Edward;
Selective solder stripping is not an easy process to do consistently well.
However in my opinion and the opinion of the customers that are prepared to
pay for the added cost it is a superior finish to HASL or HASL
alternatives. Our next best option is omikron white tin. What is your
opinion of the HASL alternatives currently available? ENIG, SILVER, WHITE
TIN, OSP.

To answer your question below.
I don't think the type of dry film photoresist is an issue. We use Dupont
9600. We found better results with a thicker emulsion than normal. 2
instead of 1.5 mil. This will help to eliminate your leaking problem.
Now you must be prepared to cheat a little.
Unless you can produce 100% perfect registration at all stages of
manufacture, you will have either bare copper at the pad edges or reflowed
solder under the mask. We do an expansion/contraction of the solder
mask/solder strip film which leaves a small annular ring of solder under
the mask. Size of the ring will depend upon your registration abilities.
Reflow the tin/lead after masking.
If you are proceeding with further testing, I will appreciate your comments.
Jac Kroeker

At 10:34 AM 09/15/1999 +0200, you wrote:
>Hi Jac,
>I am just wondering how your selective stripping is working?Probably you are
>using selective masking ( photoresist) to protect the areas destignated for
>reflow.What kind of photoresist are you using and what about overlaping
>between tin-lead stripped pattern and solder mask pattern?
>In the past we made trials and we faced the following problems:
>1) leaking of tin-lead stripping solution along lines.This resulted non
>clear boundary between the sections that were destignated for reflow and
>those destignated to be covered by solder mask.
>2) as result of the previous,we got exposed copper ar the boundary of the
>"reflow side" and tin-lead on the "solder mask" side.
>Regards
>
>Edward Szpruch
>Eltek , Manager of Process Engineering

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