Bismuth Tin Alloy

Do you ever have a need for a "low temperature" solder (meaning an alloy that melts at less than 175C)?

You may have delicate components that cannot withstand standard reflow temperatures, or maybe you are looking to reduce costs by lowering the reflow temperature, or you may be step soldering.  Whatever your reason, there are two unique metals that are used extensively in low temperature solder alloys.

The first one I am sure you can guess: Indium.  The other one is Bismuth. While these two elements are used extensively in the over 100 alloys available in the 50C to 175C range, they couldn't be more different from each other.

Indium is a very soft, malleable metal and remains so even at cryogenic temperatures. It melts at 156C.  Bismuth, on the other hand, is very brittle, even at room temperature, and melts at 271C.  But both lend themselves very nicely to solder alloys that melt below 175C.

Let's look at the two most common alloys in these families.

The two alloys:

• 52In 48Sn (Indalloy #1E) Melts at 118C
• 58Bi 42Sn (Indalloy #281) Melts at 138C

What they have in common are:

• Both are lead-free
• Both are tin-based
• Both are eutectic (liquidus and solidus temperatures are the same, with no plastic range)
• Both can be made into a wide variety of solder forms and can be used in low temperature applications

But the indium-based alloy will give you better compensation of coefficient of thermal expansion (CTE) mismatch than the bismuth alloy.  The bismuth alloy has greater tensile strength but has a lower shear strength than the indium alloy and is generally not recommended in applications where the product has potential to be dropped (like cell phones).  The indium alloy will give you greater thermal conductivity than the bismuth, as well.  The bismuth will give you a cost advantage.

So, which alloy do you use?  Well, that depends on the metallizations you are working with and the environment in which your final product will be operating. For example, if you are soldering to two different surfaces that expand at different rates, then you will want to go with the indium alloy - to keep your solder joints from cracking.  But, there are a lot more considerations when choosing a low temperature solder, and we can help you sort through them.  Check out our Low Temperature Solder page on the web or contact us at AskUs@indium.com or contact me directly at cgowans@indium.com and we can answer your questions or put you in touch with one of our local experts to review your entire process for the best solution.

Let us help!

Carol Gowans

Here is a list of tricks to help you overcome the issues that can arise while hand soldering silicon-based solar cells (and other applications as well). Some of these ideas are obvious for most, but all the suggestions can help you form a better solder joint - and build a better final product:

1)    Use the correct soldering tip. I’ve made the mistake of using an inappropriate solder tip before, and so have many of my customers. It’s a frustrating problem you will only let happen to you once: everything is set up perfectly but nothing will melt, until you notice the solder tip is not the correct size or shape. This has happened to many of my customers who were initially using cone point soldering tips when they were working with 2mm wide solder coated tabbing ribbon. Simply changing the tip to a 2mm wide chisel point made all the difference, and promoted soldering readily. Why such a big difference in performance? The chisel tip allows heat to flow across the ribbon, instead of only heating a single point. More heat flow = more heat in your solder joint.

2)    Pre-tin the soldering iron. Just as an appropriately sized soldering tip will distribute heat across the soldering surface, a bit of molten alloy can help create a thermal interface to maximize heat transfer. Remember to melt a small amount of solder onto the tip of your iron before soldering, and be sure it’s the same alloy you are soldering with. (Leave the custom alloying to us ;)

3)    Consider the alloy you are soldering. All the heat your typical soldering iron can produce will not be enough to melt some of the highest temperature alloys. Be sure to have a good understanding of the alloy you have selected. In some cases with low-temperature alloys (like bismuth or indium alloys), excessive soldering temperature can de-wet the alloy and char low temperature fluxes.

4)    Use the correct flux. Fluxes are quite different, I’ve spent my entire soldering career trying to get that point across. There are fluxes for high temperatures or low temperatures, cleaning with water or not cleaning at all. There are specialty fluxes for specialty alloys and there are fluxes for different soldering surfaces. Use the correct flux. If you don’t know what the best flux for the application is - just ask; that’s what I am here for.

5)    Use a bottom side heater. Silicon is known to pull heat away – that c-Si solar cell that needs to be soldered is a heatsink! Some solder equipment vendors also provide underside heating pads to help prevent excessive heat loss.

