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April 1998

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Subject:
From:
"Vaughan, Ralph H" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 23 Apr 1998 11:25:33 -0700
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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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