Chris,
These are really assembly questions and can be complex trade-offs.  It all
depends on who is doing the assembly.  We find a large soldermask window
around each row of pads easiest, but that might not match the assembly
capability.  All types are ordered.  There is also an issue of where the
0.5mm devices are placed if the panel is large.  FR-4 moves with any heat
cycle in assembly and the solder paste may end up between the pads.  For
this reason, customers have us put the solder paste on the board, reflow
it, clean it and then cullender it so the solder is flat.  All they have
to do is flux, place the parts and reflow.

There is no bottom to the pitch that might require solder mask.  We do a
lot of flip chip packages that are 0.2mm pitch with SM opening that 0.114
+/- 0.01mm for SM defined pads.  We have experimental boards  now that are
0.152mm pitch and require soldermask.
Its not our job to force an assembler into something he doesn't want, but
rather, to give them the price alternatives of what is available and let
them figure out the SYSTEM COSTS.

Happy Holden
Westwood Associates





Chris Robertson <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
12/15/2003 11:33 AM
Please respond to Chris Robertson


        To:     [log in to unmask]
        cc:
        Subject:        Re: [TN] .5 mm pitch components


Happy,
What is your recommendation if someone doesn't want to pay the premium
price
for a manufacturer that can't hold that?
Are you an advocate for the all mask or none approach? This is the feeling
that the cost outweighs the fallout from bridging?
This isn't a trick question. I just know people that have a block of
mask clearance and others that feel that the fallout from that is to
costly.
Is there a breaking value? I mean is there a pitch that you hit that
almost MUST
require masking?

Chris Robertson
[log in to unmask]

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