6)    Keep your soldering iron clean. That black crud that builds up on your soldering iron tip, it’s not helping you form a good solder joint. Those oxides and charred flux residues can easily be removed by wiping the hot iron across the wet sponge (that should be at your soldering station). A clean tip will lead to better heat transfer, and it will make the fluxes you use more effective.

This is the soldering station I use, it’s a PS-900 supplied by OK International. Just about any soldering iron will work, but they won’t all work as well – or come with as good support.

I’m still learning all the tricks to hand soldering, so feel free to share any you have learned over the years!

~Jim

Folks,

I have occasionally written on calculating solder alloy density, as there is surprisingly more interest than I thought there would be in this topic. Recently, it occurred to me that it might be beneficial to compare the calculated densities to actual densities of a few alloys to see how accurate the correct formula is (for the derivation of the correct formula see below). The formula assumes “perfect mixing” (i.e. no interactions between the alloy elements). The alloys we investigated were tin-bismuth-silver, tin-silver, tin and tin-bismuth.

To measure the density, I obtained a few alloys from Indium Corporation. My student, Evan Zeitchik, determined that a good technique to measure density is to machine the alloy into a rectangular parallelepiped (see photo), weigh it, and calculate its volume from its dimensions.  The results agree with the correct formula to about 1 to 2 %. Some people would ask why there is any difference. The reason is that all alloys form different phases, and some form intermetallics. These phases and intermetallics would typically have different densities than that calculated for the alloy. I will have more detail on this work in a future post. 

Here is a derivation of the correct density formula:

Many people incorrectly assume that if you have an alloy of x % tin and y % silver, that the density of this alloy would be 0.x*Density tin +0.y*Density silver. This intuitive linear formula is incorrect however, as density has two units (mass and volume).  An easy way to understand the derivation of the correct formula (proposed by Indium Corporation engineer Bob Jarrett) is to consider a 96% tin, 4 % silver example.

Lets assume I have 1 g of this alloy, 0.96 g is tin and 0.04 g is silver.

The volume of the tin is 0.96 g/7.31g/cc = 0.131327cc

The volume of the silver is 0.04g/10.5g/cc = 0.00381cc

So 1 g of the alloy has a volume of 0.131327 + 0.00381 cc = 0.135137 cc

Hence it's density is 1g/0.135137cc = 7.39989g/cc

Hence, the general formula is:

1/Da = x/D1 + y/D2 + z/D3

Da = density of final alloy

D1 = density of metal 1, x = mass fraction of metal 1

same for metals 2, 3

The formula continues for more than 3 metals.

I have developed an Excel spreadsheet that calculates density automatically. If anyone wants a copy, send me an email at rlasky@indium.com

Folks,

 A few people asked some questions after my last post on bismuth solders. Here they are:

1.      The low melting point of these solders is encouraging. What are realistic field use conditions?

Bismuth solders tend to be brittle, so drop shock environments such as mobile phones would not be recommended. However, thermal cycle performance from 0 to 100C is good, so stationary office equipment, televisions, desktop computers, etc may be good candidates.

2.     I am working with your colleagues on an automotive application and I am curious whether you have any idea how this alloy will perform between -40 and 0°C? We have not been reviewing bismuth-containing alloys due to their lower sheer strength, but may need to look at them in the future.

We can find no information on thermal cycle performance at these low temperatures.

3.     I hear that bismuth is rarer than silver, if we start using bismuth in solders couldn’t that make it very expensive.

An old number from Prismark puts the world solder use at about 50,000 metric tons (MT) per year.  Assume bismuth solders took a 5% market share (I think this would be the highest) that is 2,500 MT of bismuth solder (Bi57Sn42Ag1) or 1,425 MT of bismuth.

Although bismuth's occurrence in the earth's crust is 0.009 ppm (silver is 0.075 and gold 0.004 ppm), about 22,000 MT are produced each year.  In comparison, about 2,000 MT of gold, 20,000 MT of silver, 400 MT of indium and 5 MT of rhodium are produced each year.  In comparison to more common metals, total lead production is 8,000,000 MT/year and tin a little less than 700,000 MT.

 Realistically, it would seem to me to be unlikely that use of bismuth in solder, at 1,425MT/year out of 22,000 MTs,  would affect the price much, especially if the adaptation rate is more like 1-3%, instead of 5%. 

