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April 1998

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Subject:
From:
John Waite <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 23 Apr 1998 13:16:13 -0400
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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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