Tom,

Digital ran a set of experiments on via in lands and tangential lands a few 
years ago and established the following design rules.

1. Preferred design practice calls for a 0.013" of soldermask between the via
land and the SMT land, 0.010" minimum. (that's a dispersion etch length of 
(0.013")

2. Tangential via lands that has a slight overlap with the SMT land must have a 
maximum via hole size of 0.020" to prevent solder paste from filling the via 
and starving the SMT land; your 0.015" hole size exceeds this requirement. 

3. Re-reflow (when the wave solder process reflows the SMT solder joint) can 
be prevented by keeping the dispersion etch/soldermask bridge at least 0.030"
for SMT devices a lead pitch of 0.025" or less.

4. SMT pads with holes is not recommended because of the drill sizes and Land
If you have a FAX number, I'll be happy to send you a copy of the etch 
dispersion rules.

Harry Parkinson
Digital Equipment Corporation
603-884-6760
Internet   [log in to unmask]

From:	US2RMC::"[log in to unmask]" "MAIL-11 Daemon" 27-JUL-1995 13:22:50.09
To:	[log in to unmask]
CC:	
Subj:	Unidentified subject!



	I am looking for any information concerning the minimum distance between
	a via to smt land (1206, 0805, and 0603). Currently we are using a 
        .015 hole / .030 pad non-tented via. Is anyone out there burying the
	via in the center of the land? The wave side is not as much of a concern
        as the reflow side. We are planning to run some test boards with several
	configurations. I would appreciate any feed back.


	Tom Vaillancourt
	Philips Consumer Electronics Co.
	615-521-3482
	Internet: [log in to unmask]




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