Hi Kelvin -
There are a number of things that can affect the shininess of the joint.
Most of the time, in my experience, shininess of the joint is related to grain
structure - the finer the grain structure of the joint the shinier the
appearance and, accordingly, the grainier the joint, the duller the joint.
Finer grain growth can be promoted by quicker cooling of the joint,
particularly once you are past the solidus temperature (183 C in 63/37 and 179
C in 2% Ag).  This is accomplished with the cooling modules on the reflow
equipment.  Some machines, particularly older or lesser expensive models are
only equipped with fans on the off-load and these are blowing ambient temp air
on the board.  Cooling modules incorporate heat exchanger systems and blow
cooled air on the board, directly after reflow and thus expedite cooling.  Be
careful not to cool too rapidly - generally I suggest keeping your speed limit
during cooling the same as you used during heating.  If you heat at a max of 4
deg. C per second, you should cool no faster than -4 deg. C per sec.

Regards,
Phil Zarrow
ITM, Inc.
Durham, NH  USA
www.ITM-SMT.com

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