Tin and Lead Alloy

Folks,

Pity the copper age smelter of 3000BC.  He had to get his wood fire to 1085°C to smelt or melt copper, sometimes he couldn’t get that high a temperature.  Even when he was successful, his copper didn’t flow well and was soft. 

But the winds of change were occurring about that time, news of tin was in the air.  When tin is mixed with about 90% copper, the melting temperature of the resulting bronze plummets to 850°C, this temperature drop, of over 200°C, is a big deal.  Not only did the lower temperature make it easier to melt the bronze, the bronze would flow better in molds.  In addition, the strength and hardness of bronze is many times that of copper.  From the figure above, you can see that a 10% addition of tin to copper produces a bronze that has 3 times the yield strength.  The Bronze Age had begun. Can you imagine the joy of the early metal smiths as they transitioned from copper to bronze, not only was bronze harder and stronger, but it was much easier to process and required less precious wood in the furnaces.  On the downside, tin was then, and still is, rarer than copper, so the cost of bronze is higher than copper alone.  Poor man’s bronze is brass (copper and zinc).  Since zinc is cheaper than copper, brass is less expensive, but from the chart (left), the materials properties are typically weaker than bronze.

Because of its greater strength and hardness, bronze was an important material for war.  If you had equal fighting ability to your enemy and he had a bronze sword and shield to your copper weapons you would lose every time.  So bronze smelting and manufacturing was likely an early military secret.

An equally important benefit of tin, is that when tin was alloyed with lead, a very low melting material was created that would bond to bronze and other metals.  Soldering  was invented.  Those of us that use solder everyday often don’t recognize the miracle of soldering.  When we solder electronic components to a PWB we are essentially bonding copper to copper (which melts at 1085°C) at a temperature of less than 250°C.  We do this metallurgical bonding in the presence of thermally delicate plastic.  So without solder, we would not have the electronics industry as it is exists today.

Tin does all of the “work” in soldering.  It is tin that forms the intermetallics Cu₆Sn₅ and Cu₃Sn with copper. The other solder alloying elements such as lead, silver, and copper play important roles in wetting, spreading, and the ultimate strength of the bond, but only tin metallurgically interacts with the copper.

So when you pick up your mobile phone, type on your computer, or watch TV today, remember - without the “Miracle of Soldering” you wouldn’t be able to!

Cheers,

Dr. Ron

The Image is from Askeland's The Science and Engineering of Materials.

Folks,

Some time ago I wrote a post, “In Search of Tin Whiskers,”  Michael responds below.  He makes some good points.

Dr. Ron, I'm responding to your  blog regarding tin whiskers. I actually have a failure analysis report I did a couple of years ago in which failure of our product was due to this issue and occurred on a part that came into RoHS compliance only 3 months prior.
 

I'm not sure that your question of identifying whisker issues in product that proper steps have been taken to mitigate the problem is a constructive one. The fact is that many of the component manufacturers from overseas jumped into compliance without any thought or regard to this issue thereby flooding the industry with components such as plagued my company. We have not had this issue since we've specified an alternate finish.

These whiskers are so delicate that most problems disappear when the technician starts to work on the failed unit and the problem never re-appears so it is written off as an anomaly, loose/bad connection and not investigated any further. It was only my own curiosity as to the number of "no problem found" failures of our keypads we had suddenly encountered that caused me to dig deeper and when I looked into the connector I was amazed at the crystal city staring back at me. I couldn't believe what I was seeing after all of these years.

After seeing this problem first hand I became, and am, quite convinced that there were and are people who will be losing life, limb, and property because this forced compliance with its risk was not given proper worldwide attention.

Michael.

