You have describe a common practice. It is not necessary to change solder
alloys. Most parts are held in place by wetting force and surface tension.
The solder wants to stay on the wetted surfaces of the board and component
and the solder wants to say in one fillet. There are a couple of formulas to
predict if the part, based on mass and termination area, will stay on the
board. Search the archives.

> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Macko, Joe @ IEC
> Sent: Friday, August 06, 2004 12:33 PM
> To: [log in to unmask]
> Subject: [TN] Double sided boards and reflow
>
> Good Morning,
>
> Many of our board designs are double sided with components on
> each side.
> When I explained the build process, a few of the design guys
> expressed concern about the 1st side seeing another reflow
> when the board goes through again for the 2nd side and asked
> me to look into the practice of using solder paste with
> different melting temperatures for the 1 st side reflowed vs.
> 2nd side reflowed.  For example, use a solder paste/alloy
> with a higher melting temperature for the 1st side reflowed
> and a lower melting temperature for the 2nd side so the 1st
> side reflowed does not reflow again (or minimize the impact
> on the 1st side) during 2nd side reflow.  Some of the guys
> remember seeing CMs changing solder paste for the 2nd side assembly.
>
> Would anyone like to share their experience with this
> practice or comment on its potential value.  thanks again.
>
> joe

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