The copper foil used for printed wiring laminates is treated on one side with an oxide coating. This oxide coating gives the copper surface a crystalline texture for adhesion to the glass epoxy bonding sheets and laminate. The foil is bonded under heat and pressure using B stage bonding sheets.
It goes like this:
Oxide treated copper foil with the oxide side facing down
B-stage prepreg bonding sheet(s)
C-stage laminate
B stage prepreg is epoxy impregnated glass cloth that is cured until "tack free" but not fully cured. It will flow out and bond the layers under heat and pressure.
C stage is fully cured laminate such as FR-4 etc. that is commonly used.
There are many types of prepreg materials available with various resin and glass makeups. There are also many types of oxides available. Black, brown and red oxides are widely used.
I used brown oxides when I made multilayer boards due to the finer crystalline structure. The foil had a brown oxide coating as well. Sometimes you can see the oxide coating inside of a multilayer board. It's easiest to spot on larger internal ground planes. That's why the internal copper layers will often look darker and not a copper color.
Tom
--- In Homebrew_PCBs@yahoogroups.com, "Dave M" <dgminala@...> wrote:
>
> I've followed a thread on the Tekscopes group for several days, hoping that
> someone would ask the question that's eluded me for years. My question is
> in regard to the raw PCB manufacturing process; hopefully, someone can
> provide a clue.
>
> My question is: what process is used to prepare the surface of the copper
> sheet prior to bonding to the bare fiberglass (e.g., FR4) board? If you've
> ever lifted a trace from a PCB, you've probably noticed that the copper
> surface that was bonded to the fiberglass board had some kind of brownish
> coating on it. Logic would lead one to think that this coating would help
> the copper to securely bond to the fiberglass.
>
> I've noticed that on some boards, the copper-fiberglass bond is quite
> tenacious, requiring a good bit of effort to break it. Applying heat and
> pressure is the easiest method, but physically pulling a copper trace form
> the board can be difficult if you're trying to salvage the trace.
>
> I would like to know how the copper sheet is prepared prior to bonding to
> the board. Anyone know?
>
> Cheers,
> Dave M
>
>
> [Non-text portions of this message have been removed]
>