A popular topic Re my blog is solder density calculations. Rhonda writes……

Hi Dr. Lasky,
I am a precious metals recycler and would very much appreciate your verifying the validity of an equation that approximates the Karat Value of various alloys of gold based on S.G. which I will call density or "D," and the Karat Value is "K." The equation is seems to hold relatively true even when the exact composition of the alloy is unknown, although the percent of error obviously will increase as density decreases. I would also appreciate not only verification but also more specific information on percent of error for densities below about 14 or 15 g/cc. Here is the equation:

K = 0.0089D^3 - 0.550D^2 + 12.5299D - 77.06

Thank you so much for whatever assistance you can provide.

Rhonda

These types of equations can only work for one alloying metal with the gold.  This one is only for copper.  It is also calibrated in Rhonda’s favor as it reads the karat level about 10% low.   I was able to determine this by using the Excel Solder Density worksheet that I developed. If the alloy was gold and lead, a 50% by weight gold (12 karat) would show as 15.7 karat with this equation and Rhonda would lose her shirt.

In response to my blog post on copper as the precursor to civilization, Harvey writes about pollution from early mining operations…..

Also interesting, early copper mining and processing led to the first examples of human induced environmental damage. There are documented sites in the Alps where copper processing by prehistoric peoples has left areas treeless to this day, due to heavy metal contamination.

Harvey

Mining and smelting were very tough businesses in ancient days.  In addition to pollution, many workers died from toxic fumes.

Dr. Ron

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

Maria Durham, Indium’s new Technical Specialist in Semiconductor and Advanced Assembly Materials, has been doing some research on indium lead (In/Pb) solder alloys. We chatted about her findings this week. 

 [Andy C. Mackie: ACM] Which indium/lead solder alloys are most common, and what are their properties?

[Maria Durham: MD] Firstly, the use of lead-(Pb-)containing solders in some soldering applications is restricted due to local environmental and RoHS compliance, but there are still many applications where they are  allowed. Many military, aerospace, and industrial equipment uses, as well as many applications related to vehicles, are exempt. The table below shows the most common indium/lead (In/Pb) alloys (pink) and their properties, sorted by liquidus temperature; the higher of the two melting points (solidus and liquidus) seen for non-eutectic alloys. In blue are three comparison materials.

Indalloy 205 is the most commonly used, probably because it has the closest liquidus temperature to the tin/lead eutectic (183°C), 63Sn/37Pb (Indalloy 106). This means it can be reflowed using a standard Sn/Pb eutectic profile. The next most common alloys that are used are Indalloy7, 204, and 206.  Besides the melting range, indium has comparable thermal and electrical conductivity to standard materials.

[ACM] What makes indium-lead (In/Pb) solders so attractive, and why have we seen a recent resurgence in their usage?

 [MD] One main attraction to using indium/lead (In/Pb) solder alloys in soldering to precious metal surfaces is that, unlike tin-containing solders, they do not leach gold. That is, gold does not dissolve in them to any appreciable extent. During discussions at Semicon West in 2011, one of our California customers reported going through 8 simulated reflows with Indalloy 205 in contact with a gold surface with no loss of joint strength and no joint embrittlement. That is pretty impressive. Note that embrittlement is often caused by gold-intermetallic formation. It has been noted that even at 250°C, 50In/50Pb dissolves Au at a rate 13 times slower than it does into 63Sn/37Pb, although this, of course, is a kinetic, not a solubility limit, study.

The higher melting Indalloy 164 (92.5Pb/5In/2.5Ag) has the lowest coefficient of thermal expansion (CTE) of all of the In/Pb solders and is able to withstand the higher temperature excursions that can be seen in step-soldering type applications (where a very high melting solder is used to form the first joint, followed by a next lowest melting alloy, and so on). This is seen in applications such as power electronics assembly, where the first step solder is often used for die-attach either as a solder paste, wire, or preform. The high melting point helps the solder withstand the operational temperatures associated with under-the-hood electronics, in applications such as engine control modules, where Indalloy 151 (92.5Pb/5Sn/2.5Ag) or Indalloy 163 (95.5Pb/2Sn/2.5Ag) are most commonly used. In/Pb solder is excellent on very rigid structures such as ceramic-to-metal or ceramic-to-ceramic. The desired solidus / liquidus temperature range can be adjusted by changing the indium:lead ratio, making it very easy to “dial in” the alloy to a specific reflow process.

