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Conformal over No-Clean

Tom B.

#19647

Conformal over No-Clean | 23 April, 2002

Howdy Netter's

I was talking with my Paste and Flux Supplier about concerns of conforaml coating over "No-Clean" flux.

Supplier informs me that this should not be a problem, but I wanted to poll the industry.

1. Does anyone here conformal coat over No-Clean? If so what do you use Acrylic, Urethane, Silicone or Blend?

2. Have you had any major problems doing so? No adhesion or Low adhesion?

3. Does anyone have long term reliability data of this process?

Thanks Everyone!

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#19678

Conformal over No-Clean | 24 April, 2002

We do not conformal coat [CC] over NC flux residues. [Not to say that you cannot do it if you want to do it.] We know that the ability to do this properly depends on the NC flux res and the CC. Some work well and others work like a POS. 'The proof is in the pudding.'

We have talked about this previously, although I�m not sure about the level of detail, maybe search the fine SMTnet Archives.

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#19692

Conformal over No-Clean | 26 April, 2002

We conformally coat over NC solder paste. No problems. We're using a platinum catalyzed addition cure silicone. The only concern with this conformal coating is cure inhibition. You must test the coating over the residues to be sure there is no inhibition (coating won't cure). If there is no inhibition, absolutely no problem.

We usually have 50 - 90 psi lapshear strength, depending on the soldermask. We're looking at changing masks and there was a difference in the lap shear values dependant on the mask.

Again, we've been using NC paste for years with no problems directly attributed to the flux residues.

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Jim M.

#19693

Conformal over No-Clean | 26 April, 2002

I worked for a telecom company that had sites in the deserts, jungles, mountains and regular built up area's. All boards were built using Aim 291/293 No clean solder paste. (all boards built to Bellcore standard)

The boards were coated with Dymax urethane confroaml coat for obvious reasons.No adhesion problems. In the five years at the telecom company i did not see any boards returned for confromal coat adhesion problems. There were units operating in the field for 10 or more years that were built with no-clean solder paste, coated with urethane coating.

No-clean flux off the wave solder is another problem. If the flux sprayer was turned up to high on the wave solder. the bottom of the board was saturated in no clean flux.Vague white marks could be seen on the edge of the boards where the no-clean flux collected and dripped off.(i turned down the flux sprayer level without taking away from solder quality).If the majority of this built up of no-clean flux residue were not removed (a gentle wipe with a cotton swab soaked in alcohol), bubbles would form in the conformal coating after heat curing the urethane conformal coat. The conformal coat adhered nicely to the area's without the excess no-clean flux residue.

Verification for thickness and adhesion was performed on the conformal coat as per IPC-650 test methods.

If more info is required, post your E-mail address or phone number and i will get back to ya.Hope this helps.

Jim M.

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