Wave solder flow over back plate

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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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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