Another attraction to using In/Pb solders is that they exhibit good fatigue resistance in thermal cycling from -55°C to 125°C.  In testing, the 50In50Pb solder joint fatigue life is about 100 times greater than that for 63Sn/37Pb.

 [ACM] What fluxes are used in these applications, and how are they formulated differently?

 [MD] The fluxes most compatible with the lower melting point (<200°C) indium-containing solders are NC-SMQ-80 (solder paste) or the lower-tack TacFlux® 012 (suitable for use with wire, preforms, and spheres). These are no-clean fluxes, specifically formulated for lower temperature reflow.  Under appropriate low temperature reflow these fluxes leave behind benign residues that do not need to be cleaned off (“no-clean” flux), although they are often cleaned off in most practical applications, usually to ensure reliable wirebonds absent of flux spatter.

 To learn more, please contact us.

 Cheers!  Andy

Two categories of solder are available to choose from when the in-service environment for a device reaches above 125°C either in continuous operation or thermal cycling accelerated life testing. These categories are those comprised primarily of lead, and those of gold. From the electronics industry’s drive to eliminate lead, many manufacturers who have traditionally used lead solders are treading cautiously, looking now at the gold solders, primarily at Indalloy 182 (80Au20Sn).

The most common concern regarding this switch relates to the strength of AuSn, which is much higher than the lead solders. The degree that this should be of concern however, should be realized within the scope of the application.

For instance, review this case scenario:

Indalloy 159 (90Pb10Sn) was used in a device for years to adhere high temperature sensors to a calibration probe that is slowly cycled in operation from 350K (~75°C) to 500K (~225°C). The solder joins a nickel and gold plated Kovar™, or platinum or platinum coated, nickel lead to a tinned copper lead. The solder joint is not placed under tension or shocked.

Considering the high temperature solder options in this scenario, the AuSn would be mechanically preferred.

Why?

Well, tin-bearing soft solders will leach gold from gold metallizations during soldering, creating a brittle Au-Sn intermetallic layer within the solder joint. The more gold available, the more consumed, and the greater the thickness of the resultant intermetallic layer. The brittle nature of this layer, situated intimately next to the relatively soft PbSn solder layer, creates differential stresses that promote crack propagation upon thermal cycling.

AuSn was not considered previously because the engineers were familiar with its hardness and, given the cracking failure described using a softer solder, they did not anticipate improvement. It was a pleasant surprise to them to find that switching to a lead-free solder would not sacrifice the quality of their device. AuSn is a brittle alloy but, unlike the description above, no differential stresses are involved. 

Note: Eutectic gold solders have been used for many years to solder nickel plated Kovar™ lids to high reliability ceramic packages and have a good history of fatigue performance.

Folks,

I thought I would post a few short thoughts as the new year begins. Here it goes:

1.    A billion hours ago the stone-age was the future, a billion minutes ago Caesar ruled Rome, a billion seconds ago Jimmy Carter was President, a billion passives ago you took your last break (about 4 hours ago). As exciting as the latest quad core microprocessor is, the largest number of components that we assemble is passives, approaching two trillion per year. That is about 6 billion a day. If you lined up all of the 7 billion people in the world, each year you could give every man, woman and child several hundred passives from all of the passives that are produced. If two trillion passives (assume 0402s) were lined up end to end they would circle the earth 50 times!

2.    Schools in Indiana are no longer required to teach cursive writing. Key board skills are considered more important.  Yikes! I’m all for keyboard skills, but I want my grandkids to be able to write in cursive. If not, how do they write their names? Are we less than a generation away from people writing their names as an “X?”

3.    Thoughts on lead-free solder reliability in long term mission critical environments from a NASA study:

        “Test vehicles assembled with lead-free materials (notably tin-silver-copper) exhibited lower reliability under some test conditions.”

