SMT expert Phil Zarrow interviews Carol Gowans, incubator market manager for Indium Corporation, about the benefits of using low-temperature alloys bismuth and indium.
Phil Zarrow: When we think of solder alloys out there in the world, we immediately think, of course, Sn63, Sn62, and obviously the SAC alloys. What can you tell us about low-temp alloys?
Carol Gowans: Well, at Indium Corporation, we have over 200 solder alloys in our solder alloy directory. And we’ve categorized that into six solder families. The first one is gallium, which is a metal that’s liquid at room temperature, so we don’t use that a lot in the solder. And we have, in increasing temperature, the bismuth family; the indium family; of course the tin family, like you mentioned; the lead family; and the gold family. And what I’d like to focus on today are the bismuth and the indium.
Phil Zarrow: Okay. Bismuth. Whenever I think of bismuth, the first thing that comes to mind is embrittlement.
Carol Gowans: Yes, it can be a very brittle metal to work with. That’s why we have a lot of experience with working with it. And over the years, we’ve learned that you can add silver to it to make it a little easier to work with in our part of the process. But those challenges are definitely overcome by the fact that it’s a lead-free, it’s fairly inexpensive, and it’s also a lower melting temperature at 138 to 140°C, so it can be used in a wide variety of applications that require lower temperatures.
Phil Zarrow: But, then there’s indium and indium alloys.
Carol Gowans: Yes. Indium is very interesting. It has so many different kinds of personalities to it, that it has a lot of advantages. It can be used very well in sealing applications, particularly hermetic sealing, because it will allow for the differences in surfaces. Some surfaces aren’t always perfectly flat, and so it will bond to that. You can use it in quartz and ceramic sealing, and a lot of connector applications. It’s very thermally conductive so it will conduct heat away from components. Again, there are a lot of lead-free options. It also works very well in surfaces that you’re bonding together that have different CTE, coefficient of thermal expansion, that expand at different rates, so it will allow them to expand the way they want to without cracking the joint.
Phil Zarrow: What are some of the actual applications that you’ve seen the indium alloys applied to?
Carol Gowans: Well, it’s very good in step soldering, so if you have a component that’s sensitive to the higher temperatures of the SAC alloy, you can put it through a second reflow with a lower temperature and it won’t disturb the components that you’ve already put down. It can also be used in sealing applications where you need a hermetic seal if you have two surfaces that are not 100 percent smooth, and there’s some areas that might have deformation on it. The indium will fill in there and flow nicely, whether you’re reflowing it or just using it as a seal with a compression seal. And it’s also very good in thermal interface areas that are used a lot in thermal management to reduce heat from a particularly high heat component.
Phil Zarrow: What are some of the configurations these preforms can be manufactured to?
Carol Gowans: Well, definitely, it can be made into paste. You need a special flux because of the lower temperature, but it can definitely be used in a lot of applications that are still surface mount. But there’s also solid wire that’s available, and ribbon, and most interestingly, the performs. We can do a lot of different shapes and sizes and thicknesses. And we can make them exactly to your applications so that it fits your components and your boards and everything. We do a lot of fusible alloys that are more solid, and frames that are for hermetic sealing, and washers that are used in connectors, and just all kinds of different options.
Phil Zarrow: Carol, what resources are available at Indium for an engineer to learn more about low-temperature alloy solder preforms?
Carol Gowans: Well, we have a lot of application engineers who have a great deal of experience in looking at these alternative methods. If you call us up or send us an email, we would need to know the simple description of your application and what you’re trying to solder together. We also have a lot of resources, including blogs and technical papers at indium.com. And I have a blog myself there that I post things to, and I’m also glad to help anybody, personally with, by going to cgowans@indium.com.
Phil Zarrow: Carol, excellent. Thank you very much. I really appreciate it.
Carol Gowans: Thank you, Phil. I appreciate it.