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December 1997

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Subject:
From:
"Devitt, Ken" <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Thu, 18 Dec 1997 11:13:28 -0500
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Mr/Ms Kgchafin

I am joining your discussion late, and have perhaps missed many of the
prior responses

But let me add -

We produce a variety of single sided surface mount PCB assemblies for
avionics applications.
They are single sided because after assembly and bottomside In Circuit
Test (ICT) we laminate two together to a common heatsink.

We have many existing designs which rely upon dry film soldermask to
tent the top side of vias, leaving the bottomside open for access of the
via pads during ICT testing.  Tenting the topside facilitates vacuum
pull down on the Genrad Test equipment.

Recently, our fab houses have complained about using and developing film
on one side only, and the use of dry film mask buttons covered by LPI
has been strongly recommended by AQC, Melbourne (Ref: Frank Gordon
407-752-8815) .  However, I believe developing of LPI from one side only
would still represent a fab problem.

Like you, I also would like to know if people are using dry film to tent
via, specifically one side only, and if their is a preferred combination
of dry film and or  LPI to achieve the "topside tenting, bottomside
access" described above.

Thanks in advance TechNet

Ken Devitt
Principal Engineer
ITT Avionics
973 284-3880
[log in to unmask]

> -----Original Message-----
> From: [log in to unmask] [SMTP:[log in to unmask]]
> Sent: Thursday, December 18, 1997 8:27 AM
> To:   [log in to unmask]
> Subject:      [TN] Dry Film Solder Mask Tenting of Vias
>
> In this forum I have recently asked for your views on the relative
> effectiveness of plugging vias with LPI solder mask versus tenting
> vias
> with dry film and then applying LPI solder mask.
>
> I also asked for your views on the advisability of applying LPI solder
> mask
> over vias with no attempt to plug or tent.
>
> Each of your replies was helpful--apparently the worst option is to
> apply
> LPI over vias with no plugging or tenting.  We are reconsidering our
> original intent--tent with dry film and then apply LPI.
>
> Let me pose a few more related questions:  Do any of you tent vias
> with dry
> film before applying LPI?  If so how well does this work?  Is this a
> common
> practice in the industry?  Has any one ever heard of this practice?
>
> Dry film tenting is the only option offered by our supplier.  However
> in
> all of the replies I've received, no one has said he uses this method
> of
> sealing vias.  We want to make our switch from dry film to LPI solder
> mask
> effective with the beginning of the calendar year.  But we don't want
> to
> adopt some nonstandard, idiosyncratic practice.
>
> Thanks again for sharing the results of your experience.
>
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