Some people would respond to this statement by saying, “I told you that lead-free solder was no good.” However, another way of stating the results would be, “Lead-free solder performed better in more tests than tin-lead solder did.” The ratio, by my count, was about 5 to 3 in favor of lead-free. However, I agree that lead-free is not ready for mission critical (>20-year) service life. The main reason being that, in some cases, when lead-free solder joints failed in these types of studies, the results were much, much worse than tin-lead solder joints. These failure modes need to be understood and addressed. In addition, tin whiskers and pad cratering are looming problems in these, mission critical, long service life quadrant D applications as discussed in the Navy's Manhattan Project (http://www.navyb2pcoe.org/pdf/LFEMP_book.pdf).

4.    SACM has arrived. SACM is a SAC105 alloy that is doped with manganese. Work performed on SACM by Liu, Lee, et al was reported in a May 2009 ECTC paper, Achieving High Reliability Low Cost Lead-Free SAC Solder Joints Via Mn Or Ce Doping. The thorough testing reported in this paper suggests that SACM has promise as a material candidate for quadrant D applications mentioned in #3. In explaining the superior performance of this material the authors state:

“The mechanism for high drop performance and high thermal cycling reliability can be attributed to a stabilized microstructure, with uniform distribution of fine IMC particles, presumably through the inclusion of Mn or Ce in the IMC.”

We have had to wait awhile for this material to become commercially available as it is a challenge to manufacture doped solders like this in large quantities.  I think this paper should be on you "must read" list.

5.   I had not planned on reading Steve Job’s biography , as I thought I knew quite a bit about him from reading recent articles in Forbes, Fortune and Business Week. But I went ahead and downloaded it to my Kindle anyway. This work by Walter Isaacson is a masterpiece. To share one tidbit from it that relates to those of us in electronic assembly: 

"In almost all cases electrical engineers first design the circuits that perform the functions of some device like a mobile phone or tablet. Mechanical Engineers are then left to fit the circuits into the “box.” (Hence MEs are often called “box stuffers” by EEs). Jobs completely changed this approach. He told the engineering team how he wanted the product to look and function first, then they had to determine how to make it work that way. I’m convinced that only through this approach are the revolutionary design concepts that Jobs and Apple came up with possible."

The book also points out his many flaws (e.g. Jobs would regularly park in handicap spots, the author reports several times that Jobs just didn’t think the rules applied to him, etc.). Another interesting thought (read it and see if you agree with me) that if Steve was not Paul Jobs' adopted son, Apple would have never happened.

Cheers,

Dr. Ron

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,

In a recent posting we discussed that the higher melting temperatures of lead-free solder require reflow soldering temperatures to be higher, thus more electricity is used in lead-free assembly. However, as we calculated, this increased use of electricity is very small compared to all electricity used in the world.

An additional concern that some have voiced is the claim that RoHS, with its lead-free requirement, actually makes the environment worse because more tin and silver is used in lead-free solders.   They argue that the increased use of these metals, creates mining pollution and has driven the price of these metals sky high. Let’s examine these claims.

Prismark has estimated that approximately 90,000 tons of solder are used in electronics, with about 80,000 used in wave soldering and 10,000 tons for SMT soldering. It is important to remember that electronics solder is a subset of all solder. All solder (alloys for brazing pipes etc) uses about 190,000 tons of tin. Solder is the single largest user of tin. See Figure 1. 

Figure 1. Solder is the largest end use of tin. Tin is the base material for almost all solders. 

If tin-lead solder were still used predominantly, approximately 57,000 tons of tin (90,000 x 63% tin) would be used annually. With lead-free solder, about 88,000 tons (90,000 x 98% tin) of tin are used per year. This is an apparent increase of about 30,000 MT of tin used each year. However, an interesting thing to consider is that lead-free solder is about 14% lighter than tin-lead solder. Knowing that, and knowing that solder used in wave soldering (remember wave soldering accounts for almost 90% of all solder used in electronics assembly) is consumed by volume not weight (i.e. assuming approximately the same fillet size), about half of this increase is canceled out. 

