Optimization of the Soldering Process With ECAs in Electronic Equipment: Characterization Measurement and Experimental Design

IEEE Xplore : With the introduction of the European Directives restrictions of hazardous substances, electrically conductive adhesives (ECAs) have received great attention in the field of electronics as a possible replacement of the traditional tin-lead soldering technology. So, in this new context, the analysis and the characterization of these alternative materials represent a fundamental topic, above all, from the industrial point of view. Nevertheless, studies on ECAs have rarely been reported in the literature, but more recently research has started to focus on this specific topic.

After a comparative assessment concerning soldering materials, this paper focuses attention on their characterization through measurement in order to verify the electrical behavior of the isotropic silver conductive adhesive.

In addition, since the soldering process is affected by a large number of variables such as the thickness of the conductive adhesive film, radial velocity, and curing temperature, the optimal selection of the factors is carried out through experimental design theory and the dual-response approach by means of generalized linear models.

In this paper, the experimental and comparative studies on soldering made up of epoxy adhesives are carried out, in particular, the adhesives constituted by metallic particles (silver), normally in the form of flakes, in a polymer matrix are considered. The novelty of the kind of adhesive considered is the Ag filler loadings of 50-65% by volume. At these loadings, the materials achieve the percolation threshold and are electrically conductive in all directions after the materials are cured. Two different types of conductive adhesives, characterized by different chemical structures and compositions, are experimented and tested.

Then, since the lead-free soldering process is characterized by several critical factors, a statistical approach is used to optimize this process. Experimental results obtained by testing samples with ECA materials prove the validity of this paper.

The value of this paper is in the application of a statistical approach to these adhesive materials in order to achieve the optimization of the soldering process with a small number of treatment combinations, satisfying at the same time the stringent requirements and achieving robust electrical interconnections.

Can Conductive Epoxies Replace Solder?

Circuitmart?: It's a question that's been asked many times over the ages, especially since the original advent of RoHS appearing on the horizon. It's something we've touched on before in one of our other diatribes on conductive epoxy. Fundamentally yeah, you could consider it, but from a practical standpoint, not really.

This was originally examined back in the early days of getting ready for RoHS. Motorola Schaumberg IL, did some major investigation into this at the time and they decided absolutely no. There's a number of limitations with conductive epoxies regarding how they fare in terms of robustness compared to tin lead solder. Jim, do you want to elaborate on that a little bit?

Jim

Probably the most predominant concern is that the conductivity of the material degrades over time. That is, the net contact resistance goes up. So, for many applications that can eventually damage your signal flow through your circuit. That's the biggest one.

Other issues that are raised are difficulties reworking, and some of the structural properties. Depending upon what your failure modes are, in some cases the epoxies can be more robust. But in others, cycling or vibration and so forth, they are less robust.

The final issue is cost. Ca-ching, ca-ching. Yeah, most of the ones that work successfully are filled with silver or gold. Silver is just plain expensive. Phil, did that Motorola study have a rough estimate, even back in those days when silver was cheap?

Phil

Yeah, the study was done at least 12 or 15 years ago and the cost of silver-filled conductive epoxy was at that point about 10 times the cost per gram of tin lead solder. Can you imagine what gold-filled epoxies were going for?

Jim

One of the possibilities that has been tested many times is a copper-filled epoxies, but nobody has mastered those to achieve the reliability level of silver-filled.

Phil

So, one the surface, the idea looks really good. It's a no clean, it's lead-free, you don't have to worry about VOCs, no cadmium, chromium or mercury.

But when you start cross sectioning, at least in its present state it's not a great idea. But remember, things tend to change and improve, so, you never know. And with that, we say thank you for tuning in to Board Talk and this is Jim Hall and Phil Zarrow, The Assembly Brothers, saying whatever you do –

Jim

Don't solder like my brother

Phil

And don't solder like my brother. And quit snorting the conductive epoxies, too. It's not good for you.

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Comments

Interesting talk on conductive epoxies. We have looked at this for many years, we used high volume Ag filled epoxies from the 1970's. This has reduced over the years due to changes in chip packaging/hybrid assembly. We saw no significant problems with using Ag epoxies even in extended life trials. Problem is price. Price problem is embarrassment. Majority of conductive epoxies are used in mil/hi-rel applications.

The few main suppliers have milked this business since 1960's. I can only use the last data I have (from 2001) but at that time the main US supplier was selling their Ag epoxy for about $1.75 per gram, main Asian source US$1.35 per gram. Both said these were competitive prices! At that time I was buying high purity electronic grade epoxies for about $0.003 per gram and Ag flake or powder for $0.15 per gram.

I spoke often to the main manufacturers to try to get them to reduce pricing to an acceptable level. I compared total cost of ownership of 96S solder or Ag epoxy as solder replacement for lead attach. The process savings using epoxy was enormous. No fluxing, reflow,cleaning just relatively low temperature curing.

Bottom line was we were willing to pay over $0.50 per gram. Volume would have been huge. No main supplier was interested. 4 times we had small epoxy manufacturers OEM the material to our specifications.

Each supplier either went out of business, was taken over or had "changes in strategic direction" after a few months manufacturing. Basically we gave up and looked at other ways to increase soldering efficiencies. I don't think we will ever have reasonable pricing in this sector due to the high historical margins for the suppliers.

Allan Dowie, BI Technologies Ltd