Ofer - Why not use .0098" drill with .020" pads?  11:1 aspect ratio plating
should not be a problem for high-end board fabricators.  You should also be
teardropping the pads for this combination to ensure .002" at the
pad-to-trace junction.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ofer Cohen
Sent: Wednesday, March 09, 2005 3:09 AM
To: [log in to unmask]
Subject: [TN] Shapes of pads

Hello, guys,
The problem - BGA with pitch of 1 mm and components in 0402 packages just
under the BGA.

The thickness of the PCB - 110 mils (2.7 mm), hence of through vias drilling
diameter is 12 mils (aspect ratio - 1:9). To keep annular ring of 5 mils the
pads diameter is 22 mils. To place components between two adjacent vias I
allowed 24 mils square pads over the via holes with deep hole plugging (50%
to 80% hole length). The plugging was pretty good (as seen in cross
sections) and the soldering results were also not too bad, although required
some touch up. This was acceptable for the production of low to medium
volume.

We are going now to manufacture in high volume. The solution is not
economical any more. Hence, I am considering three options, all of them
requires us to place pads for the components between the vias holes:
1.      Round 24 mils pads.
2.      Round 20 mils pads with tear drop to enhance the area
3.      19 mils square pads

Since the design will port in no time to LF soldering, but I can't test it
now due to lack of time and components - can you advise which is the best
solution?

Regards
Ofer Cohen
Manager
Quality Assurance, Reliability and Production Technologies
Seabridge Ltd. - A Siemens Company


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