Hi Ralph,
Good question, hopefully I can give an acceptable answer. There are a
few reasons why vias are tented/plugged (past and present). Different
applications have different designs.
One of the main reasons is for ATE (Automated Testing). Most systems
draw a vacuum and cannot have a lot of open areas or the board will not draw
down properly and intermittent faults are detected. As vias become smaller,
the hole size is more difficult to control and they may not clean properly
or have a shadowing (where the wave does not contact the pad to initiate the
wetting process) and result in partial or no filling during wave solder (if
through hole), effecting ATE and potentially leaving residues (sometimes
corrosive) in the holes, resulting in LTR (long term reliability) concerns.
Another reason involves via hole placement. If left open and placed
under a component, a solder migratory short potential exists. (SMT&TH)
Another situation that I've seen is when an escape pad is placed very
close to an SMT pad, non common conductor (another via), or circuit, The
soldermask operation may not be able to hold a barrier and result in a
short at the soldercoating operation or during assembly.
Some Aerospace applications are extremely weight sensitive and solder
weighs it down (IE- Boeing, Just busting them a little Ralph! :-).
Vias that are cleared are typically used for testpoints at SMT Testing.
Noticed that I said cleared. It is not advisable to plug these holes due to
the potential of the plugging material encroaching on the test probe area.
Yes, I've seen this one also!!
Just off the top of my head, (no comments please), This should give you
an idea of what issues are present. If you want to call me sometime, I can
talk better than I can type. Just Email me, or give me your number and we
can discuss. JOHN
Vaughan, Ralph H wrote:
> John,
>
> Enjoyed your tent/plug dissertation (I know, I gotta get out more). I'm
> just wondering about the need to do either. Why not leave them open, or
> let them fill during wave solder?
>
> Ralph Vaughan
> Boeing-atlanta
>
> > ----------
> > From: John Waite[SMTP:[log in to unmask]]
> > Reply To: TechNet E-Mail Forum.;John Waite
> > Sent: Thursday, April 23, 1998 1:16 PM
> > To: [log in to unmask]
> > Subject: Re: [TN] Plugging via holes
> >
> > HI Joe,
> > There are many techniques being used for via hole
> > plugging/tenting.
> > First, Tenting is a "covering" of the hole with a material
> > (typically a D/F type), and
> > plugging is the attempt to "fill" the hole completely. We need to
> > distinguish between the two
> > due to acceptability and application techniques.
> > tenting is easy: The soldermask artwork is void of any clearances
> > for the Via holes and by
> > way of process, the d/f is exposed/polymerized and stays in place
> > during the developing phase. I
> > have also used a "high conformance soldermask material" (no names
> > implied) in conjunction with
> > LPI for some applications. I would not recommend this as a primary
> > alternative by any means.
> > Plugging is typically performed by a screen/stenciling operation.
> > Many people are using a
> > thermal cure soldermask, but I have had very good success with a UV
> > material that has been
> > designed for via plugging. (the pigmentation is lower, allowing for
> > better UV energy exposure to
> > the center of the plug). I also like the use of these types of "via
> > plug" materials due to the
> > solids percent. Most thermal masks are approx. 70-80% solids and UV's
> > are close to 100% solids,
> > thus less shrinkage of the plug during curing.
> > The next issue is to plug before or after lpi, before or after
> > soldercoat. I liked to plug
> > before LPI. In doing so, We laid down a better profile for the lpi to
> > cover, and minimized the
> > amount of lpi pulled up during exposure from the via area due to the
> > thickness of the LPI versus
> > exposure energy at the hole. This situation was due to a screen
> > printing process (flooding of
> > the hole) and was minimized when we incorporated an alternate
> > application process (in our case,
> > spraying of the lpi). By applying prior to LPI, we also cleared up
> > any aesthetic concerns our
> > customer had due to the different color "dots". Before implementing
> > this, prove out your
> > preclean process. I won't bore you with details unless you ask this
> > (long subject).
> > Some people will apply after soldercoating due to the theory that
> > the copper should be
> > covered with solder and any skips or missed plugs will not be as
> > detrimental. Also another long
> > subject of debate. My personal thoughts (as small as they may be) are
> > that It is even more
> > difficult to clean a hole after soldercoating since it is smaller and
> > has a greater potential of
> > flux residue to be entrapped.
> > In any event, however you decide to plug/tent. The following are
> > critical areas to check:
> > 1) insure that any tin (or tin/lead) residue is removed during the
> > stripping stage. I would
> > recommend that tin be used as an etch resist, since it is easier and
> > more environmentally
> > friendly to strip than tin/lead.
> > 2) profile your preclean. Different methods may create issues,
> > dependent upon when and how
> > you plug.
> > 3) run some tests with different size vias to find you limitations
> > for stripping, cleaning
> > and plugging. If you're plating the hole down to .008", then plugging
> > could become a Moot point.
> >
> > 4) Get data from your customers as to how they like the plug
> > process to be done. I've had
> > accounts that insist on double sided via plugging. This is extremely
> > difficult with thermal
> > masks due to the outgassing that will occur on side 2 of the plug and
> > the "volcanoes" remaining.
> > this could interfere with cleaning/assembly operations. I have had
> > some pretty good success with
> > 2 sided via plugging of UV materials, but am not saying that this is a
> > proven industry process.
> > I guess I could give some more concerns/ideas/comments, but I'm
> > sure that our peers also have
> > some success/failure stories to share. If you would like any more of
> > my long winded
> > comments/ideas, Email me direct and we can discuss off-line where I
> > can mention some material
> > types. Good luck. JOHN WAITE
> >
> > Hurst, Joe wrote:
> >
> > > Hello Persons:
> > >
> > > Any common ways of tenting and plugging standard via holes
> > > (0.016) ?
> > >
> > > Thx
> > >
> > > Joe H.
> > >
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