Try cooling the boards more rapidly as they exit the solder wave. 
There is an excellent paper by Rockwell-Collins (Hagge and Mather 
were the authors) in the early China Lake Soldering Seminar 
Proceedings on problems with grainy-looking or "cold" solder 
joints from vapor phase soldering.  They found that slow cooling 
allowed the grain structure to become quite coarse, which gave 
the "grainy" look.
In the paper, they reported an accelerated cooling profile which 
gave reliable joints with the desired fine grain structure, which 
gave the desired look to meet cosmetic criteria.   

If running a scrap board through the machine and using a hair 
dryer to rapidly cool these large joints gives the desired look, 
then I think some pull strength tests would settle the issue.
The paper was given at the '82 China Lake meeting, but only the 
abstract is in the proceedings.  The paper was published 
elsewhere.

--Bill Kenyon
Global Centre for Process Change, Inc.
302-652-4272/-5701 Tel/Fax

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