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May 7, 2007

Wave Solder fluxing

What is the maximum preheat temperature for alcohol based flux used in wave process? We are finding dirty sticky material on the bottom of the boards after the wave, can you offer a suggestion to this problem?



M.K.

Experts Comments

Two reasons for the preheat are to remove carrier solvent, and reduce thermal shock as the PCB hits the wave.

IPA boils at 82C, but volatilizes well before this. Wave temperature is around 250C, but it's not a good thing to subject components to this temperature for a long period!

Of course, another reason is to prepare (clean) the PCB to facilitate solder wetting. The Arrhenius equation militates towards high temperature, but preservation of activator militates against high temperature.

Against this background, and experience, we recommend from 80 - 100C topside, depending on conveyor speed. But, as you can see, it's a question of balance. And there are other factors. For example. a large heavy PCB will demand higher temperature as it will behave as a heat sink.

Post soldering residue may reflect poor (insufficient) preheat that leaves unreacted activator. But another source is simply over application of flux. Increased wave activity can help as it washes activator/residue from the PCB.

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Doug Dixon
Global Marketing Director
Henkel Electronics
Mr. Dixon has been in the electronics field for over twenty years and is the Global Marketing Director with the electronics group of Henkel. Prior to joining Henkel, he worked for Raytheon, Camalot Systems, and Universal Instruments.

This may have more to do with the choice of flux than any particular wave soldering parameter. When you identify a dirty, sticky material on the bottom of boards after the wave, this is usually simply the residue that is left behind from some flux types.

All no-clean fluxes are not created equally, and some no-clean fluxes include a small proportion of rosin (the sticky stuff) in the formula to assist with the heat stability of the flux as the board reaches wave soldering temperatures. (Among liquid flux activators, rosin is among the best at retaining some level of chemical activity at the elevated temperatures during contact with the wave.)

It is highly likely that the sticky material on your boards is just the rosin portion of the residue that remains after soldering.

It is almost certainly not a reliability issue but if your customer is wanting to see this remedied, you can either switch to a non-rosin (or possibly a lower-rosin) liquid flux or potentially clean the boards after soldering.

If you choose the path of a rosin-free no-clean flux, note that the lack of rosin may impact solderability performance and this may not be advised if this is a lead-free assembly. Leaded wave soldering can more easily be accomplished with a rosin-free no-clean flux than a lead-free application can.

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Brian Smith
General Manager - Electronic Assembly Americas
DEK International
Mr. Smith has been supporting customers in the electronics assembly industry since 1994. His expertise is focused on solder paste printing and reducing soldering defects. He holds a BS in Chemical Engineering and an MBA in Marketing. He has authored several papers in trade magazines and at industry conferences. He is an SMTA Certified Process Engineer.

As often as not, the sticky material is excess flux residues from the soldering process. The best option is to remove these residues before they cause problems. Aqueous cleaning systems, benchtop cleaning with a brush, or vapor degreasing with a nonflammable solvent are your best options, depending upon the equipment you have and the volume of boards expected.

Here at Micro Care, our most popular recommendation is to clean with our Flux Remover C, a nonflammable, fast-drying solvent that's safe for people, the planet and the budget.

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Mike Jones
Vice President
Micro Care
Mr. Jones is an electronics cleaning and stencil printing specialist. Averaging over one hundred days a year on the road, Mike visits SMT production sites and circuit board repair facilities in every corner of the globe, helping engineers and technicians work through the complex trade-offs today's demanding electronics require.
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