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Wave solder flow over back plate

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#68259

Wave solder flow over back plate | 20 February, 2013

A question for all you wave experts out there:

We run our wave with a constant flow over the back plate. I know it's recommended that we set the back plate so that the solder sits just at the top of the back plate with surface tension keeping it from flowing over until a PCB/pallet hits the wave and the solder cascades over. What is the basis for this? Is it just so we get the proper dwell time?

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#68265

Wave solder flow over back plate | 21 February, 2013

The backflow in the wave started to become very important with the development of no-clean/no residue fluxes. These fluxes don’t have (or at least shouldn’t have) rosin, resin or other vehicles in the formulation which encapsulate surface oxides from the wave. The way you describe in the post is the ideal setting for the backflow, but if you have a wave where these settings are not possible it’s better to have an ongoing backflow than no backflow at all at least when using no- or low residue fluxes.

Function of the back flow with no residue fluxes: A. Remove surface oxides away from the board. B. excess flux is drained away from the board by the solder surface. C. Physical peel back of solder to minimize shorts.

For more on this see papers below (written in the begin 90’s but still valid today).

http://www.interfluxusa.com/Technical/No-Residue_implementation.htm

http://www.interfluxusa.com/Technical/No-Residue_Paper.htm

I wrote this reply in a hurry if it didn’t answer your question let me know.

Patrick

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Pete B

#68268

Wave solder flow over back plate | 21 February, 2013

We agree with all of what Patrick has said. Hopefully your equipment is capable of the ideal setup you describe with no back flow until the product hits the wave and then back flow until the product emerges the other side.

One other reason to strive to achieve the ideal setup is that minimising back flow when product is not present significantly reduces dross generation (unless you have Nitrogen).

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#68277

Wave solder flow over back plate | 22 February, 2013

Pete,

It would even be better if the wave machine manufacturers would add a feature that allows the wave to drop in rpm when a board passed the wave, and slowly get the wave back to normal rpm when a new board approaches. This would be like a reset for the backflow every time boards approach the wave and would also further reduce oxidation in a regular (no nitrogen) environment.

Patrick Bruneel Interflux USA, Inc.

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#68278

Wave solder flow over back plate | 22 February, 2013

Our new Soltec Delta 3 has this feature (they call it mainwave pushup) where you can add up to 60 rpm to the pump after the leading edge passes over the wave. Unfortunately, our old Delta Wave doesn't have this feature.

Ideally we'd like to set it up with no flow over the back plate until the pallet hits it, but since we vary the lead clearance (and therefore the pump speed to get the solder at the right height) this would necessitate constant back plate height adjustments. That's not really an option for us.

The whole reason I was looking in to this was because we are having a hole fill problem on a handful of assemblies, but it doesn't sound like constant flow over the back plate would have any affect on that. Time to look elsewhere...

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#68281

Wave solder flow over back plate | 22 February, 2013

Caurbach,

Higher preheat and longer dwelltime will fix your problem.

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