This is all a bit confusing however, so it may be best to just to look at tin use. According to the United States Geological Survey (USGS), about 300,000 tons of tin are mined each year. Figure 2 is a graph of world tin production at mines per year (this graph does not show recycled tin.)  The amount of refined tin used each year in the US is depicted in Figure 3. Figure 3 includes about 15,000 tons a year of recycled tin. Recycling solder is very cost effective. Scott Mazur just pointed out (Printed Circuit Design and Fab and Circuits Assembly, p 36, August 2011), that recycling solder dross is 10 times as cost effective as recycling aluminum cans.

Looking at these graphs, it is hard to say that the amount of tin used has gone up since RoHS. It would appear that tin use is likely more affected by the economy and that it is really difficult to see an effect from RoHS’s July 2006 enactment.

Figure 2. World Tin Production at Mines. 

Most wave soldering solders have low or no silver. So, about 3% of the 10,000 tons of SMT solder, or 300 MTs of silver, are used in electronics. This is about 1.5% of the 22,000 MTs of silver produced each year. Silver use in electronics does not make anyone’s list of top silver usage.

Figure 3. US consumption of tin has decreased since RoHS was enacted.

So electronics solder use since RoHS has not caused tin use to increase, nor is it a significant factor in silver use. Therefore it is highly unlikely that electronics' use of tin or silver has been a prime driver in their stunning price increases in 2011.

Folks,

An obvious disadvantage of lead-free electronics soldering assembly is that the oven must be hotter and therefore will use more electricity (versus 63Sn37Pb soldering). But is the extra amount of electricity significant? Bill O’’Leary claims that a typical SMT oven uses $7K of electricity a year at $0.072/Kilowatt hour (Kwh) or about 100,000 Kwh. That number strikes me as about right, as a household uses about 5-20,000 Kwh per year.

In the late 1990s there were 35,000 SMT lines in the world, at a 3% growth rate that would be about 50,000 lines now. So worldwide SMT reflow oven use would be about 5E9 KWhr (50,000 ovens x 100,000 Kwh/per year) world wide.  

With most heat loss be due to convection, the increase in energy use will be approximately proportional to the difference between the oven temperature and the room temperature (25C). An oven processing tin-lead solder would run at about 210C versus lead-free’s 250C. So the added energy for a lead-free oven would be about (250-25)/(210-25) or about 22% more. So if all assembly lines in the world are SMT the added energy use would be about 0.22x 5E9 Kwh = 1E9 Kwh. The cost of this extra electricity would be about $100 million (US) at $0.10/ Kwh. The electronics industry generates about $1.5 trillion in sales. So this added cost would be about 0.0067% of sales. Since world electrical use is about 150,000 E9 Kwhr per year, this increase is about 1/150,000 of all of the electrical use or 0.00067%.

So although more electricity is used, the increase is not significant to the value of the electronics sold or the total world use of electricity.

Thinking about higher temperatures reminds me that my Indium Corporation colleague Dr. Ning-Cheng Lee is presenting a paper this week on a high melting temperature lead-free solder based on a BiAgX alloy system. Higher melting temperature solders are often needed in what is referred to as a solder hierarchy. Solder hierarchies have solders that melt at decreasing temperatures in multiple soldering steps, starting with the highest melting solder.

Cheers,

Dr. Ron

• High thermal conductivity (33W/mK)
• Higher melting point than SAC alloys (221C)
• Low tensile stress, so suitable for large die (5800psi)
• Excellent thermal cycling properties (-55 to 125C)

1.  Preform (a specially-shaped solder piece) with TACflux® used to hold the preform and die in place
2.  Solder paste, which holds the die in place with no extra materials added 
3.  Soft solder die-attach wire, a fluxless type of solder wire, which is melted onto the substrate metallization under an inert cover gas, and the die directly mounted onto the molten solder pool, then allowed to cool.

