Joe, Kester makes a solder paste that is meant to go through the reflow
twice without having to wash in between. Nearly all double-sided
assemblies have the primary side go into reflow during the secondary
reflow. It is pretty much unavoidable. You cannot see any difference in
the solder joints between side one and side two.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Macko, Joe @ IEC
Sent: Wednesday, March 28, 2007 3:34 PM
To: [log in to unmask]
Subject: Re: [TN] Double sided reflow
I have always been concerned about reflowing BGAs twice so I prefer to
reflow the side with BGAs as the 2nd side. I have always notice that the
solder joints that are subjected to 2 reflows do not look as bright and
shiny as those that only see one reflow, which I suspect is because the
1st side reflowed is heated again without new flux applied.
Best Regards,
- Joe
-----Original Message-----
From: Dehoyos, Ramon [mailto:[log in to unmask]]
Sent: Wednesday, March 28, 2007 1:28 PM
To: [log in to unmask]
Subject: Re: [TN] Double sided reflow
You are going to try out something that is reverse from what
ordinarily is done. A complete reflow has been accomplished on the big
part side and you are trying to have those parts upside down on the
second run ( glue them so they do not fall down, an extra step ). The
board goes through chip shooter and places the small parts. A lower temp
profile will reflow the small parts and barely reflow the first side.
Try the concept on a scrap board. It might work.
Regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
Sent: Wednesday, March 28, 2007 4:13 PM
To: [log in to unmask]
Subject: Re: [TN] Double sided reflow
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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