Norm Dill asked:

>    Our HASL solder thickness spec. is 0.3 to 1.5 mils.  Is it unreasonable
>to expect using a horizontal HASL process that 1.5 mils will be the actual
>max. on 100% of the pads on both sides of the board, even with fine pitch (<
>= 20 mils)?

Of course.

>    My guess is that the surface tension of the molten solder on the fine
>pitch pads, its low mass and the surrounding geography, make it difficult
>for the air knives to do consistant leveling.

I'm with you. You have some problems with the orthagonal nature of pads,
those in X axis usually vary a lot from the Y unless the panel is run
through on a bias.

>    Some of our product mix here is Mil Spec work.  Converting the boards
>with fine pitch on both sides to Au/Ni may be more pain than it is worth.
>    Any inputs on this subject will be greatly appreciated.

But isn't all "Mil Spec" work painful and expensive? (forgive the derisive
rhetoric).

One very nicely written spec I have seen required that no more than 5% of
the surface could have thickness less than 60 uin, a mean thickness of 320
uin, and no more than 5% of the surface area SnPb thicker than 2,400 uin.
Thickness measurements are to be taken in the center 80% of length and
width (a marginal 10% lineal boarder is not measured) to accomodate the
natural "doming" due to surface tension.

The same spec called for a "class A" thickness for fine pitch SMT in which
the absolute minimum thickness is specified at 160 uin, mean 320 uin, and
the absolute maximum is 1,000 uin, again measured as above. This will
probably be the best you can ask for.

If you are asking for an absolute 300 uin minimum it would appear to be one
of those bizarre "you're kidding" spec items most of us have on our
drawings. Everyone with real understanding (a small minority) knows they
are impossible, and therefore blatantly ignored with appropriate winks,
unless involving those on the lunatic fringe willing to pay exorbitant
prices to achieve some arbitrary spec of dubious lineage. Most of those
(fortunately) do not know how to make such measurements.


;-)


cheers,

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