Jim;

   Have you checked for vibration inside the oven at reflow zone ?
   Have you checked air flow at bottom side at reflow zone ?
   Have you checked inside the oven to certificate that there is no oven
parts that touch the second side of the board ?

I hope this help
Jorge Dourado de Santana
Maintenance / Process Engr
Microtec - Brazil



> -----Original Message-----
> From: d. terstegge [SMTP:[log in to unmask]]
> Sent: 17 de Abril de 2000 10:07
> To:   [log in to unmask]
> Subject:      Re: [TN] DOUBLE SIDED SMT REFLOW
>
> Hi Jim,
>
> I really don't understand how the melting solder at one end of an already
> soldered 0402 can result in tombstoning. I have no idea what can be
> causing this problem, but I see very little relationship with the normal
> mechanism of tombstoning. But maybe you can explain a little more ?
>
> Kind regards
>
> Daan Terstegge
> SMT Centre
> Signaal Communications
> Unclassified mail
> Personal website: http://surf.to/smtinfo
>
> >>> Jim Bergenthal/MKT/HQ/KEMET/US <[log in to unmask]> 04/17 2:46 pm
> >>>
> As you probably know, there are at least three keys to preventing
> Tombstoning of 0402's.
> These are pad design, accurate placement, and most important a solder
> profile that results in the solder at both ends becoming molten at the
> same
> time.
>
> Doing a second side reflow probably puts the last item at risk.  It could
> be very difficult to obtain simultaneous reflow of the solder on the first
> side when soldering the second.
>
> Recommendation,  put all the difficult parts to solder, like 0402's on the
> last side to be soldered.
>
> Good Luck
>
>
>
>
> "Marsico, James" <[log in to unmask]>@IPC.ORG> on 04/12/2000
> 07:40:45
> AM
>
> Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please
> respond
>       to "Marsico, James" <[log in to unmask]>
>
> Sent by:  TechNet <[log in to unmask]>
>
>
> To:   [log in to unmask]
> cc:
> Subject:  [TN] DOUBLE SIDED SMT REFLOW
>
>
> Hello TechNet:
>
> We've just ran our first production board designed with double sided SMT
> components.  Everything worked well (bottom side components actually
> stayed
> on!) except for one thing.  During the second reflow operation, some of
> the
> 0402s on the bottom tombstoned.  These all soldered fine the first time
> around.  My first thought is to raise the heat slightly, but I thought I'd
> check with you guys/girls first.  Any recommendations?
>
> Thanks,
> Jim M.
>
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