Homebrew PCBs

>One thing I wasn't clear about after reading the excellent descriptions
>is exactly how the 'sandwich' is made up.   I had expected that for a
>4-layer board one would use two double-sided boards with an insulation
>layer between -- but it sounds as though in fact one uses a single
>two-sided board (for the two inner layers) and two single-sided
>boards for the outer layers?   Is there a particular advantage in
>this?

Yes, the main advantage is that it's easier to handle. No need to
isolate. I.e. if say at some spot there is no glue in the inner layer,
it would not necesairly be catastrophic. It really boils down of
applying glue to both inner layer sides, tap the outer ones on it and
press it together as good as you manage.

The inner "double sided" material is 0.5mm sized and the outer ones
0.3mm together with the glue you end up with something about the
thikness of a standard PCB.

Markus