As far as flat surfaces go, I've heard many comments from PCB customers
about the benefits of Omikron with regard to surface flatness.  The
disadvantage is the decrease shelf life over HASL (about 50%).  I've also
heard that horizontal HASL gives better surface flatness consistencies.
I'm not sure about immersion Silver though...


On Mon, 21 Jun 1999 13:27:00 -0400 KELLY M SCHRIVER
<[log in to unmask]> writes:
>Hi Mike -
>
>Here's my two cents worth, from an assemblers viewpoint:
>
>I've tried to spec the same thing in the years gone by, only to learn
>that the
>HASL process doesn't have the degree of control that many folks would
>like.
>After a number of attempts, we finally learned that the paramount
>charactaristics are SOLDERABILITY and UNIFORMITY.   Specifying
>minimum
>thickness led our suppliers to be absolutely sure they had enough
>thickness to
>meet our minimums.  This often gave us boards with tall solder crowns,
>which
>affected both past application and component placement.  We also
>found
>substantial variations: on individual boards, from side to side; on
>the same
>side; from board to board; and from one production lot to the next.
>
>To make a long story short, we now spec solderability requirements and
>tell
>the supplier our preference on thickness, without it being mandatory.
>
>Regards - Kelly
>
>PS - we're still looking at the white tins by Omikron (Florida
>CirTech) and
>Dexter in hopes of getting that ultimate solderable board with flat
>pads!!!!