I agree with the "process control" reasons stated in earlier mail, they are 
absolutely necessary.  Additionally, anyone who has been in SMT for any length
of time knows that proper land pattern design is a must.

BUT, even though we do all of those things well, our solderpaste output ppm
is not zero on finepitch and bga devices.  While it is true that not all PCB
suppliers are capable of mask webs between finepitch lands, it is important
to find one who is.  My primary board supplier, Merix, is capable of web
thicknesses down to 2.5 mils with none of the flaking defects described.

Now, for the data, we experimented with webs between fine pitch lands and found
a step function ppm reduction in our paste defects on fine pitch devices.

If these tools are available to us, why not utilize them?


Regards,

Dean Barham
DFM Mfg. Eng. Mgr.
Teradyne, Inc.