In a message dated 99-07-02 13:43:56 EDT, you write:

<< 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.
 [log in to unmask] >>


In working with a large board house last year - we were able to manufacture
blind vias as small as 4 mil diameter. But again, there was a lot of tweeking
to do. Starting from drilling all the way through wet process.

1. Boards were 0.020 finished - we still used a pre-etch process.
2. Using CO2 did ablate at a more even rate - but without verification of
your set-up on laser - somewhere in the middle of the laser drilling one side
wall - ( was usually the left one ) was indented about 2 to 3 tenths more
than the right one. Almost looked like someone was trying to dig a tunnel
over to the next via.
3. Ash that is formed in the via - can be removed in etchback without the use
of plasma - but the whole etchback ( not desmear ) process has to be
realigned - from chemical make up and dwell times - to the introduction of
mechanical vibration. ( This will play havoc in consumption and soforth -
till someone gets a hang of the process - but once it's operating - can speed
up production as opposed to plasma)

4. They were using an electroless coopper to metalize the vias - this had to
be revamped from the start also. ( Introduction of mechanical vibration does
help - but again - it creates a nightmare in wet processing )

5. We tried a brightner with leveling as opposed to a straight micro-leveling
brightner and a run in someones pulse tank.  Micro-leveling brightner - had
to be plated twice to get a minimum of 0.50 mils in the via - macro-leveling
took two goes to get 0.75 - pulse had worked one-pass - but again - it needed
to be striked.


All tanks had eductors ( from what i witnessed - eductors on micro & macro -
straight DC plating - didn't really display much of a difference - but we
used them to make sure the copper ion was being replenished in the via.)

Setting up a system to run blind via's under 0.60 diameter - is not as simple
as we thought - pulse was probably the best bet - but then there are about an
additional 25 0r 30 parameters that have to be met with pulse ( both
mechanical and chemical ) before a decent blind via can be produced.

Everything that Joe outlined in point 2 had occurred and worse. But we got
over it eventually.

Best of luck to all that are trying their hand at blind vias 6 thou and under
- if i cud be of any help - just email me - [log in to unmask]

Rich Fudalewski