For those interested in how bismuth is produced, this Wikipedia quote may be of interest:

"According to the United States Geological Survey, world 2009 mine production of bismuth was 7,300 tonnes, with the major contributions from China (4,500 tonnes), Mexico (1,200 tonnes) and Peru (960 tonnes).^[11] World 2008 bismuth refinery production was 15,000 tonnes, of which China produced 78%, Mexico 8% and Belgium 5%.^[9]

The difference between world bismuth mine production and refinery production reflects bismuth's status as a byproduct metal. Bismuth travels in crude lead bullion (which can contain up to 10% bismuth) through several stages of refining, until it is removed by the Kroll-Betterton process or the Betts process. The Kroll-Betterton process uses a pyrometallurgical separation from molten lead of calcium-magnesium-bismuth drosses containing associated metals (silver, gold, zinc, some lead, copper, tellurium, and arsenic), which are removed by various fluxes and treatments to give high-purity bismuth metal (over 99% Bi). The Betts process takes cast anodes of lead bullion and electrolyzes them in a lead fluorosilicate-hydrofluorosilicic acid electrolyte to yield a pure lead cathode and an anode slime containing bismuth. Bismuth will behave similarly with another of its major metals, copper. Thus world bismuth production from refineries is a more complete and reliable statistic."

So I don't think bismuth supply and price would be affected by its use in solders.

Folks,

When the industry was preparing to transition to lead-free solders almost ten years ago (can it have been that long), tin-bismuth solders were serious candidates. Their low melting point, of about 138C, made these solders interesting candidates to replace tin-lead solder. However, if contaminated with lead, tin-bismuth solders can produce a eutectic phase that melts at 96C. In such situations the resulting solder joint exhibits poor performance in thermal cycle testing. Since early in the transition to lead-free solders it was expected that there would be numerous components and PWBs with lead-based surface finishes, this property made tin-bismuth solders unacceptable.

Another aspect of tin-bismuth solders is that they expand on cooling. This phenomenon can result in fillet lift in through-hole solder joints.

However, as we are now well into 2011, almost no components or PWBs have lead-containing finishes and many portable electronic devices have no through-hole components, so it may be time to reconsider tin-bismuth for some applications.

Some years ago, Hewlett Packard (HP) had performed work to show that adding 1% silver to tin-bismuth solder enabled this alloy to outperform eutectic tin-lead solder in 0 to 100C thermal cycle testing. Even at these low reflow temperatures, HP demonstrated solder joint strength with SAC BGA solder balls that was 65% that of tin-lead solder. Expanding on this work, Indium Corporation's Ed Briggs and Brook Sandy performed stencil printing and reflow experiments consistent with the requirements of current miniaturized components using this 57Bi-42Sn-1Ag solder. All of their results were promising. Ed presented a paper at SMTA Toronto,summarized the Hewlett Packard work, and reviewed the results of this new work.

So for applications consistent with 0-100C thermal cycling, 57Bi-42Sn-1Ag solder may be something to consider if the high temperature of SAC solder paste is an issue to components or PWBs in a product

Cheers,

Dr. Ron 

PS: Read my follow-on posting about bismuth.

Although most scientists today feel that alchemy has been widely discredited, and I have been taught to agree, the idea of it is whimsical and exhilarating.  Of course, I don’t have a hope of changing the makeup of bismuth or transforming it into another metal, but in a modern way, it’s very interesting how bismuth can be used to change the properties of other metals significantly - through alloying. In my last post on bismuth, I outlined its physical properties, some of which I find rather unusual. The main reason I originally researched bismuth was because of its viability for use as a low temperature Pb-free alloy.

BACKGROUND:
I'm not an alchemist like Newton, I can't transmute bismuth to gold like Seaborg, but I can use bismuth and metallurgy to transform an alloy.

I just read a fascinating article about Sir Isaac Newton titled, “Moonlighting as a Conjurer of Chemicals”. Newton is widely regarded as one of the most important people in the history of science, and he was very devoted to his work. The revelation in this article about the depth of his interest in alchemy left me somewhat awestruck. In my previous reading about Newton, I remember perhaps a mention of his interest in alchemy, but I guess I figured it was because science and alchemy, at that time, were fairly closely related. As scholars are starting to translate more of his diaries, they are discovering that his passion was alchemy and he saw it as the path to complete control over the natural world.  

