There are lots of factors which will effect solder shorts on SOIC device during
wave solder. Solder thieves do improve yield by pulling the extra solder from
the proceeding pads. Here are the facts the solder thieves must be three times
minimum bigger than the previous active pad as the pads have a pin on it and
increase the surface tension of the solder between the last two active pins.
Think about it as a tug of war if there is more strength on the active pins then
they win and a short remains.

The reason that many people see very few shorts on of there boards may be
superior machine or set up but it is more likely to be correctly designed SOIC
pads. The most common pad width for SOIC 0.050" parts is 0.026" wide which is
too large. Dropping the width down to 0.020" will virtually eliminate the
problem of solder shorts on a standard process.

SOIC devices are always mounted parallel to the direction of travel so that both
sides of the leads see the wave at the same time to provide the ideal drainage.
The best way of seeing this in action is with a glass plate. Mount some parts
with adhesive on the glass plate run it through the wave and watch the direction
of flow of solder round the parts. Try getting two copper repair foil pads and
place one at the end of the SOIC with a little glue and watch them work.

But what happens in the real world, some times the wave is not perfect, the wave
height drop a couple of mm, the spray fluxed sensors do not see the whole board,
the flux being used is not the best performer. Answer solder thieves can open up
the process window again and make a well designed board very robust during
manufacture that is what DFM is all about. Many people eliminate the problem by
not having any active parts on the base of the board just chips which most
people should be able to solder all day long. It gets a little more fun
processing QFP devices on a 0.025" pitch which are mounted at 45 deg to wave
contact and the solder thiefs get a little bigger.

My company has been running a wave solder workshop every month for the last
eight years on behalf of Electrovert UK in Henley On Thames. I have worked with
Soltect, Sensby, Blundell, Hollis and all the other equipment over the years and
can only say that the extra pads make life a little easier. 

It is not time consuming to design in solder thiefs they should be part of the
libary footprint for ewave solder with a different identification for a reflow
footprint. In contract it is just difficult asking for things to be changed
after the event. They should be there to start off with.

Wave soldering to many is not eligant but it gets the job done and I continue to
learn many tricks to obtain near to zero defect.

I will be running a workshop on Wave Soldering at Nepcon West 1997. I will be
offering a workshop to the IPC for thief event in March and look forward to
working with any design engineers to get the best out of any wave solder
process.


Bob Willis
Process Engineering Consultant
Electronic Presentation Services
2 Fourth Avenue, Chelmsford, Essex CM1 4HA. England.
Tel: (44) 01245 351502
Fax: (44) 01245 496123
Home Page: http://ourworld.compuserve.com/homepages/Bwillis
Email: [log in to unmask]

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