Indium Corporation Blogs

There have been a variety of people who were key to the development of indium metal in general, and Indium Corporation in particular, over the years.

People such as:

• Hieronymus Theodor Richter and Ferdinand Reich who discovered indium metal in 1863
• Daniel Gray, William S. Murray and J. Robert Dyer who formed the original Indium Corporation back in March of 1934 and who hold patents on processes and applications involving indium
• Frieda Nojeim who joined the company in 1966 and was elected vice president in 1971

 

And, in 1972, Charles E.T. White joined the Indium Corporation as a vice president (he was elected executive vice president in 1981).  I had the pleasure of knowing Mr. White before he retired.  He was indeed a character, but he also knew a LOT about indium.

In 1986 (a lifetime ago in the electronics industry) Mr. White published an article called: Indium: High Technology Metal in Advanced Materials and Processes magazine.  I ran across a copy of it the other day and, after reading it, was interested in how relevant it still is today, even though technology has marched forward.

Of course the physical characteristics of indium are still as valid today as they were then. 

• Resistance to thermal fatigue
• High thermal conductivity
• Wetting of non-metals (glass, quartz, ceramics)
• Malleability and ductility, even at cryogenic temperatures
• Electrical conductivity for a variety of screens
• Indium does not work harden

 

But one might expect the technology described in an article from nearly 30 years ago to have evolved or gone entirely away, resulting in the elimination of the need for the indium.  The truth is, many of applications that Mr. White mentioned still exist today:

• "Conforming gasket material for cryogenic vessels."
• "Indium is present in every wristwatch and computer screen that uses a liquid-crystal display."  Okay, so no one wears wristwatches anymore, but the screens on our phones (the new time-telling devices) have indium tin oxide coatings.
• "...used in lens blocking and in temperature-overload devices such as safety links, fuses and sprinkler plugs."
• "Many solder alloys containing indium have been developed to take advantage of indium's enhancement of thermal-fatigue resistance, reduced gold scavenging, and resistance to alkaline corrosion."
• "Glass sealing alloys containing indium ...have been developed for electronic device packaging where high temperatures cannot be used."
• "Indium's use in solder alloys is likely to increase as specialty solders become more important in electronic assembly techniques."
• "The whole area of conductive films of indium oxide and indium-tin oxide has good potential for growth.  This includes solar cells: silicon-cell efficiency can be improved with an indium or indium-tin oxide coating."
• "New applications such as solar cells made of indium-copper-diselenide/cadmium-sulphide are under active development."

 

And while these indium applications still exist today, R&D continues to find new opportunities for this very unique metal.  We have several Research Solder Kits that you can use to evaluate the value of indium in your process.  Just go to our e-commerce page or contact our engineers to see how indium can work for you.

Carol Gowans

Since the NanoBond^® process is almost instantaneous, fluxes are not used. (They just don’t have enough time to heat up to their activation temperature and remove oxides.)

So, because this is a fluxless soldering application, surface choice and preparation become very important. We no longer have the chemical power of a flux to break down surface oxides, instead we must make sure our surfaces are ready to be joined.

The first choice to make is: will you have solder on the parts to be bonded, or will you use solder-coated NanoFoil^®?

If you decide to use bare NanoFoil^®, the parts must have a solder finish such as pure indium, SAC 305, or tin. If you choose to use a solder-coated NanoFoil^®, you can bond gold and silver metallized parts.

 

Need help figuring out what to do? Ask us: AskUs@indium.com

*This post is part of the NanoBond^® Process series

The first time I was taught how to solder (as a child), I was told: “All the surfaces need to be mechanically cleaned and chemically cleaned.” The person who told me this was referring to pipes, I was learning about plumbing. (I would have never thought we'd be using nanotechnology to create solder joints!) Although your application is probably far from a plumbing job, the basics of soldering remain the same. The best solder bonds are formed when oxides and contaminants are not present.

