Rick,

It is likely that the fabricator does not have the
equipment necessary to provide a reliable dam between
the surface mount pads.  Assuming that he is using an
LPI there are a lot of process points that would or
could make this a difficult proposition.  Remembering
our first attempts at damming (and damning) the
tighter pitched devices it was really hit and miss.

We have found that there are some newer masks that
make damming easier. Lea Ronal OPSR5900 and Vantico
(formerly Ciba Geigy) 77MA or 77MA-1 make processing
these tighter geometries easier.

Don't know if this helps but I thought I'd put my two
cents worth in...

Pete Menuez

--- Rick Thompson <[log in to unmask]>
wrote:
> Hi,
>
> We have a customer who has a board with 20 mil pitch
> QFP's on it.  The
> actual pad size is 11.8 mils wide and there is a 2
> mil gap in the soldermask
> around the pads leaving 4.2 mils of solder mask
> between the pads.  The board
> house that is building these says they can't
> maintain the soldermask between
> the pads and is asking for a window around all leads
> in the group or a
> tightening of the soldermask openings around the
> pads.  Before I go to our
> customer with this, I'd like to get some feedback on
> whether or not this is
> unreasonable?  Are these dimensions acheivable or
> should I ask our customer
> for a deviation?
>
> Thanks in advance,
>
>
> Rick Thompson
> Ventura Electronics Assembly
> 2665A Park Center Dr.
> Simi Valley, CA 93065
>
> +1 (805) 584-9858 voice
> +1 (805) 584-1529 fax
> [log in to unmask]
>
>
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