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December 6, 2017

Epoxy Bonding Problems on RoHS Boards

We are having problems with epoxy bonding components to RoHS circuit boards.

We never had this problem on non RoHS boards. We're sure the epoxy has not changed. Is there something about RoHS compliant boards that makes them more difficult to bond to?

The boards look sparkling clean to the eye.

I. S.

Experts Comments

This is related to the surface energy of the solder resist.

High surface energy is good, low surface energy is bad.

For example a non stick frying pan is low surface energy, not easy to paint!

There are many different types of solder resists and the surface energy varies with all of them. Over curing solder resist tends to lower the surface energy as silicones can be forced to the surface.

I suggest that you get a set of dyne pens, these are inks of a known surface tension. The point at which they wet will indicate the surface energy of the board.

As a guide 36 to 38 dynes is average, a no clean assembled board would be about 32 dynes and anything less than this bad.

These levels are what we (HumiSeal) consider good, bad and average for conformal coating purposes. You have probably moved PCB manufacturer or are now using a different solder resist to the one you are used to.

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Chris Palin
European Manager
HumiSeal
Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.

It all depends on the composition of the new board material vs. the old, as well as the surface tension of the new boards vs. the old.

There are several factors which could contribute to this adhesion problem: Surface tensions, "blooming" of trace contaminants would be at the top of my list.

I'll be very happy to talk with the user to offer some free consulting.

Jim Williams
Chairman
Polyonics, Inc.
Jim Willimas is a PhD Chemist in Polymers and Materials Science. He specialize in printing, cleaning, inks, and coatings used in electronics manufacturng operations. Williams has more than 30 years experience.

Depends on the surfaces you are bonding to.

If the bonding is to the green soldermask, there could be a change in the finish mechanically on the solder mask. We will specify matte finish to help with mechanical adhesion of adhesives.

If bonding to metal plating, the are several solder finishes which are RoHS compliant. One of which would be OSP, but since you used the word shiny, I'd assume there is a SAC HASL finish on the PCB and that should not interfere with your bonding material.

If you are bonding to metal surface. You might prep the surface with etch or alchohol, to ensure the surface is prepped and clean as it looks prior to epoxy application.

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Rodney Miller
Capital Equipment Operations Manager
Specialty Coating Systems
Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.

Even if the boards look clean, you should do a Control test to investigate the surface energy of the boards by using dyne pens or a contact angle test, where you measure the contact angle of a droplet to see how much the droplet beads up or wets out onto the surface.

I suspect when you do the droplet test, you'll find the beads are quite round and do not wet, indicating low surface energy and poor bondability.

Next, as an experiment you should do a suitable gas plasma treatment on some boards (usually using argon and/or oxygen gas), and repeat the surface energy tests. I think you'll see the drops wet out quite a bit at that point.

That's because the plasma treatment activates the surface of the board and improves bondability and adhesion. (Using this technique, one can even make non-stick surfaces like Teflon wettable and bondable.)

Finally, you can bond actual parts to both Control and plasma treated boards to confirm the improvement in adhesion.

If you have access to a plasma system in your facility, we can advise you on how to set up your DOE. Once you confirm increasing surface energy with plasma resolves your bonding issue, the final step is to figure out how to implement the treatment into your production environment.

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Scott D. Szymanski
Global Marketing Manager
Nordson MARCH
Mr. Szymanski works to expand strategic alliances, strengthen partnerships with equipment suppliers, and develop future product offerings tailored to the semiconductor market.
NOTE: Mr. Szymanski is no longer working at Nordson MARCH
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