4 layer pcbs
2007-11-19 by DJ Delorie
I had this idea for home-brew four layer pcbs, and finally got around to trying it. It seems to work, too! You need a few special items: * Thinner than usual PCB stock. I got some 16 mil (1 oz) and 8 mil (0.5 oz) SS off eBay. * Sheets of double-sided tape. I got some 3M 7953 "membrane switch spacer" which is about 4 mil thick. I etched the four layers separately. For testing, I just did a grid of vias of varying sizes and drills, with thermals to the ground and power layers. The stackup looks like this: 1/2oz copper 8 mil FR4 tape 1oz copper 16 mil FR4 tape 16 mil FR4 1oz copper tape 8 mil FR4 1/2oz copper I included small vias in the corners for registration, but next time I need to be much more careful about etching and drilling those, as well as the technique for lining them up. Anyway... The theory does like this: You don't use the inner layers for signals, just power and ground. That way, you never have to connect both inner layers together. Through vias are done as usual; etch, tape up the board, drill, solder in a wire. I use wire wrap wire, which fits in a 13 mil hole. Oh - the drill "press" I built works great, I drilled a couple HUNDRED 13 mil holes on this test board, and the only bit I broke was the one I dropped on the floor. http://www.delorie.com/pcb/dremel-stand/ But with the above stack, you get blind vias almost for free! For this, drill ONLY the outer layer, and make sure your inner etch doesn't etch where you'd drill - you want a solid land under the via. The hole needs to be big enough to get your iron in there; for mine that means a 22 mil drill and 42 mil copper. You put the tape on the outer layer, leaving the other backing on, and drill out all your blind vias. Then peel off the backing and tape the layer on. Now, poke the iron into the hold and solder, and the solder jumpers the two layers. To connect to the opposite power layer (i.e. connect layers 1+3 or 2+4), just drill a bigger hole on the other side (44 mil in my case) before taping to expose the copper, then drill your via hole from the other side after taping for the wire. The 8 mil clad is thin enough to be see-through, so taping those to the inner layers is pretty easy, just line them up and press. Taping the two halves together is harder, I think next time I'll drill the registration holes *after* taping up the halves, as it's more important to line up the outer layers with each other than to line them up with the inner layers (through vias have smaller annuluses). The resulting stack measures 57 mil thick (the math says 64), but still slightly flexible, unlike a the rigid board you get when you epoxy the layers together. You could use standard 32 mil DS for the inners if you already have experience getting the two sides to line up. Just tape on the extra layers afterwards. I wanted to try it this way to see if it was easier to line up the sides, and to see if I got better results from my laminator with thinner stock.