Folks,

I have often pointed out that SAC solder's poor wetting is both a curse and Godsend.  It is a curse when trying to fill a through-hole in wave soldering, and a Godsend when assembling close lead spacings as shown in the image (below). Indium Corporation colleague and friend, Mike Fenner (image below), pointed out that, when I say that, "SAC solder doesn't wet well", I should be saying, "it doesn't spread well". His explanation follows:

SAC is different from SN63, and I think it is helpful to explain the difference by making a subtle differentiation between wetting and spreading.

The way that solders spread and wet to a surface is a balance of competing forces. We have surface tension acting to make the molten solder shrink into a ball, and wetting forces trying to make it spread across the surface. Wetting is also the action of the solder dissolving into the surface to form an intermetallic. This intermetallic is essence of the solder joint. The balance changes with different alloys, surfaces, and processes.

Most people are very familiar with the way that tin lead solders behave - and that governs their expectations. The different balance in SAC means the solder tends to spread less for the same wetting and, therefore, can give the impression of a lower quality joint. This lack of spread is usually expressed as 'poor wetting'.

I would explain this by saying the “active ingredient” in both solder families is tin. SAC alloys have a ~50% higher concentration of tin than the Sn63 solder alloy. This gives them a higher surface tension which increases the balling (coalescing) force. At the same time, the less dilute tin, in SAC solders, dissolves into a surface faster. So the final SAC joint can have a well formed intermetallic, but not high spread. These relationships will vary with surface finish and, of course, flux chemistry and process conditions come into play, but that’s for another day. Meanwhile I hope this simplified explanation helps.

Thanks Mike!

Cheers,

Dr Ron

The solder image is courtesy of Vahid Goudarzi of Motorola.

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.

Folks,

I was at SMTAI (Surface Mount Technology Association International) from September 24 and 27, 2010.   As I mentioned, I chaired a session on Alternative Alloys from 2:00-3:30PM on Tuesday 26^th.

At this session, Greg Henshall presented a paper on the Low Silver BGA Sphere Metallurgy Project. This paper was a collaborative effort of six companies. In addition, Richard Coyle presented an overview of the work of three companies entitled The Effect of Silver Content on the Solder Joint Reliability of a Pb-free PBGA Package. Both of these projects evaluated lead-free thermal cycle reliability as a function of silver content and compared the results to tin-lead reliability.

Both papers concluded that as far as thermal cycle reliability is concerned SnPb<SAC105<SAC305< SAC405. Coyle’s paper summed it up the best:

Each of the SAC alloys outperformed the SnPb eutectic alloy in every test including the long, 60 minute dwell time test. This tends to diminish the argument that SAC is less reliable than SnPb. (See Coyle’s figure. Data curves to the right are more reliable.)

Henshall’s paper also showed that the addition of dopants, to improve shock resistance, in SAC105 does not reduce thermal cycle life.

So, it appears, at this time, that, from a thermal cycle and drop shock perspective, it is looking more and more like SAC based solders out perform tin-lead solders in these two reliability arenas.

At the end of the session a noted lead-free curmudgeon came over to introduce himself.  We have had a jovial disagreement on several blogs etc. in the past re: lead-free status and issues, but had not met in person.   I should mention that this person is a college graduate, a former technical leader at several influential technological companies, and he owns a PE license. I asked him what he now thought about lead-free reliability after hearing the talks. He claimed that he is a little less likely to think that lead-free reliability is a disaster. He still refuses to purchase any lead-free products. He buys old units (pre-2006) on eBay.

I mentioned that over $2 trillion of electronics has been placed in the field since 2006 with no unusual reliability issues.   I then went on to say that a RoHS-compliant product is much more likely to fail due to a non-RoHS related issue. He did not disagree. So then I asked him why he won’t use RoHS compliant electronics. His answer: “I just don’t trust them.”

Cheers,