These two points are taken care of in traditional electronics soldering by using a flux. Flux can flush away light contaminants like dust, and reduce oxides on certain metal surfaces. But, in the NanoBond^® process, we aren’t using flux.

Luckily, NanoFoil^® can power through the soldering process as long as the proper surface finishes are used, and they are ready to be soldered to. Different surfaces are prepared in different ways. Here is a list of some common surface finishes and what preparation they require:

• Gold – Wipe with isopropyl alcohol if aged
• Silver – Wipe with isopropyl alcohol if aged
• Tin – Remove oxides with 10% HCl if aged
• Solder coating - Remove oxides with 10% HCl if aged
• Aluminum – Add solder coating
• Molybdenum – Add solder coating
• Titanium – Add solder coating
• Naval Brass – Add solder coating

And for any surface you don’t see, feel free to contact askus@indium.com so we can find a solution for you.

 

*This post is part of the NanoBond^® Process series

One of the biggest misconceptions about NanoFoil^® is that it is a form of solder. While it may contain a solder coating if specified (usually tin), it is really a heat source. A NanoBond^® requires solder, whether it comes from a plating on the joining surfaces, additional solder preforms, or on the NanoFoil^® itself.

Surface Coating

There are many ways to deposit solderable coatings onto parts that will be NanoBonded. Sputtering, thermal evaporation, thermal spray, plating, and HASL (hot air solder leveling) are just a few of the options. Coating the parts that will later be bonded tends to make assembly a bit easier.

Solder Preforms

If the parts have a gold or silver surface finish already, a thin solder preform is a very simple way to apply solder in the assembly. Preforms are sold as custom-shaped foil for your application.

NanoFoil^® Coating

Although Sn is the most popular solder coating for NanoFoil^®, it has been custom plated for individual customer applications with indium, traditional solder alloys, and even Au/Sn.

By the way, make sure there is solder on both sides of the NanoFoil^®. I almost overlooked this very point today while I was bonding a set of industrial batteries. There was solder on the battery terminal, and I was about to use bare NanoFoil^® to bond it to a gold plated board. Luckily we had some tin plated NanoFoil^® that I used instead – to ensure there was sufficient solder on the board side of the interface.

 

*This post is part of the NanoBond^® Process series

Individually, indium and gallium each have some pretty interesting characteristics.

Gallium is liquid at 30°C (86°F) and, because it is less toxic than mercury and has a lower vapor pressure at higher temperatures, it is used as a mercury replacement in thermometers and other applications. 

Indium, as I have discussed in previous blogs, has many unique characteristics including high thermal and electrical conductivity, resistance to thermal fatigue and reduced scavenging of gold in soldering.

But, combine the two, add a little tin, and the resulting alloys are liquid at, and below, room temperature (8°C to 25°C) and are very effective in conducting or dissipating heat away from temperature sensitive components. They can also conduct heat and/or electricity between metallic and non-metallic surfaces.

A recent study published by researchers at the McCormick School of Engineering, working with scientists around the world, discusses the use of an indium-gallium based alloy (EGaIn) to make stretchable electronics.  The indium-gallium content overcomes the loss of conductivity that occurs when the material is stretched.  The liquid alloy allows the "electricity to flow consistently even when the material is excessively stretched".

In 2009, researchers at the North Carolina State University used InGa to form antennae that would not break.  Again it is the flexibility and the electrical conductivity of the liquid alloy that make this work.  Michael Dickey and Gianluca Lazzi who headed the research, indicated they "were surprised" that the alloy operated at about 90% efficiency, similar to the efficiency of copper.

So whether on their own or combined, indium and gallium can be the solution to a variety of electronic challenges faced today.  The Possibilities Are Endless!

Carol

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

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’m taking a few moments from Wassail Weekend , held annually in my village, Woodstock VT, “The prettiest small town in America”, to write a post about last week’s workshops at ACI.

