It can be done. The solder joint is much weaker, and generally even though
the temperatures are not high enough to qualify as "a good process temp" the
joints will be melting or very close to it. So, if the parts are heavy they
can disturb the solder joint. This is a defect. Better to go all the way
into reflow and relieve any stress in the joint. If the parts are heavy
enough to actually fall off you need something like Loctite BGA
corner-bonder. It is pretty cool stuff.

We did something like this once. I bet some three leg SMT control pots would
fall off on second side reflow. Had to buy my co-workers lunch. They stayed
and looked perfect. 


****************************************

 New address & phone numbers below

Guy Ramsey

R&D Assembly
1660 East Race Street
Allentown PA 18109-9580
(610) 443-2299
FAX (610) 443-1622

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
Sent: Tuesday, March 27, 2007 3:43 PM
To: [log in to unmask]
Subject: [TN] Double sided reflow

Dear colleagues,

let's take a board with SMT on both sides. Let's say that for particular
reasons we have to solder the more thermally demanding side first.

The question is: when reflowing the second side, the one that requires lower
temps because the components are less demanding, may the reflow recipe be
colder? Will this affect the other side, that will reflow again, but at
lower temps and for not as long? I mean the IMC is already formed; is there
anything else that could go wrong?

Or, to generalize, a PCB with SMT parts that were properly reflowed once,
can it be reflowed again at lower temperatures, without adverse effects on
the metallurgy of the joints?

Thanks for your help,

Ioan

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