In addition to the points listed below, might I ask what the plasma gas
composition is that is needed for via formation? I believe that some of the
chemicals used may be in violation of the Montreal protocol for CFC reduction.
RSVP
Dave Rooke
Circo Craft - Pointe Claire
>
> \0
> TO: I4235700 IBMMAIL new address for ipc technet 25.6.96
>
> FROM: DSTEWART EX2 D.Stewart - Product Development
Manager.
>
> DATE: 22 October 1996
> SUBJECT: re-microblind vias laser v plasma
> REFERENCE: design
> DATE: 22 October 1996
> SUBJECT: RE: PLUGGED VIAS (AND LASER VIAS AND PLASMA VIAS)
> =
>
> Just a couple of corrections re-plasma drilling of microblind vias-
> Most circuit shops do NOT have a plasma machine, only those who use
> teflon boards or other exotics which do not like/react with
> permanganate for desmear. Even if they do have a plasma machine, it
> is unlikely that the distribution of the etching process is
> sufficiently tight to guarantee all vias etching completely, and
> anyway, they are likely to infringe Dyconex's patent or sail close
> to the wind, unless they have forked out =A3600K for a license.
> =
>
> Microblind vias by plasma ARE cheap on the basis that they all etch
> simultaneously, but we decided to follow the laser route for the
> following reasons:
> =
>
> 1. Plasma etch is isotropic ie. it etches sideways as well as down,
> so what starts life as a 0.1mm (4 mil) opening in the copper acting
> as the mask, ends up as a 0.3mm (12 mil) opening by the time the
> plasma has etched down through 3 mil of dielectric to expose 2 mil
> of the copper on the layer below, and then the copper has been
> etched back to remove the overhang. (Based on experience not
> theory) At 12 mil we could mechanically drill this!
> Using laser the cost goes DOWN as the hole size gets smaller, and
> the positional accuracy is superlative - +/-0.5 mil, so the
> manufacturing tolerance for the laser drilled blind via is reduced
> to around 8mil total - so now you have a 0.1mm via in a 0.3mm pad,
> compared to the plasma 0.3mm via in a 0.5mm pad. Loads more routing
> space.
> =
>
> 2. The dielectric MUST be a homogeneous organic substrate for
> plasma etch, whereas laser can ablate through copper, and glass
> fibre reinforcement, so materials do not HAVE to change, although
> there are advantages to using the same materials as plasma.
> =
>
> 3. Board manufacturing materials are priced in relation to their
> global volume use, hence FR4 is still the cheapest, aramid (once
> its impregnated by a laminator) can be anything from 3 to 10 times
> more expensive, PTFE is extortionant, and polyimide foil is
> probably 3 to 6 times more expensive. They are also more difficult/
> impossible to bond by most fabricators, so the option of FR4 looks
> more likely to be accepted.
> =
>
> =
>
> 4. Finally, there is the Dyconex license fee, which is a
> particularly galling stone to swallow, as Exacta have examples of
> their own plasma etched via products made back in the 70's, but
> that's not sour grapes, just a comment.
>
> As far as volume manufacture goes, all these new technologies are
> in the early phases of volume ramp, Plasma etch machines can take 6
> panels at a time if you buy the biggest machines, laser drills only
> take 1 panel at a time, but obviously this will change as time
> moves on.
> =
>
> For a final note, we are also working on the SLC type processes
> (now known as Sequential Build Up -SBU or HDI- High Density
> Interconnect) with some success. These are limited currently in
> terms of the minimum via size for exposure, and dielectric
> seperation, but the investment needed for most circuit shops is
> minimal in comparison to laser or plasma, and the volume production
> is not limited in the same way as the alternatives. There are
> always advantages in different ways to make boards, so
> fundamentally, the design rules should allow fabricators their own
> choice as to which process they use to produce the board.
> =
>
> D.Stewart
> Product Development Manager
> Exacta Circuits
> Scotland
> =
>
> The views expressed above are mine and do not neccessarily reflect
> those of my employer.
>
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