Multilayer PCB Prototyping

Overview

The primary intent of fabricating a multilayer printed circuit board is to provide one more degree of freedom in the selection of suitable routing paths for complex and./or noise sensitive electronic circuits. Such considerations as layer to layer impedance aside, what we are really trying to do is stack two or more circuits on top of one another and establish a reliable set of predetermined interconnections between them. There are a number of well established techniques that are routinely used by the printed circuit industry to accomplish this. However, they all suffer from their reliance on a large number of chemical processes (as many as 15) to condition the substrate, activate the through-holes and electrolyticly plate copper between the adjacent layers. The following method reduces this dependence on "wet chemistry" to a single acid copper electroplating bath and relies on a small set of easily mastered processes to eliminate the need for much of the complexity normally encountered in multilayer board prototyping.

The technique begins with a departure from conventional processing in that all of the layers are drilled and plated before they are laminated together. The two innermost layers will comprise a conventional (if somewhat thin) two sided PCB and the various outer layers will be fabricated as separate single sided PCBs. Prior to lamination, the inner layer board will be drilled, plated through, imaged, developed, and etched. The drilled outer-layers (signal layers) will be plated through in such a way that uniform donuts of ductile copper are formed on the underside rims of the through-holes. (The underside is the side without copper foil) During lamination of the various layers into a composite multilayer, these donuts bite into pads on the surface of the opposing layer to form gas-tight, wave-solderable interconnections. Sounds pretty easy so far, eh?

Getting ready

Equipment needed

In addition to a conveniently laid out work area with good ventilation, a source of hot and cold water, and adequate UV proofed lighting, you will need:

• high-speed drill (or a good drill press with min. 3500 RPM capability)
• 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.
• If you will be hand drilling your substrates, press the button {hyperlink to hand drilling/formatting, on-line access only}
• electrolytic copper plating cell
• dry-film laminator or photopolymer compatible silkscreen
• ultraviolet exposure unit
• photopolymer developer
• copper etcher
• laminating press

Before you start

Although the techniques presented below will work equally well on most of the common materials used in PCB substrates (e.g. FR4, FR2, polyimide), the description will specifically deal with various forms of FR4, the most commonly used PCB material. Key considerations include:

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.