Hi Yuan,
A high cooling rate after either rework or reflow soldering is not a good
idea. As John Guy, EMPF, has pointed out, fast cooling will result in a finer
grain structure of the solder joint. And when one performs accelerated
testing on solder attachments, one find that finer grained solder joints will
result in longer life. This to my knowledge was first reported by Roger Wild,
IBM, at an Electronics Manufacturing Seminar at China Lake, CA. The problem
is, that this apparent reliability gain fizzles to insignificance for real
product. The reason: accelerated testing takes typically less than 6 months,
real products have lives measured in years. As Guenter Grossman, ETH Zuerich,
and John Guy have previously pointed out, the grain structure of tin/lead
solder is unstable even at room temperature. Colin Lea in his book: 'A
Scientific Guide to Surface Mount Technology', states that after about 1 year
at room temperature, initially fine and coarse grained solder joints are
virtually indistinguishable-->consequence: no reliability gain.
Fast cooling rates, particularly after reflow soldering, can and have
resulted in fractured solder joints due to tensile stresses on the solder
joints caused by thermal shock warping of larger components.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]