Conductive Epoxy: Pros and Cons

Phil
Welcome to board talk. We are the Assembly Brothers, Jim Hall and Phil Zarrow, of ITM Consulting.

Today's question is from a Mr. R.H. We don't know where he's from, but it was Mr. R.H. Jim, what's his question?

Jim
His question is what are the advantages and disadvantages of electrically conductive epoxies? We have a lot of questions about this, Phil, and I know that this is something you have a deep and significant knowledge and wisdom about. So please enlighten us.

Phil
I would be glad to. I probably have this stuff running through my veins, especially ever since I cut myself. Conductive epoxies are typically polymers that contain conductive materials. Those materials can be tin, copper, graphite, gold, and silver, there's another one. The idea here is that they're used as a connectivity compound for forming electrically conductive joints, for lack of a better word, but we're not really talking about joint connections.

Typically, they're in the form of a liquid, a viscous liquid, that you would apply by either dispensing or printing. There's another type. It's a film type that we'll talk about another time. But concentrating on those liquid ones. Most commonly, they're printed on and as I said, they can be dispensed.

Some of the advantages of them. Well, first of all, one advantage is they're lead free. How about that. 

The second advantage is that they're also no clean. We're not reflowing. We're not using a flux. We're basically setting up this material, curing it, if you will. Another advantage is, typically, that cure temperature, typically, the schedules call for temperatures that are much, much lower than reflow temperatures. We've seen quite a few that cure in under 100 degrees centigrade.

So those are the advantages. Now, of course, your immediate reaction is, "Wow, wouldn't this be a fantastic material to use instead of leaded solder?" Well, yeah, maybe. But there are some limitations.

One limitation is it tends to be very, very expensive. I don't know the what the current pricing is, but I know a number of years ago, when we seriously evaluated it as a possible substitute for leaded solder, the price for the silver filled conductive epoxies was approximately ten times per gram that of tin/lead solder. So that's kind of a limitation right there.

Second of all, a lot of these epoxies work very well for steady state temperature environments particularly at extreme temperatures. However, in terms of applications where you have thermal cycling, typically, they don't do as well.

The third disadvantage is that there is sometimes an inconsistency not only from vendor to vendor in terms of the conductivity and resistivity of the materials, but even from batch to batch.

But it's a good material to know about. It's a good material to have, particularly if you're ever in a situation where you have a thermal limitation in terms of dealing with a material.

Jim
What about rework, Phil?

Phil
They could definitely be used for rework.

Jim
But can you rework them when they've been put down initially?

Phil 
Qualified, yes. There's basically two types of conductive epoxies. One is what's called "thermoset." That is a very rigid plastic. That is very, very difficult to rework.

Jim
It's like a two-part epoxy or something like that.

Phil
Exactly. Those of you who have ever had to use them for holding passives for your pick and place, that's the type that you have to basically use a chisel to get the component out.

The other type, which is thermo-plastic, is more of something where you reheat it and it becomes pliable and you can actually remove the component. It kind of resembles Tupperware, if you will. It's in a set state.

Jim
Tupperware, another scientific evaluation from Phil Zarrow. 

Phil
Yeah, well, you know, what can I say? Just common science of the common man. Well, anyway, that's about as far as I'm going today.

Jim
Another thing, getting back to the cost issue, the traditional ones that have been used successfully in aerospace and so forth are silver and gold filled. They are much more expensive. A lot of work has been done and still remains a question of whether you can do it with copper filled. Obviously, if you can it would be a lot cheaper. But I've not heard anybody who's gotten complete confidence in using a copper filled conductive epoxy.

So that's it for us today. So this is Phil and Jim, Pick and Place, the Assembly Brothers, signing off. And remember, whatever you do ...

Phil
Don't cure like my brother.

Jim
... and don't cure like my brother. And keep the kids away from the conductive epoxy jar.

The fundamental problem is that almost all surface mount components have tin bearing terminations or in fact are pure tin. Tin oxide quickly forms in heat cycling or damp heat. This is physically disruptive to the joint plus leads to high resistance and eventually open circuit. there are some Ag/epoxy products which claim to overcome this but are too expensive for mainstream SMT and also claims are exaggerated.Never use conductive epoxy unless there is absolutely no alternative.

Mike - The problem on tin is galvanic corrosion between the tin and silver which oxidizes the tin and increases joint resistance. Materials can be modified to prevent that effect. For copper surfaces you need to OSP treat or chromate treat the copper first. Copper oxide is weakly held and while adhesion to it is fine the copper oxide will delaminate from the base copper during t-cycle or with other stresses on the joint. a note there are now lower cost conductive adhesives available and all thermosets are not rigid.

Regarding the use of conductive epoxies on solder coated surfaces, the answer is NO. Although there are new formulations that are advertised as solder substitutes for PWB assembly, the most commonly used conductives carry a prohibition against use on solder or tin coatings. Components of the epoxy can react with the tin surface, causing increased electrical resistance. Please carefully evaluate any adhesive you intend to use in PWB assembly for this effect, even if it's advertised as a solder substitute.

Conductive epoxy is good for reworking QFN devices, actually, the only good way to manually place these on a board that is already populated.

Regarding the uses and applications of conductive epoxies; Is it wise to apply conductive epoxies onto surfaces or components that have materials that oxide very quickly such as copper or even 60/40 solder? What are risks of developing a high resistance at those interfaces. Are there types of conductive epoxies that are more suited to reliably attach to a tinned solder surface vs. others?