I suppose if it was still socially acceptable to be an alchemist that is what I would have wanted to be; it just never seemed to be a viable option. What I have chosen to do now kind of makes sense considering chemistry/metallurgy is about as close as you can get nowadays. 

Reading this article reminded me of some interesting information I had come across while researching bismuth a couple months ago; namely that, although bismuth wasn’t one of the seven central metals in alchemy, it has an "alchemical" symbol (#52 in the image to the left) and was frequently used, although it’s not known for what purpose. I also came across this bit of information:

“In 1980, a scientist named Glenn T. Seaborg was able to transmute a minute quantity of bismuth into gold at the Lawrence Berkeley Laboratory, via nuclear collisions.” 

Seaborg is a fascinating scientist in his own right and discussion about him could fill quite a number of blog posts. Is it possible that alchemists underestimated bismuth and should have focused more on turning it into gold?

BACK TO MODERN TECHNOLOGY:
The eutectic alloy of 58Bi/42Sn has been used since the Pb-free transition as a low temperature (138°C liquidus) option for soldering products used at ambient temperatures - such as consumer electronics.  Note the low melting temperature of this alloy, despite the individual melting temperatures of bismuth and tin, 271°C and 232°C, respectively. Although bismuth is typically known to be quite brittle, this alloy has been shown to perform similarly to the SnPb eutectic solder (in response to a comment on my last post, for further data-based information, please feel free to contact me directly). In cases where more ductility is desirable, 1% silver can be added, further improving thermal shock and fatigue resistance. Perhaps the similarity in performance makes sense because of bismuth’s proximity (right next to) lead on the periodic table, although they differ in several other qualities such as toxicity.

The more I learn about bismuth, the more interested I become. Now if only I could find some in-depth alchemical information about it from Sir Isaac Newton.

Okay, I have a confession to make: I’ve always had a grudge against bismuth, ever since I started recommending thermal interface materials. It is the polar opposite of my favorite element (indium) – well, as much as a metal can be. These 2 elements (indium or bismuth) are added to almost every solder with a lower solidus temperature than Sn/Pb. The choice for most thermal interface applications that I have dealt with was indium or an indium alloy, but now I am starting to become very fond of my new friend bismuth for solar applications.

Bi/Sn and Bi/Sn/Ag are now available as a solderable coating for our Tabbing and Bus Ribbon. After getting a feel for this material, I must say I find it pretty nice to work with. Both alloys melt at 138-139degC, with the Bi/Sn/Ag having a greater tensile strength (which is not necessarily a good thing for tabbing ribbon). With a little bit of lab time I have isolated an existing flux that works very well with these alloys. So far GS-5454 has formed good solder bonds down to 160degC. This is great news, because it allows you to minimize the reflow temperature (and stresses) of your C-Si/tabbing ribbon interface. 

~Jim

The binary eutectic alloy of bismuth and tin (Bi/Sn) is well suited for many low temperature applications due to its melting point of 138°C, but the alloy is known for being rather brittle for a solder alloy.
All hope is not lost though - there is a similar alloy, known as Indalloy 282, which drastically reduces the brittle nature of the Bi/Sn alloy with the addition of just 1% Silver (Ag).  This 57%Bi/42%Sn/1%Ag alloy has a 1°C higher melting point.  Although you won’t be able to notice this reflow change, you will immediately notice the physical difference if you compare solder wire made from each alloy.  The Bi/Sn wire is likely to break upon bending, unlike the Bi/Sn/Ag alloy.
The easiest (and most economical) way to experiment with these alloys is with a solder wire kit . With a wire kit you can obtain both wires in the same diameter, and you get 5 fluxes to help with soldering.  You can also get up to 10 different alloys if you want to experiment-your-heart-out and compare Bi/Sn and Bi/Sn/Ag to other alloys like Sn/Pb or Sn/Ag/Cu.

When you think of solders, you generally think of tin, lead, copper, silver.  But have you ever considered bismuth? 

Bismuth as a pure metal has a melting temperature of 271C but when bismuth is alloyed with other metals, it can bring the melting temperature of the resulting alloy down considerably.  For example 58 Bismuth 42 Tin alloy is a eutectic alloy that melts at 138C.  And there are a whole host of bismuth contained alloys that fall below the 100C range.

