Conductive Epoxy: Pros and Cons

Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting. Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes

Phil Zarrow With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes. Jim Hall A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript

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.

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.

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