Indium colleague Ed Briggs and I gave a 3 hour presentation on “Lead-Free Assembly for High Yields and Reliability.” I think Ed’s analysis of “graping” and the “head-in-pillow” defect is the best around. 

There was quite a bit of discussion on the challenges faced by solder paste flux in the new world of lead-free solder paste and miniaturized components (i.e. very small solder paste deposits.) One of the hottest topics was nitrogen and lead-free SMT assembly. There seemed to be uniform agreement that solder paste users should be able to demand that their lead-free solder paste perform well with any PWB pad finish (e.g. OSP Immersion silver, electroless nickel gold, etc.) without the use of nitrogen. Not only does using nitrogen cost money, but it will usually make tombstoning worse. However, in the opinion of most people, nitrogen is a must for wave soldering and, since it minimizes dross development, it likely pays for itself.

After Ed and I finished, Fred Dimock, of BTU, gave one of the best talks I have ever experienced on reflow soldering. He discussed thermal profiling in detail, including the importance of assuring that thermocouples are not oxidized (when oxidized they lose accuracy). He also discussed a reflow oven design that minimizes temperature overshoot during heating, and undershoot when the heater is off. Understanding these topics is critical with the tight temperature control that many lead-free assemblers face.

Fred Verdi of ACI finished the meeting with an excellent presentation on “Pb-free Electronics for Aerospace and Defense.” Fred’s talk discussed the work that went into the “Manhattan Project.” A free download of the entire project report is available.

There appears to be agreement that acceptable lead-free reliability has been established for consumer products with lifetimes of 5 years or so, but not for military/aerospace electronics where lifetimes can be up to 40 years in harsh service conditions. These vast product lifetime and consequences of failure differences are depicted in the Fred's chart (above). Commercial products are in quadrant A and military/aerospace products in quadrant D.

One of the greatest risks faced by quadrant D products is tin whiskers. Fred spent quite a bit of time discussing this interesting phenomenon. One of the challenges of this risk is that there is no way to accelerate it, so you can’t do an equivalent test to accelerated thermal cycling or drop shock. Fred mentioned that there have now been verified tin whisker fails, the Toyota accelerator mechanism being a confirmed one.

In addition to tin whiskers, lead-free reliability for quadrant D products (with a service life of up to 40 years) in thermal cycle and other areas remains a concern.  I mention that tin pest was not on the list of issues for this quadrant.

Fred and the Manhattan Project Team have identified many "gaps" that need to be addressed to determine and mitigate the risk of lead-free assembly for quadrant D products.  They plan to start this approximately $100M program in 2013.

For those that missed this free workshop, ACI host Mike Prestoy is planning another one in 6 months.

Cheers,

Dr. Ron

Intermetallics are a necessary evil in the metal-to-metal bonding world, which definitely includes soldering. There are two basic ways that metal will "chemically" bond to another metal: 1) solid solution 2) intermetallic. We will focus just on intermetallics for the moment as that is the most pertinent to the soldering world.

Many people confuse or interchange "wetting" for intermetallic formation (bonding). Wetting is just wetting. Just because a solder "wets" to a surface does not mean that an intermetallic "bond" has been formed. For example, and I have done this myself, 55.5Bi 44.5Pb can be melted onto a piece of copper. The molten BiPb will flow and "wet" to the surface of the copper. However, upon solidification (cooling) of the alloy, the BiPb can be peeled off. Why?... because no intermetallic was formed between the BiPb and the copper surface.

