[log in to unmask] asked:

>     Typically, what is the minimum spacing between printed circuit solder
>     pads that would allow for enough LPI soldermask to prevent bridging
>     for a wave soldering process ?

Several ways to answer. Since I think bridging in wave solder is a three
dimensional issue (some designers forget this), no amount of mask is enough
to prevent bridging, unless is as thick as the leads or covers them. And,
if they are spaced far enough apart, the chances of bridging will be
reduced without the mask. Don't forget that solder can flow over that mask,
and that the mask has not been invented that will block all briding. IMO,
mask does not inhibit solder wetting any more (and probably less) than bare
laminate. Space alone is the more significant predictor of whether you will
have bridging.

But, for SMT designs with narrow pads, we find that 0.5 mm pitch SMT parts
using 0.010" pad width and space is about the limit to allow a "web" of LPI
mask between the pads. This will be with a minimal mask web width and no
mask on your pad. I would say chances of bridging such pads in wave solder
(even with mask) are extremely high, and noticeable even for properly
printed paste post-reflow.

The major reason we use this design is to improve the gasketing of the
stencil during fine pitch solder paste stencil printing, which does
indirectly affect bridging.

>     Is there a design guideline that specifically addresses this ? If so,
>     can you point me in the right direction.

Not sufficiently to cover all the implications of your question. Go to your
fab supplier and lay the cards on the table. They should work with you, or
find one who will.


cheers,

Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com/