Dr. Traian Cucu Discusses Low-Temperature and Lead-Free Soldering

Estimated reading time: 7 minutes

In an interview with Dr. Traian Cucu from Alpha Assembly Solutions, we discuss low-temperature and lead-free soldering applications and advantages, and their recently published I-007eBook on the topic.

Happy Holden: This is Happy Holden at SMTAI, and I'm here to chat with Dr. Traian Cucu from Alpha Assembly Solutions. He is the group leader of the Global Applications and Technologies Expert Group (GATE)—R&D—within Alpha. He is also one of the authors of the very successful low-temperature soldering (LTS) book by the Alpha team that is available for free download. Good to meet you, Dr. Cucu.

Traian Cucu: Pleased to meet you, and thank you for giving me the opportunity to talk a little bit about our book and LTS in general. As you said, we recently published a book on the topic and have received a lot of interest from the industry on it. For obvious reasons, everybody is interested in it because the topic of the day is LTS. Everyone in the assembly industry is looking at LTS and lead-free soldering for many critical advantages that we can achieve by moving to a low-temperature process, which is outlined in the book as well. The lower thermal energy that goes into a process allows for less mechanical and thermal stress on the components, which will have an impact on the long-term performance and life of the device itself automatically. Again, the lifespan of an assembly will have an impact on the bottom line for all manufacturers.

Many people ask, “Why are you looking at low-temperature lead-free soldering now? Why not go directly to LTS when we moved from tin-lead to lead-free?” These are good questions. The problem then still exists today, but it is not as big of a problem anymore. The fact that one of the main constituents of low-temperature alloys is bismuth, which creates a low melting phase with tin and lead that actually melts around 98°C, created a lot of headaches when the transition to lead-free was happening. Due to that, when the move was made from tin-lead to lead-free, every alloy that contained bismuth would not make a good candidate at that point in time.

As many of you might remember, there were a lot of discussions and work done during the transition period on the compatibility of lead-free alloys with lead alloys. Basically, there were lead finishes on the components and lead-free alloys used for solder paste, and that's when the tin-silver-copper (SnAgCu, or SAC) family came out on top. This was the one that was adopted with all the variants and flavors, like SAC 305, SAC 405, SAC 387, etc.

Today, I would venture to say that the transition to lead-free is almost 100%, meaning that there are few lead-containing encounters in the industry. The bismuth-containing alloy actually became interesting again, and, at this point in time, we can actually use them. We have seen a lot of work being done in the past couple of years on the first generation of low-temperature alloys. We have also seen a lot of adoption happening in the industry, and the advantages that everybody was looking for have been proven and demonstrated. Now, we are seeing the second generation—we might call it the second wave—of low-temperature alloys, and the third generation, which is looking to improve on those further. With the second generation, we are already close to the performance of the joint mated with a low-temperature alloy, and that of a SAC alloy. The third generation is already looking to improve even further for more benefits—including mechanical and thermal—on top of all the advantages that we see by moving to a low-temperature assembly process.

Holden: Are we approaching the performance of our older tin-lead solder, or are we starting to exceed the performance?

Cucu: We can exceed in certain areas, and even the SAC alloy family exceeds that of tin-lead. Tin-lead—for all the bad that lead does—actually does very well in alloying with the tin, so I think it will always be close to or better than it in specific ways, and we will have to work on others. However, because this transition to lead-free brings in different types of alloys, in many cases, the requirements that we saw during the tin-lead time do not exist anymore. The new requirements are part of the emerging technologies that are coming into play today. For example, if you think about the automotive industry moving from a combustion engine to an electric engine, you get some of the constraints out of there, but new constraints are coming in as well.

Even on some of those applications, it's even harder to say that tin-lead would be better than lead-free. We are living in a different time today and are looking at different complications that didn't exist during the tin-lead time. We are off to a bright future for the assembly industry. A lot of innovative materials are coming into play. We see people being interested in and trying to cope with and get ahead of the curve with what is being thrown at us from the assembly landscape.

Holden: Traveling a lot, I certainly am interested in the reliability of a solder joint on an aircraft in its environment, and also on automobiles as they move to increasingly better electronics in cars. In the past, it might not have been as critical as it is today, so it's good to find out that scientists and engineers understand and are fixing the problem with lead-free. How long has tin-lead soldering been around? Does it go back to the Egyptians?

Cucu: Yes, something like that. That’s the reason why nobody could patent it actually because there was always someone in the past who used it. As I said, lead was a good thing in the solder. It's not good for us as humans or the environment, but it did very well when alloyed with other materials.

We definitely are looking at all of these challenges in addition to innovative assembly materials and technologies to make sure that we are ready for these new applications, such as electric vehicles and the drive in the automotive industry towards electrifying everything that can be, for good reasons, and moving away from the combustion engine for a sum of reasons. We are also looking at the motherboard and telecommunications segment. They are all trying to cram in more and more parts and smaller components, and utilize higher speeds and frequencies on the board, which lead to more challenges. Further, the thermal release of specific high-power components and surface insulation resistance (SIR) electromigration issues that we need to all be aware of and consider are always there and need to be taken into account. These are all factors that need to be considered, and material suppliers are continuously pursuing to improve and work on to come up with new solutions for our materials.

Holden: I’ve driven a hybrid for the last five years. I average about 47 miles per gallon, but my sons tell me to be cautious of those orange cables in the car, which are 800 volts.

Cucu: I drive a hybrid as well. They are great, aren't they? Imagine all the electronics in there just to make it feel like we—the end users—are driving a regular combustion engine. You don't see any difference. Mine gets around 45 miles to the gallon.

Holden: I’m surprised because I have had seven free software upgrades, and with each upgrade, I get more performance and better mileage. There are not too many products you can talk about after five years that are better than when they were brand new.

Cucu: Yes, and that's the beauty of today's systems where you have the software and hardware in one system that works, and we are responsible for the whole car. We need to make sure that this one works flawlessly because with an upgrade, as you said, you can move it on to the next generation. You don't really have to change it after a short period because a better one came out. We need to ensure we are up to the task, which we are. LTS will play more and more of an important role in the next generation because of all these requirements and basically, as I said in the beginning, less thermal energy put into the board translates into higher reliability for the assembly.

Holden: I recommend that everyone should download the low-temperature soldering micro eBook from Alpha. It's approximately 50 pages, which is a relatively short read. Unless you studied LTS, there is a lot in it that is not common knowledge about lead-free soldering. With that, I want to thank Dr. Cucu for his time and expertise today. Do you have any final or recommendations for any first-time users of LTS?

Cucu: As you said, read the book because it's an excellent and important resource for LTS. You get the basics there and a couple of more reasons why to go to the LTS route. Feel free to contact us too. We’d be more than happy to discuss it with you and see how we can help.

Holden: Thank you very much.

Cucu: Thank you, Mr. Holden. It was a pleasure.

To download your copy of Alpha Assembly Solutions’ micro eBook, The Printed Circuit Assembler’s Guide to… Low-Temperature Soldering, click here.