Hello fellow Blaser (Laser),
There are more "experts" on laser drilling than there are lasers
out there (if you get my drift).   Some basic facts help with this
strategy.  The ablation rate is a combination of energy and time.
If you review the graphs pertaining to CO2 or UV/Yag you will
see that in the UV systems there is a large variance between glass
and resin, but in CO2 there curves are close.  If you can live with
the print/laser registration issues CO2 may be your best bet.

My issues with FR-4 ablation are as follows:
1. Using a UV/Yag system the alignment is excellent, but the sidewall
is not smooth.  This can be a real problem when trying to produce
vias  with aspect ratios > .5 (dielectric) to 1 (Top Dia. of hole).
Look at the weave pattern of both 2313 and 106, neither is completely
uniform.  If you look at hundreds of holes at a time you'll see large
variations from your "optimum hole" to your "worst hole".  Also in
Lasering the vias the spot size of the spiral cycle must be small
enough to overlap without creating the classic copper finger in the
capture pad which causes partial voids during plating.  Double
cycling is recommended for this.  Even a third trepan cycle helps
(the suppliers don't recommend this because the throughput numbers
stink!).  The vias/ sec. are about 10 (ugh!).

2. Using C02 the holes are ablated at a more even rate.  They require
a pre-etch process unless you are using very thin copper (we haven't
had much success with the thin copper either).  Registration is
critical as you will ablate right past the pad if misaligned.  Also
the heat created from this process causes a "ash" at the base of the
hole that needs plasma removal.  Once all of these steps are done the
hole looks more like at football than a tea cup.

You didn't ask, but I tell you anyways....
I'd run 1 ea. of 106 and use RCC with a UV/Yag.  The RCC resins are
UL approved and are FR-4 materials.

Hope this helps,
Joe Dickson
Dir. of Engineering
Tyco Intl.
Santa Clara, Ca.
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> I have a product that requires a .006 dia. laser ablated via from lyr 1 to
> 2.  I'm required to use glass re-inforced FR-4 as a dielectric.  The
> dielectric is approx .0035. I can either use a single ply of 2313 or 2 plies
> of 106 prepreg.  Which would be the most favorable construction in terms of
> laser hole quality and subsequent processing?
>