While this wide range of lower melting temperature alloys will not put them in the mainstream of electronics assembly, there are plenty of other applications including step soldering where they will be very useful.  Stay tuned to this space for more ideas on using bismuth.

For more information, go to Bismuth.

A couple of my colleagues, Jim Hisert and Andy Mackie recently published an article about the use of solders in chip packaging. The article begins with the historical notion that the practice of soldering dates back around 5000 years where it was used to make metal jewelry. It is interesting to take notice that although soldering has been done for so many years, it is not a science which is completely understood. On top of that, many people are still lost in the soldering basics.

It seems that the best place to begin addressing the common misunderstandings or lack of knowledge regarding soldering is to begin with the soldering basics. In Subsequent postings we can build information from this framework.

Soldering Basics:
• Soldering is different than brazing or welding. Soldering is a technique to attach surfaces together using metal fillers (solders) which melt below 350ºC.
• A solder joint is the result of an intermetallic formation between the elements of the low temperature solder (typically some mixture of tin, lead, silver, bismuth, gold, indium or copper) and the substrate metallization being soldered. A comprehensive list of popular solder alloys can be found on Indium's Website.
• The presence of oxides over the metallic surfaces may prevent the ability for solder to co-mingle and form an intermetallic with the substrate metals. For this reason, fluxes are utilized to remove the oxides off the metal surfaces. Fluxes are a chemical mixture which may contain rosin, acids, or halides.

More information on soldering basics is available in the Indium Corporation Technical Library under Soldering 101.

Image courtesy of www.forsythedesigns.com/GigPage.aspx?PageID=1256

Annika writes:

Dear Dr. Ron,

I'm a chemist and a PhD student working in the field of atmospheric corrosion of lead and lead-tin alloys in historical organ pipes in Europe.
I read your article on the web about tin pest and Napoleon's Buttons and Lead-free Soldering. In the article you write:

As a general rule of thumb, metals readily soluble in tin suppress or eliminate tin pest formation. Examples of these alloying metals are bismuth, antimony, and lead. As little as 0.5% bismuth or antimony by weight essentially eliminates tin pest, while about 5% lead by weight is needed. Since tin-lead eutectic solder is 37% lead, tin pest has not been an issue in soldering with traditional lead-based solder paste.

Do you think you can give some more information about this "rule of thumb" and that 5% lead will stop the pest?

Annika:

Tin pest in church organ pipes was one of the first known examples of the phenomena. Sadly, little work has been performed on this important topic in recent years. It is an issue of considerable concern for the current move to lead-free solders. I wrote a posting on it some time ago. For those interested, I have a paper with quite an extensive tin pest bibliography, that has the papers with the tables from which I got the "5% lead" information. Send me a note if you would like a copy, rlasky@indium.com.

Cheers,

Dr. Ron

Folks,

I read with dismay this week, an advertisement in an electronic assembly trade journal touting a "drop in" solder paste. The solder paste uses a patented alloy, that has four elements (e.g. tin, copper, silver, bismuth) versus the NEMI, IPC, JEITA etal recommended tin-silver-copper (SAC) alloy. The ad suggested that one need make no changes in their tin-lead based process to assemble PWBs with this "drop in" solder paste. Just as with the desire for RoHS exemptions for all your products, "hope springs eternal."

Why am I so down on the "drop in" solution? Here are the major points:

1. The alloys used don't really give a "drop in" solution. They typically melt at a little over 200C, admittedly a lower temperature than the mainstream SAC alloys (217C), but still about 20C higher than tin-lead. The 20C difference is just too much to enable a "drop in," most folks would need to develop a new process for this paste.
2. Four element alloys are hard to manufacture with consistency.
3. The alloys do not have a broad data base for reliability or processability.

The third point is the real kicker. You are on your own. The world is going lead-free using the SAC alloy. The vast majority of the reliability and process data in the world will be on SAC alloys. Even though preliminary data are good for both processability and reliability with SAC, as with all soldering systems there will be issues. We still have surprises with tin-lead solder after almost 100 years of use in electronics! If you are going to have issues with your solder, it is nice to have them with a solder that has a vast data base. That lead-free solder will be SAC.

Cheers,
Dr. Ron