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Subject:
From:
Carey Pico <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 16 Sep 1999 22:11:22 -0700
Content-Type:
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Edward
My experience is based on my 5 years experience with laser drilling
development in a laser supplier as well as customer support, so I understand
some of your issues.  I do not have PWB shop experience, but I've interfaced
much with them as well as paper studied the process flow in detail.  So,
I'll try to answer you questions:

1) The common via plate process depends on the method.  With photoimaging,
both Cu plating and metal paste fill are being investigated.  I have the
impression photoimageable dielectrics are not manufacturable as yet.  I'd
like to hear from Continental Circuits, McDermid, etc. on this.  Issues are
adhesion, defects, and a host of others.  Alignment is very difficult (lots
of stretch and shrink in the panels before imaging).  Also,
thickness/chemical uniformity causes variation in dielectric quality and via
size.

2) CO2 lasers are great if you have their parameters under control.  I get
mixed reviews from those using them.  It depends on the CO2 laser type as
well as via size.  Cleaning is critical in these situations, but it can be
done.  Alignment of vias to layer2 pads is a problem with CO2, but not as
much as the "other" laser folks make it out to be.  Via sizes below 4 mils
(100 um) are difficult for a variety of reasons from what I hear, but I
think this can worked out.

3) UV lasers can drill down to 1 mil vias, but those are not truly
manufacturable except in specialized situations.  The quality of the vias
are excellent and plating is relatively straightforward using a standard Cu
process for vias down to 4 mils.  Below 4 mils, one has to get careful about
plating, etc.  The smallest practical size is 2 mils, and with much effort
and specialized materials.  The advantages are they can go through the top
layer of Cu, so alignment to layer2 is good.  The disadvantages are
throughput being limited, variations within a panel in via quality because
of variations in the panel quality, variations from laser system to laser
system, and a not-so-reliable machine.  There's been some improvement on the
system-to-system variation.

4) Personally, I've been seeing a shift from UV to CO2 for via drilling.
That may be to do with the slower growth to sub-5 mil vias and the lack of
need for a UV laser down to that size.  It may also be that CO2 is better
for FR4.  Since most shops are afraid of committing to RCC or similar
materials, FR4 is an issue (along with variation in resin thickness/hardness
in RCC-like materials).  This can all be worked out either way.

5) Overall, I don't know of any shops "flat" plating the vias.  I'd like to
hear.

6) I don't understand what you mean by a blind via within a pad.  Is that
connecting from the top layer (call it layer1 down to layer2 and/or layer3?
It's been done, but one has to make it work with the laser technology they
buy (not always an obvious choice).

7) Lastly, the customer can say all they want.  But the bottom line is cost.
Your customer will live with what you supply if it works.  If there is
competition that has a more attractive product to your customer, you need to
find out.  Get past the "wish list" and talk... is it still Shekels ($$$)?

There are job shops using different lasers (I don't know of a photoimageable
shop).  Or you can evaluate boards from the system suppliers and have them
do a test batch of vias.  My impression is it pays well for the cost of the
equipment, regardless of the technology.

Carey
ps- Thanks, and yes, I'm one.  I just want to get past this Monday without
too much guilt or hunger.

-----Original Message-----
From: edward Edward Szpruch <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Thursday, September 16, 1999 12:17 AM
Subject: Re: [TN] BGA micro-blind vias


Carey,
thank you for your reply.
In my company we are making tests in the area of microvias.We are
investigating different processes for creating of microvias (
photovias,different types of laser drilling as well and controlled deep
mechanical drilling using flat bits). Metallisation of the holes is based on
conventional permangatate  desmear followed by copper electroless process
and the final copper thickness of 20 to 25 microns is build by pulse
plating.We got good results with laser drilled holes down to 4 mills
diameter and mechanical drilled blind vias 5 mills diameter.
The question of the finishing arose during technical discussion with one of
our customers concerning test vehicle.The customer spoke about pads with
blind hole within the pad.The customer stated,that from his point of view ,
he preffers flat pad .I do not know,how essential is this request,since I
know nothing about assembly problems.From my own experience I know,that this
can be achieved using traditional additive lamination ( external layer
pre-drilled,holes plated and laminated) ,which assure the hole completly
filled with the resin and then the outer surface is again overplated
creating "no-hole".
May be now my question is more clear concerning the practice.
I does no seams to me feasible to fill such hole ( 4 mill diameter drilled
hole after metallisation it will be  2 to 2.5 mill internal diameter,less
than 2 mill deep) with LPI or any kind of fill material due to registration
problems.To fill it with electrolytic copper is may be possible,but very
problematic to achieve.
Best regards
Edward
P.S.
Following your Shana Tova I presume you are "one of ours",so I am wishing
you Gmar Hatima Tova


Edward Szpruch
Eltek , Manager of Process Engineering
P.O.Box 159 ; 49101 Petah Tikva Israel
Tel  ++972 3 9395050 , Fax  ++972 3 9309581
e-mail   [log in to unmask]

> -----Original Message-----
> From: Carey Pico [SMTP:[log in to unmask]]
> Sent: ד ספטמבר 15 1999 20:10
> To:   TechNet E-Mail Forum.; edward Edward Szpruch
> Subject:      Re:      [TN] BGA micro-blind vias
>
> Edward
> There is no "common practice" for BGA's.  It depends on the PWB shop and
> customer requirements.  Also on the BGA density (of connnections).
> Microvias range in size from 2-8 mils diameter.  Plating is typically
> electroplated copper (although the photovia process is being demonstrated
> by
> conductive inks provided by ORMET).  Most don't require microvias below
> 4-5
> mils (because of the cost of manufacturing).
>
> There is much more detail to go into, but your question is vague.  Do you
> need to locate a laser drill shop, board supplier, etc?  Or is this
> educational?
>
> Shannah Tovah
>
>
> -----Original Message-----
> From: edward Edward Szpruch <[log in to unmask]>
> To: [log in to unmask] <[log in to unmask]>
> Date: Tuesday, September 14, 1999 5:06 AM
> Subject: [TN] BGA micro-blind vias
>
>
> >Hi,
> >What is the common practice concerning blind-microvias located on BGA
> pads:
> >a) open ? ( 2-3 mills diameter, 2 mills deep)
> >b) filled with what ( fill materials,LPI solder mask)?
> >c) filled with electroplated copper ?
> >Pls advice
> >
> >Edward Szpruch
> >Eltek , Manager of Process Engineering
> >P.O.Box 159 ; 49101 Petah Tikva Israel
> >Tel  ++972 3 9395050 , Fax  ++972 3 9309581
> >e-mail   [log in to unmask]
> >
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