Here are two methods of supporting boards during wave soldering all that is
needed is a design for manufacture approach and it can save money and its better
than a jig. It is is response to a previous question on Technet.

Even if a jig supports a board on all four sides the board will still tend to
sag in the centre as the wave sucks it down. This will always increase the
possibility of shorting in the centre and back portion of the board. A simple
way of reducing this is to use a wire fixed and recessed on the underside of the
solder jig. This will support the board during processing and minimise
distortion. Any support wires like wave supports need to be considered at the
PCB (Printed Circuit Board) design stage. 

During layout a no go area should be placed along the center of the board on its
longest dimension. This no go area should be a minimum of 2-3mm wide and should
not allow any placement of component termination points or component bodies on
the base of the board. It is perfectly acceptable for circuit tracking to be
placed across this area as the tracking will be protected by the solder mask.
Ideally there is a line placed down the centre of the base of the board and a
small line on the top side at the front and back of the board. This can be put
in as part of the legend or if legend is not used reversed into the solder mask.
The mask marks aid set up of the support when used in the wave as opposed to on
the jig.

This method does provide much better support to large boards and they do not
sag. It does not however get ride of jigs so an alternative is the use of ski
slopes or cheese wires. The cheese wire is either passed through the whole
machine or just positioned at the wave. The wire may be fixed or set up to be
adjustable across the wave to accommodate different design, some suppliers have
mechanised support which does put the price up its better to do it yourself.

The wire will have support pillars on the fluxer, preheat and solder wave and
then have a small piston providing a tension to the wire to keep it rigid. One
of the problems with the wire approach is the oxides that will build up on the
wire as the solder flows passed it in the leading edge of the wave. This can
over a couple of hours build up and then become detached and leave dross shorts
on the board, in nitrogen waves this is not an issue. 

The alternative ski slope is a titanium blade placed in the wave and fixed in
position on the back and front of the wave. The blade will be scalloped in the
chip and main wave so the blade is lower than the wave as the board enters the
wave this prevents oxides building up to fast and causing dross shorts on the
base of the board. They will still build up in an air environment but can be
knocked off ever couple of hours.

It is fair to say that if more consideration was given at the front end of the
design cycle many products would not require traditional jigs. 

Any more information may be obtained at my Wave Soldering Workshop at Nepcon
West or one of my video tapes on design or wave soldering.

Further information may be obtained from my good friends Bill Down at
Electrovert or Marc Dalderup Soltec in the USA.

Have a great day !!!!!!!!

(Part of an article to be published by Peter Swanson and Bob Willis some time in
the not too distant future ??????)

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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