Multilayer PCB Prototyping

1. Overview
2. Getting ready
3. Formatting
4. Drilling
5. Hole wall activation
6. Board cleaning
7. Through-hole Plating
8. Photoresist lamination
9. Image inner layer
10. Etch inner layer
11. Laminate multilayer board

Overview

Getting ready

equipment needed

1. high-speed drill (or a good drill press with min. 3500 RPM capability)
2. Some board shops will drill boards for much less than they charge to fabricate a finished PCB. Call around, it might be worth your while. Be sure, however, that you clearly understand all of the costs involved. Ask if there are things you can do to the design and/or the CAM file that will lower the cost to you.
3. If you will be hand drilling your substrates, press the button {hyperlink to hand drilling/formatting, on-line access only}
4. electrolytic copper plating cell
5. dry-film laminator or photopolymer compatible silkscreen
6. ultraviolet exposure unit
7. photopolymer developer
8. copper etcher
9. laminating press

before you start

1. If possible, panelize your design so that you get more than one board out of your panel. If this is your first multilayer board, choose a small, simple design that will allow you to make at least two individual boards. That way, if one is defective, you might still end up with one good one. As you gain experience, this is less of a consideration but, making more than one copy of your design is always a wise practice.
2. Select the substrates for the various layers based on the impedance and signal isolation requirements of your design and the physical limits placed on the total thickness of the laminated panel.
3. For a four layer design with a finished thickness of 0.063" (1.6mm), copperclad with 0.0007" (17) copper foil on both sides (so called half ounce/half ounce or "half over half" copperclad) and a substrate thickness of 0.025" (.64mm) is a good selection for the innermost PCB (designated .5/.5 0.025" FR4).
4. The outer layers will be constructed from copperclad with half ounce foil on one side and a substrate thickness of 0.017" (0.43mm). The substrates used in these outer layers are generally comprised of laminae of glass fabric held together with a partially cured resin that will remelt during multilayer lamination. Remelting causes the resin to become VERY tacky and adhere to the inner layers. This is the bonding mechanism that lies at the heart of multilayer fabrication. Copperclad based on these partially cured resins is referred to as "B stage" laminate. B stage is also now available with a polyester release liner lightly adhered to the resin side to protect the bonding surface during all pre-lamination processing.
5. The thickness of substrates used in PCBs with a higher layer count should be scaled accordingly (if the finished thickness is constrained to some preset value). One of the real advantages of the following technique is that, assuming that you can drill all of the layers simultaneously, in a single stack, you can pretty much stack as many layers as you want.
6. Set aside all of the drill bits that you will need, with spares if possible. This is especially helpful if you will be manually drilling the board. Hand drilling is hard enough without having to interrupt the work flow to go look for a drill bit. There is a fairly substantial body of scientific evidence that states that many of the most notorious mass murderers in history started out as mild mannered PCB drillers.
7. You will need at least 2 ea. 0.125"(or 3mm) dowell pins. If these are unavailable, broken 0.125" (3mm) shank carbide PCB drills will serve admirably. Make sure that you grind down any sharp edges to avoid cutting yourself and your counter tops during drilling.