, using a unique symbol to represent each drill size. As such, it forms an excellent roadmap for manual drilling.
- Peel off the release liner on the back of the adhesive and carefully apply the drillmaster to the entry foil on the top of your stack. Make sure that the "label" goes on without wrinkles or voids. Once the drillmaster is in place to your satisfaction, you are ready to manually drill the stack.
Remember that, if you are making a design that requires buried or blind vias, you will need to reserve unique symbols for each unique layer. As a matter of practicality, drill the through-holes first. It is then safe to pull the stack apart and assemble a substack containing the backing material, the layers that need further drilling, and the entry foil with the mounted drillmaster.
Note: If your design is small enough, or your label stock and printer format big enough, you can include the dowell holes drilled above in the drillmaster and mount it to the entry foil before assembling the stack. This has the advantage of locating the dowell holes with your PCB layout software where you can insure that the holes do not conflict with any circuit element or mechanical mounting holes.
included by permission of Think & Tinker, Ltd copyright 1986 to 2012