Phil,
you are right if we consider a somewhat balanced top-bottom component distribution.
What I am talking about is the case where the heavy stuf, BGAs, heatsinks, transformers, etc, are all on the first side that is soldered, whereas on the second side there are small discretes only, e.g. 0603 and 1206. In this case you can solder the discretes at lower temperatures, but these temperatures are not enough to properly reflow the joints of the first side.
Don't ask me why would I do such a stupid thing, reflow the havily loaded side before the light one. I am simply trying to brainstorm on how to reduce setup times at the reflow oven. And the basic rule of brainstorming says that nothing is stupid a priori.
Thanks,
Ioan
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Phillip Bavaro
Sent: Wednesday, March 28, 2007 3:45 PM
To: [log in to unmask]
Subject: Re: [TN] Double sided reflow
Ioan,
Someone can't see the forest because the trees are in the way....lol.
The second side reflow is actually more thermally demanding because it has
to account for both sides of the board with respect to mass. Set your
reflow profile up for the double sided reflow only and use it for both sides.
Phil
At 03:42 PM 3/27/2007 -0400, Tempea, Ioan wrote:
>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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