In order for an intermetallic to form, some amount of the surface metallization must dissolve into the molten solder. For this reason, Sn (tin) has long been a critical component of solder alloys. Molten Sn (tin) is an excellent solvent of many other metals. And, conveniently for us, those "many other metals" include elements like copper, gold, silver and, to a lesser degree, nickel. The rates at which these other metals dissolve into molten tin (solder) will differ. Gold dissolves readily into solder; whereas nickel does so slowly. So, because the rate of dissolution is different for each metal, the rate of intermetallic formation is also different. I have dealt with companies that have a long history of soldering to copper, and, for whatever reason, they are forced to switch to an ENIG (Electroless Nickel / Immersion Gold ) surface. (It is important to note that the gold layer is very thin and only applied to protect the nickel from oxidation. This gold layer readily dissolves completely into the molten solder and the "bond" is actually made to the nickel surface). When they make the change they sometimes encounter a number of issues such as incomplete wetting, poor bond strength, etc. and do not know why. They are not aware that the same reflow profile (time and temperature) that yielded a good (intermetallic) bond to copper is not sufficient to get the same intermetallic bond to nickel. Once they adjust their profile (more time and/or higher temperature) to allow for sufficient intermetallic formation , they are able to achieve acceptable solder joints. Keep in mind that dissolution, the phenomenon of a solid dissolving into a liquid, is effected by both time and temperature. Generally speaking, more time and more temperature allows for more dissolution and, hence, more intermetallic formation.

As mentioned in my opening line, intermetallics are a necessary evil. Why "evil"? Because they tend to be the most brittle part of the solder joint. Some intermetallics are more brittle than others. (This should be taken into consideration when choosing a solder alloy for a particular metallization).  For example, intermetallics that form between Sn and Au are often extremely brittle.  Being brittle, they can be subject to fracture, etc. This is a case where more is not always better. Yes, you need an intermetallic to get a "bond". Too thin of an intermetallic layer can be bad; but too thick of an intermetallic layer can be just as bad, if not worse. Believe it or not, the solder may not adhere well to its own intermetallic layer. Intermetallics are generally crystalline and chemically-stable structures....they do not really react with anything else once they have formed. If you have ever looked at a fractured solder joint, you may have noticed that the fracture likely took place right at the interface between the intermetallic layer and the bulk solder.

One other possible outcome of an excessively thick intermetallic layer is "voiding" at the interface. Why? Well, we first need to look at the reaction products. There are two basic types of reaction products that form the intermetallic layer between Sn and Cu. They are Cu3Sn and Cu6Sn5. In the first case there are 3 Cu atoms to every Sn atom and in the second case 6 Cu atoms to every 5 Sn atoms. In both cases the Cu is being consumed faster than the Sn atoms. Because of this disparity in the reaction, in an exaggerated scenario, little holes or vacancies ("voids") can form in the copper surface.

Intermetallic formation is not only limited to the solder process. Metal atoms can diffuse even in the solid state. And that movement can cause the metal atoms to interact, react, and form intermetallics or cause the existing intermetallic layer to thicken. "Ageing" experiments are often performed to measure how much the intermetallic layer will change and what effect it will have on the mechanical nature of the joint.

It is well beyond the scope or purpose of this blog post to provide an exhaustive discussion of intermetallics. Whole books could be written on the topic. So, I am far from doing justice to the topic of intermetallics. I can only hope to shed a little light on the subject.

Comments or questions are very welcome.

Folks,

I was at SMTAI 2011 last week and, as usual, JoAnn Stromberg and team did an amazing job.

I think SMTAI's technical program is the best around, offering scores of topics and world class speakers.  I chaired a session (MFX4) Alternate Lead-Free Alloys, with papers by Dr. Ning-Cheng Lee, Srinivas Chada, and Jasbir Bath.

I also co-authored three papers:
 
The technical sessions were extremely well attended, with 30-60 people in each.  An emerging trend is that the tech sessions are  swamped and the show floor not so much.  I think the Internet allows people to get a sense of products online, while the technical talks enable one-on-one discussions with experts in the Q&A after the papers.  It is tough to beat this interaction, even in an Internet world.

The new hot topic, to me, is the interest in "Conflict Minerals."  I participated in a panel discussion on this topic (see image).  It appears that the Dowd-Frank act will require publicly held companies to show "due diligence" in investigating their supply chain to determine if their tin, tantalum, gold, and tungsten come from "conflict" mines.  This requirement will likely ripple up and down the supply chain.  So we all need to become knowledgeable in this topic. Indium Corporation is very involved in this.

