PCBs delaminating?

[Andi]

Morning gents. As I think I've mentioned, I make my own PCBs with a CNC router. In general the quality that comes out is superb, but on a couple of recent ones the pads have lifted rather easily. Given that there's no etching I wouldn't have thought that this is a production problem but rather something wrong with the PCB blanks. I bought a bundle of them some (4?) years ago from Maplin and they've been sat on a shelf since, being used up as needed.

Do PCB blanks delaminate, or is it more likely that I just have one bad sheet? Or is there something else I'm missing?

Ta.

[R.G.]

Probably a bum sheet of stock. Could be because that sheet is less well glued together. It's unlikely to be age.

PCB stock is made by pressing copper foils onto a film of epoxy-coated substrate, then heat-curing the epoxy. The epoxy grip does not tend to weaken with age to any noticeable degree. Excessive heat can soften it with soldering, but aging does not make it lift. Otherwise the circuit boards in effects from the 1960s would all be turning to goo. Could be that one sheet does not cope well with the edge effects of your CNC routing.

If it were mine, I'd get a fresh sheet, note if this new one had problems, and if not, scrap the old questionable stuff. If the new sheet does have problems, I'd look at my CNC bits to ensure they're sharp and being spun at the correct speed. They can generate excess heat when dull, too.

[Andi]

Excellent - thanks RG. The last sheet was fine, so I think it's probably one duff sheet.

[Tantalum7]

I was thinking about CNCing some boards myself at some point.  Every time that I've cut into any stock that had epoxy, I found that without flood coolant, the cutter would eventually get bogged down by the fine dust and epoxy mix and generate heat.  Also, even carbide endmills (diamond coated endmills last hundreds of times longer, but not everyone has access to or is willing to foot the bill for these (think $200 per endmill)) will eventually dull on any material that contains glass or silica.  They might still feel sharp to the touch, but under magnification, you can see that the edge has broken down some.  If you've never had a problem in the past, it could very well be a bad board or two, but if it keeps happening, try a brand new endmill. 

[Andi]

I'll change cutters - I don't use endmills though, instead they're pointy isolation cutters so you can set the width of cut by the depth limiter.