Romeo,
Our main supplier of dry film masked circuit boards has told us that they will no longer supply us with
dry film masking. This is apparently due to the continually inconsistant dry film material/process.
Because of this, we are investigating which is the best method to plug (damn incircuit testing) the holes
using an LPI mask. The Technet archives are full of opinions as to what method works best and we will be
doing some testing to see which method we like best.
If I were to have my preferance I would still be using a 2-3 mil dry film mask. We have used it for
several years with no problems "in-house." It has tented our vias well. However, I don't like to saddle
my supplier with a process that causes him excessive scrap and rework. So, I guess my guestion to Technet
is the same one you voiced "What was wrong with just "simple Dry film tenting"?? Have your suppliers given
up on dry film? Why are many looking for a good way to plug using LPI when dry film is available. I'll
take two cents from anyone thats got it.
Gary Camac
Kubes, Romeo (NM75) wrote:
> Kelley,
>
> What was wrong with just "simple Dry film tenting"??
>
> And how thick was the product after application?
>
> R. Kubes
> Honeywell DAS (Albq)
>
> > ----------
> > From: Kelly M. Schriver[SMTP:[log in to unmask]]
> > Sent: Thursday, October 01, 1998 7:54 AM
> > To: [log in to unmask]
> > Subject: [TN] LPI Filled Vias ?
> >
> > Good morning TechNet!!!!!
> >
> > Here's a challenge for the board fabrication guys and gals:
> >
> > Most of our board designs go into severe environment service. For the
> > past
> > eight ten years or so, we have been tenting vias with thin dry film solder
> > mask disks, followed by an LPI overcoat of the entire board (both sides).
> > The
> > purpose of all of this is to seal up all exposed conductive surfaces,
> > particularly those under component bodies, prior to assembly. This leaves
> > only the termination pads and plated thru holes which receive thru hole
> > leads
> > exposed. This has worked out to be a very successful process.
> >
> > We arrived at this combined mask application after having tried a number
> > of
> > different via fill approaches and just simple dry film tenting. None of
> > these
> > were fully satisfactory (bumps, open voids, ruptures, incomplete fill, not
> > process durable, too costly, not repeatible, etc.). The dry film plus LPI
> > also turned out to be the only approach that ALL of our board vendors
> > could
> > handle reliably.
> >
> > Now I am revisiting the process. We have run a few samples using LPI as a
> > via
> > plug as well as the overall solder mask. These have turned out to be very
> > durable in our assembly process. Microsections indicate small voids, less
> > than 50% of the via volume, which are not a problem to this application.
> > Board thicknesses range from .040" to .080". Finished vias range from
> > .008"
> > to .016" dia. (This covers multiple designs.)
> >
> > Now, here's the big question: Could you produce class 2 and 3 boards
> > with
> > LPI plugged vias which will not rupture or expose metal during my
> > soldering
> > and cleaning processes? (Assume that worst case will not exceed four
> > solder
> > and cleaning cycles. Cleaning agents may include DI water, long chain
> > alcohols or citric acids) Total mask thickness buildup over pads should
> > not
> > exceed .0015", in order to not interfere with our stencils.
> >
> > For anyone who is actively performing this process or assembling CCAs
> > using
> > this process - I would greatly appreciate your inputs.
> >
> > Regards - Kelly
> >
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