As for the venue, Forth Worth was nicer than I expected (not that a business traveler ever gets to see much). There was a nice restaurant area near the conference center. It reminded me of the Gaslamp Quarter in San Diego.  But for me, I longed for Disney World a little. Next year!

Cheers

Dr. Ron

If you have ever handled a piece of 80Au 20Sn solder alloy, one of the things that you might have noticed is that it does not look anything like gold (yellow lustrous metal). In fact, it does not look all that different than tin or any other tin based alloy.

And, yes, before we continue....it is the same gold (Au) used in jewelry, etc.

To the AuSn newbie, the first shipment of 80Au 20Sn solder may cause a little bit of alarm. "Did they send me the wrong alloy? This doesn't look like it has any gold in it!!!"

To the human mind, when one thinks of something that is comprised of 80% of a material, one naturally assumes that material will dominate the properties of the composite material. And, normally, that would be a valid assumption. However, in the case of a solder alloy, the composition is almost always reported in terms of percent by weight. So, in the case of 80Au 20Sn, the alloy is 80% by weight gold and 20% by weight tin. The "issue" lies in the fact that gold is more than twice as dense as tin; 19.3 g/cc versus 7.3 g/cc. 

So, let's think about this.............

If we have 100 grams of 80Au 20Sn alloy, you have an alloy comprised of 80 grams of gold and 20 grams of tin. But, it terms of volume of gold and tin, you have 4.15 cc of gold and 2.74 cc of tin. So, by volume, the alloy is 60% gold and 40% tin. The 40% (by volume) of tin in the alloy is enough to "dilute" the gold and greatly diminish any "yellowing" that one would expect the gold to impart to the appearance of the alloy. 


If anyone has ever attempted to accurately photograph a shiny metallic surface, one can appreciate the difficulty in so doing. So, the photo shows some 80Au 20Sn solder preform in comparison to a pair of stainless steel tweezers. Visually there is very little, if any difference in appearance.

 

Solder wire is generally used for manual soldering operations, including rework.  But, it can also be used in automated applications such as die-attach soldering.  Solder wire can be flux-cored, or solid with a separate flux used.

Each application can have different requirements for the wire.  For example, wire used in die-attach applications needs tight dimensional tolerances to insure an exact, repeatable amount of solder is deposited each time.  Reduced oxides are also critical to eliminate any "splattering" of the molten solder during the deposition process.

Wire can also be used for non-soldering applications. For example, indium (and indium alloys) wire are often used as a sealing material (particularly in cryogenic sealing applications) - more here) and as a thermal interface / management material.

Decades ago, 0.030" (0.76mm) diameter was the standard size, but today we are able to produce diameters as small as 0.001" (0.025mm) in tin silver (Sn Ag), tin silver copper (SAC) and gold tin (Au Sn) alloys.  Considering that a human hair is about 4X that size, that is a very small diameter!  Pure indium wire is limited to 0.010" (0.254mm), but alloys containing indium can be produced smaller than that.

The wide variety of diameters available in Au Sn make this alloy ideal for the complex applications in medical, aerospace, and other high reliability applications.  However, the Sn Ag and the Sn Ag Cu are used across a variety of standard applications that require lead-free materials.  Sn Ag is particularly good in soldering to Nitinol.

At first look, wire seems like a pretty simple product.  But specifying the right alloy, diameter, tolerances, and packaging can make all the difference.  It can help you achieve a repeatable process that gives you high yields, strong solder joints, and enhanced profitability.  For further information - contact me.

Carol Gowans

Many, many thanks to the hundreds of you who came by the Indium Corporation booth at Semicon West this year. Some of you came to hear about our recent global Semiconductor Assembly Materials Roadmap presentations, and all of you wanted to talk about your specific materials needs. Special thanks to those of you who shared the many successes you are having with our growing portfolio of applications-specific materials.


