Alejandro,

        It's more simple than what it looks. But in any case you might want
to take into account some considerations when designing or running a product
using solder paste on both sides of the board:

    1. Try to keep all the big components for the second pass, pretty much
boards are designed to have all the big devices on the primary side or top
side of the board. Most of the times on double reflow you try to run the
side with smaller components first and then the side with more complicated
devices.

    2. Despite of what I said, you can still if needed run big or heavy
components on the second pass. I've ran QFPs, PLCC, BGAs and connectors with
no problems, one thing though is that you have to maintain a ratio of
solderable land area in square inches and component weight in grams
according to the following: Formula = weight of component in grams/total pad
mating area in square inches. The resulting grams per sq. in. must be less
than or equal to 30.

    3. Any convection reflow oven works alright with double reflow
processes. The only concern here would be the ability of running the product
on a conveyor instead on the mesh, you don't want the components rubbing the
belt when the solder hits the melting point. The other consideration would
be component heights, although most of the ovens have enough clearance.

    4. Finally, the last design recommendation, if you get the chance to say
a word. All SMT active devices and all PTH should be located on the topside
and only SMT passives on the bottom side.


Good luck


Jorge Rodriguez
Process Engineer
Varian Inc



-----Original Message-----
From: Becerra Alejandro [mailto:[log in to unmask]]
Sent: Thursday, November 08, 2001 4:52 PM
To: [log in to unmask]
Subject: [TN] Information about Double Reflow



Hello to All,
My company is considering to start using the process of double reflow.
Where can I get information about this process?
What are the main factors that could affect this process.
Is it possible to perform it in conventional reflow ovens (e.g. BTU Paragon
150)?
Thanks,

Alejandro Becerra

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