Alpha's Revolutionary Approach to Low-Temp Soldering

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In an interview with I-Connect007, Morgana Ribas discusses the new technology on low-temperature solder that she has developed at Alpha Assembly Solutions.

Patty Goldman: Morgana, before we get into the details of this new low-temp solder, please tell me a bit about yourself and your background at Alpha.

Morgana Ribas: I am based in India and manage the Metals Technology Group for Alpha. I have been working as a metallurgical engineer for over 20 years and spent several years as a researcher at Tohoku University in Japan. As such, I have always been focused on alloy and materials development and analysis.

Goldman: Now to the good stuff: Alpha’s new low-temp solder product, HRL1. How low of a temperature are we talking about?

Ribas: As you probably know, Alpha has actually been working on low-temperature soldering using tin-bismuth alloys for several years; we have launched other products before, but we think our latest product is going to revolutionize the use of low-temperature soldering.

This product is below SAC305 reflow profile. There are several drivers for using low temperature solders, but the main one is cost reduction. The cost reduction can be from reducing the material cost, or from using cheaper substrates and cheaper packages.

Goldman: This is due to many solders needing special substrates because they are so hot, correct?

Ribas: Yes, the reflow temperature is the highest. There is significant cost reduction if we reduce the peak reflow temperature below 200°C. There is also a very strong technical requirement that if we keep it below 200°C we are going to reduce dynamic warpage of the flip-chip ball grid array. We can reduce the dynamic warpage and with that we can increase yields. Then really the reflow peak temperature has to be below 200°C.

Goldman: Why is there such a need for a low-temperature process?

Ribas: Low temperature solder pastes are especially well-suited for consumer products such as white goods. We do sell eutectic tin-bismuth solder paste right now, but this eutectic tin-bismuth solder paste can only be used for certain applications that do not require a high mechanical reliability or high drop-shock performance. If there is a requirement for higher strength, such as for portable electronics, then the material needs to have high mechanical shock strength. All portable devices like cellphones, tablets, and laptops are very sensitive because one drop and you may completely lose the functionality of the device. There is a high risk in using materials that do not fulfill this requirement, hence the need for new low-temperature soldering materials.

Goldman: I'm sure that requirement goes beyond some consumer products and spreads into things like automobile and military and everything else.

Ribas: The iNEMI roadmap predicts that low temperature solder paste use will grow from the current 1% to 20% by 2027. This is probably the largest growth that we are going to see in one type of soldering alloy.

Goldman: If it is low cost and it has better shock resistance, then everyone will want it, whether they need it or not, because they will want the best properties possible. I can see how that would grow.

Ribas: There is one additional requirement and that's the environmental requirement. In the Paris Agreement, several governments have committed to reduce greenhouse emissions and are therefore trying to push the corporations in each country to fulfill their corporate social responsibility by reducing greenhouse emissions. There are several ways of doing that, like finding ways to mitigate the negative impacts on the environment and reducing energy consumption that directly reduces greenhouse emissions.

By reducing the reflow temperature, we are going to have a considerable reduction in CO2 and more efficient use of materials, and that's why we are proposing using low-temperature soldering materials that can be more efficient and can considerably reduce the cost for the equipment manufacturers.

Goldman: What are the materials in this new solder?

Ribas: Our alloy has a proprietary composition, but it is basically a tin-bismuth, no-lead package solder alloy with about 2–8% additives. We have seen these additives give exactly the right combination of melting behavior, mechanical properties, drop shock performance, and thermal cycling performance.

Goldman: What, if any, are the downsides to it?

Ribas: The downside is that the industry continues evolving and new designs of microprocessors will come every year, and these requirements tend to grow. It's important for the customers to associate themselves with soldering material suppliers like Alpha that have a long tradition of R&D innovation and continue to research new materials to be ready for the next generation of low-temperature soldering materials.

Goldman: Well, I don't see that as a downside. (Laughs)

Ribas: The downside for me as a formulator is that I must continue working on it! But, the other downside is that tin-bismuth is a brittle material. It has to be used with care because the brittleness is what gives it strength, and strength is a desirable property, but it has to come with a certain minimum level of ductility. That is very important. It's already known that by reducing bismuth content from a no-lead package, one can actually increase the elongation property of the material; however, we show that reducing only the bismuth content is not enough. By reducing the bismuth, we still cannot achieve all the desired properties of that. We must be very careful.

Goldman: You have to get it just right.

Ribas: Yes. One important thing is that for the flip-chip ball grid array, we use SAC balls, and these SAC balls must be reflowed at lower peak reflow temperatures. The SAC starts melting at 217°C—that's the solidus temperature—but the reflow profile is going to be maybe 195°C. How is this possible? That is the main question. Alpha’s HRL1 solder alloy has a very high degree liquid phase even before it touches its liquidus temperature, and then it increases the inter-diffusion with the SAC during the reflow, maximizing the formation of this mixed area between the SAC and the low-temperature solder, and giving the desired strength.

When you have something that can reduce your cost, retain your reliability levels, and allow a company to advance green initiatives in the electronics industry while minimizing their environmental impact, it’s a win-win situation.

Goldman: That's certainly sounds like the right way to go. Morgana, thank you very much for your time.

Ribas: Thank you.

Visit I-007eBooks to download your copy of Alpha's micro eBook today:
The Printed Circuit Assembler’s Guide to… Low-Temperature Soldering


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