Cooling rates for solder reflow processesSo I asked one of the material
scientists here what the thought. Here's a quote:
  Guy the following statement is completely false:
  What happens below the liquidus temp. is of little consequence to the
grain structure.

  True:
  The cooling rate is the main determinant of the grain structure
immediately after reflow.  (Sophomore materials I class)
  Cooling rate also effects the dimensions of the overall joint
  Cooling rate influences the mechanical (brittleness, ductility) properties
  CTE must be considered so not to induce extreme stresses

  Consider terms like quenched and heat treated in your explanation, explain
the mechanical properties of coarsened (soft) and quenched (brittle)
tin-lead solders.
  You could reference "Materials Science and Engineering" by Callister or
solder related  "Solder Mechanics" by Frear

Guy Ramsey
Senior Lab Technician / Instructor
E-Mail: [log in to unmask]
Ph: (610) 362-1200 x107
Fax: (610) 362-1290


  -----Original Message-----
  From: TechNet [mailto:[log in to unmask]]On Behalf Of Ochenas, Tom
  Sent: Friday, August 24, 2001 12:52 PM
  To: [log in to unmask]
  Subject: [TN] Cooling rates for solder reflow processes




  Hi Technetters,

  My first post (although I do work with the estimable Glenn Pelkey so
perhaps my question will garner a response or two) to Technet:

  There is debate within my group about the recommended cooling rate (to
optimize grain structure and fatigue resistance of the solder joints) for
eutectic solder (incidentally we are using a convection type reflow oven)...
One camp holds that a fast cooling rate (~ 3C/sec) should be maintained to a
temp. of liquidus MINUS 50C to optimize grain structure. The other camp
contends that what happens below the liquidus temp. is of little consequence
to the grain structure. If anyone can resolve this argument with a materials
science based explanation I would be extremely grateful...

  Best Regards,
  Tom Ochenas
  Maxtek Components Corporation
  [log in to unmask]