Hello Phil,
This is an interesting challenge. My suggestion to you would be to
combine the single sided board with the double sided board as you
suggested below without the ABAB (I call it flip flop) design. Keep the
single sided board components on the same side as the double sided board
with the small components. The new challenge is to keep the heavier
components on the boards when reflowing the side with the smaller
components. You can achieve this by the following:
- Use a lead-free solder paste such as SAC305 to solder the heavier
components
- Use the standard Sn/Pb solder paste to solder the smaller components
and the components on the single sided board
This should work for this application as the second side reflow
(assuming 210C Tmax) will not reach the reflow temperature of the
lead-free solder (assuming 221C Tmin). The product will not qualify as a
ROHS compliant product but it may not be a requirement at this time!
Hope this helps,
Paymon Sani
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phillip Bavaro
Sent: March 28, 2007 5:13 PM
To: [log in to unmask]
Subject: Re: [TN] Double sided reflow
I have started to make my reflow profile measurement tools by combining
the
single side board and the double sided board together.
The advantage is that I get to measure the coldest connection (largest
mass
on the leading edge of the double sided board) and the warmest
connection
(smallest passive on the trailing edge of the single sided board)
Reducing the set up time at reflow is accomplished because you don't
have
any oven changeover between the two sides of the board. The same
profile
works for both sides.
Having said that, I should mention that I typically have ABAB panels so
half my BGAs see two reflows and the other half only one reflow.
Bright and shiny solder has not been a requirement for quite a
while.....especially since most have gotten rid of nitrogen atmosphere
reflow ovens and have accepted the oxidation that occurs the second time
through.
Phil
At 04:27 PM 3/28/2007 -0400, Dehoyos, Ramon wrote:
>
>
> 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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