We encountered the same problem of localized re-reflow with fine pitch parts. 
Part of the answer is to isolate the fine pitch pad from it's via by a 
physical distance, at least 0.050", and plug the via so the wave doesn't
flow up through the via and onto the fine pitch. Using small via holes also
discourages flow through the via hole onto the pad.

Harry Parkinson
Digital Equipment Corp
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From:	US2RMC::"[log in to unmask]" "MAIL-11 Daemon"  5-JUL-1995 14:22:41.73
To:	[log in to unmask]
CC:	
Subj:	Re: transition to fine pitch

Our major problems were in the areas of controlling part placement and lead 
misalignment to prevent opens during assembly.  This was mostly equipment and 
training related and essentially had to do with getting people used to the 
fragility of these leads when placing the devices (we autoplaced but VERY SMALL 
forces can move leads out of alignment (vertically or horizontally).

As we conquered this, we also had some problems on "mixed technology" PWAs with 
fine pitch on top and also wave soldered (PTH) devices.  Problem was that the 
wave solder schedule was "too hot" and tended to cause localized reflow/voiding 
under the fine pitch device leads.  These joints would look OK but have very 
little mechanical strength.  We had to lower wave solder preheats/dwells to 
overcome this.  IBM has had similar issues (I believe this was reported in a 
recent IPC Tech Review article.

Jim Maguire
Boeing Defense & Space Group
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______________________________ Reply Separator _________________________________
Subject: transition to fine pitch
Author:  [log in to unmask] at esdigate
Date:    7/5/95 8:56 AM


Hello,
Our design requirements are driving us to finer pitch technologies.  We 
currently assemble 50mil pitch devices.  My expectation is that our next 
generation of products will be 20 to 25 mil pitch.  I am interested in hearing 
war stories of people who have been through this process.  Specifically, what 
was the number one problem encountered and how was it overcome?
Thanks
Mitch Austin
Itron
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