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Subject:
From:
Jeff Deeney <[log in to unmask]>
Date:
Tue, 29 Aug 95 10:07:25 MDT
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Good summary of the reflow issues to watch out for.  Another issue with
the conversion of DIL parts to SMT is the lead plating at the point
where the DIL lead was cut.  Most often the lead is solder plated prior
to cut and form.  This results in exposed copper or alloy on the tip of
the lead.  Most often this surface will not wet, preventing a good
fillet on the tip of the lead.  Depending on the quality of the
remaining fillets, this may or may not be an issue.

-Jeff Deeney-  [log in to unmask]
Hewlett Packard Company
Computer Interconnect Operation
Fort Collins, Colorado


Brendan Mc Cormack wrote: 

> Take a look at some of the surface mountable DIP switches on the market
> (from Aries, C&K, etc.).  These are often no more than the through hole   
> DIL version with the leads cropped and bent outwards.  It should be quite
> easy to bend the leads of a DIL component and cut them to a suitable
> length, thus forming a gull wing lead type rather than a butt lead.
> 
> There are, however, some issues to watch out for..
> 
> - The formed leads must be co-planar, i.e. all leads should contact the
> board surface.  The effect of poor coplanarity is well understood and
> relates more to fine pitch technology than this much larger pitch
> situation, but don't forget about it.
> 
> - The DIL component may not be compatible with the reflow process. The 
> plastic packages may be damaged by the high temperatures and soaks. 
> Remember that these components normally sit on the top-side during wave
> soldering and are thermally isolated from the heat source, (i.e. the solder
> wave). 
> 
> - If you use a ceramic DIP package you may also have difficulty with reflow
> soldering as the component will act as a mini heat-sink.  I know one
> company where the board was designed for reflow soldering of the CERDIP
> and they had so many soldering problems that they now hand solder these
> components in place at end of line.
> 
> - The solder pad size that you use is, of course, pretty important.  The pad
> size will depend on the shape of the formed lead.  You need to ensure that
> there is space behind the lead for a good heel fillet.  You may want to do
> some calculations on pad size versus solder paste volume to ensure that
> there is enough solder to form the joint.  You may find that you need a
> pretty thick stencil to get sufficient solder in each joint.
> 
> - As for reliability data on this type of joint we have done some
> basic testing, but I suggest that you call some of the DIP switch
> manufacturers and get them to convince you about the reliability of their
> connections !
> 
> Hope this helps...  
> 
> 
> Brendan Mc Cormack
> Senior Interconnect Engineer
> AMT Ireland
> Electronics Manufacturing Centre
> University of Limerick
> Limerick
> Ireland
> 
> Tel. +353 61 331588
> Fax. +353 61 330316



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