Hi ehb,
In a message dated 10/22/97 5:30:45, you wrote:
>Thanks Werner,
>I really do appreciate your response, so often people don't respond to
requests >for information on this net. The use of square pads for BGA
intersests me. I had >thought that the reason for this was to provide an
irregular outline which would >be useful for X -ray inspection purposes.
>Solder resist has quite a large tolerance for defined windows. (A border of
3 thou >(mil) is normal and PCB fabs are telling us 1.5 is possible.) So
+/-1.5 to 3 thou.
>What are the reasons for this tolerance?
>How do different resist types perform?
>Are there any improvements forth coming?

The tolerances have to to due with (1) the alignment accuracy for the
photo-definition, and (2) the colimation of the light beam and sharpness of
the photo-defined solder mask edge. Both of these are rapidly improving with
the efforts put into HDI. There was quite a bit of talk about this at
IPCWorks'97.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]

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