I ran a mini BGA (1 mm pitch) with a 2 thou clearance. 18 Mil (thou) pads with
22 thou resist.I have used 1.5 thou (mil) before, and got away with it. But it
was a small quantity and wouldn't rely on it for a volume product.
I have seen mini BGA with oblong pads, basically two circular pads joined
together. I guess that this was to aid inspection by X-ray. But at this pitch it
meant the pads were getting pretty close together.
I guess you are doing something similar to ask questions about 6 thou widths of
resist.
There's the question here, what are the implications for reliability?
If designing in a pad shape to aid inspection presents a more fundamental risk!
The BGA irregular shape pad, square, or with a prodruding bit...whatever, is
designed in to aid inspection, which is of no benifit to the product itself. The
joints long term reliability is not considered, just the short term inspection.
I think this is missing the aim of pad design.
It's not difficult to control the reflow process, lets face it that's the only
aspect really associated with the irregular pad. Regular profiling would help
reassure you that the spheres were reflowing.
Regards
Edward Brunker

______________________________ Reply Separator _________________________________
Subject: [TN] Solder Mask Clearances on dogbone vias
Author:  "TechNet E-Mail Forum." <[log in to unmask]>           "Joy  Stephen C"
<[log in to unmask]> at INTERNET
Date:    17/02/99 16:51


We are looking at the design rules for soldermask clearances on mini BGAs.
The current rules say the solder mask will be .006" greater than the nominal
hole size.
I have 2 questions.
What is the resolution capability of solder masks? Can solder masks reliably
resolve a .006" line?
What is the current capability for solder mask registration? I would like to
spec nominal FHS+.002"/side as the solder mask opening with no s/m in the
hole.
Thanks,
Steve

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