Hi Steve,

I'm honored to get a reply from such an esteemed member of the group!

We actually use the 2% formulation for that very reason - this is slightly
different from classic tombstoning - I have included some info. from the
vendor (which we implemented in our board and stencil design) as well as a
few pix of the resultant behavior in our design (can't imagine much
proprietary stuff could be pried from the top-down shot but please treat the
photos with discretion). Note component terminations... kinda look like
pontoons.














Best Regards,





Tom Ochenas
Process Engineering
Maxtek Components Corporation
2905 SW Hocken Avenue
Beaverton, OR 97005
[log in to unmask]
desk: 503-627-3078
fax:  503-627-4651




-----Original Message-----
From: Steve Gregory [mailto:[log in to unmask] <mailto:[log in to unmask]> ]
Sent: Monday, January 12, 2004 12:23 PM
To: [log in to unmask]
Subject: Re: [TN] reflowing 0402 SMT capacitors with underside terminations


Hi Tom!

There's a few articles out there that talk about "Phased Reflow Soldering".
Here's a link to one:

http://www.reed-electronics.com/semiconductor/index.asp?layout=article
<http://www.reed-electronics.com/semiconductor/index.asp?layout=article&> &
articleid=CA274087&industryid=3026

Something I've done in the past that works really well in my opinion, is to
use a 2%
silver solder paste. Sn62/Pb36/Ag2...works in the same way that Phased
Reflow
does.
The difference is pretty dramatic when it comes to eliminating tombstones!

-Steve Gregory-


> Greetings All,
>
> I am having my first experience working with an assembly design that
> involves 0402 capacitors with underside solderable terminations (vice
> wrap-around end terminations)... The reflow process seems to cause the
> components to tilt and skew to highly unacceptable levels. Physically it
> looks as if the caps floated to the top of the molten solder volume (as if
> on pontoons) then slid down one way or the other. The vendor made a few
> recommendations including migration to a thinner (4 mil) stencil as well
as
> some pad design and nitrogen tweaks and... Even after implementing these
> recommendations we are experiencing the same problems, just to a slightly
> lesser (although still unacceptable) degree... Does anyone out there have
> any additional thoughts on how to remedy this sort of issue?
>
> We do build another part [successfully] with a similar style component,
but
> it is a 1206 form factor and is perhaps massive enough to keep from
floating
> up the way the 0402 caps do...
>
> Your collective wisdom is greatly appreciated!
>
> Best Regards,
>
> Tom Ochenas
> Process Engineering
> Maxtek Components Corporation
> 2905 SW Hocken Avenue
> Beaverton, OR 97005
> [log in to unmask]
> desk: 503-627-3078
> fax:  503-627-4651
>


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