Based on these discussions, just a few of the topics that you will be hearing about in this blog in the coming months are:

- Lead/indium paste for multiple reflow applications onto gold pads
- Tin antimony solder paste
- Fluxes for 2.5D and 3D flip-chip applications
- Waferbumping fluxes for microbumps
- Jetting epoxy fluxes
- Tombstoning in semiconductor applications

Also: a final big THANK YOU to our friends at Nordson/Asymtek for showcasing the Indium halogen-free PoP paste Indium9.88-HF which was still dispensing after over 3 days of continuous usage at room temperature: proving its hard-earned reputation as the Energizer bunny of Pb-free (lead-free) dispense pastes. Here is a picture from the final day.

We look forward to seeing you all in 2012 (Exhibits: July 10-12th, 2012).


Cheers!  Andy

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

Cheers,
Dr. Ron

PS:  Interesting thought: About 165,000 tonnes of gold have been mined throughout history. If all of this gold was gathered into a cube it would only be about 21 meters on a side. At $1550/oz, its value would be $8.5 trillion, quite a bit less than the almost $15 trillion debt of the US government.  Yikes!

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.

Cheers,

Dr. Ron

image source

Folks,

Many people have been infatuated by the price of gold in recent months, but the price of silver has also skyrocketed. In 2000 silver was about $3.00 per troy oz. In the eight years that followed, its price grew to $15/oz. Today it is trading at over $41/oz! This price is almost an all time high, except for the time when the Hunt brothers tried to corner the silver market in 1980. The aberration of their efforts jolted the silver price to just short of $50/oz, but it settled down to $11 or so after the Hunts came under margin call and other pressures.

Unfortunately, the dramatic price increase today, does not appear to be an aberration. Although we may hope that it will soon drop to more historic levels, we may not have reason to expect that it will.

Although not as dramatic, tin and copper have experienced significant prices increases as well. The price of tin has doubled in the last year to $15/pound and copper has increased from about $3/lb to $4.50.  These metals are obviously key ingredients in critical electronic materials such as solder pastes, solder bar, and solder preforms.

In addition, oil, which is used for most organic electronic materials such as PWB resins, flip chip underfill, and epoxy fluxes, has increased to $110/bbl - approaching its all time high of $145/bbl.

All of these price increases have a significant impact on the electronic materials supply chain. Although we are used to price decreases in the cost of our mobile phones and PCs, at this point in time, the price of the materials that go into these devices will be increasing.

As one materials supply chain executive commented at APEX, “It’s not like we can be clever and somehow work around the price increase of silver and these other materials, we have to pass it on to our customer, or go out of business.”

Cheers,

Dr. Ron

在这里给大家拜年了：祝愿大家身体健康，万事如意，财源滚滚，阖家幸福！

有人告诉我2011年是金兔年(Golden Rabbit Year)。我上网查了一下，还是得不到考证。但是却让我想到了，Indium公司和金gold (Au) 这种金属，还是有一定相关性的。

金gold (Au)自身的熔点是⁰C，但是如果和锡tin（Sn）在一起，做成80%Au20%Sn的共晶合金，熔点就只有280 ⁰C了。金锡共晶金属有很高的焊点强度(joint strength)，抗腐蚀能力，导热性能好(thermal conductivity)，能够与各种贵金属兼容，还符合无铅的要求；可靠性很高。

金锡共晶金属在SMT里或是电子焊接中的用途很广泛。在SMT中，如果需要分温度层焊接(step soldering)，金锡的280度正好作为第一梯度的焊接；第二梯度的焊接合金可以选择锡银铜SAC或是锡铅SnPb；如果有第三梯度，可以选择锡铋SnBi。 在IGBT，automotive， 和 Radio Frequency (Power Amplifier) 的第一层焊接中，金锡常常是首选。 在电子焊接中，金锡的用途就更广泛了，特别是在医疗器械、仪器中。比如说在catheter导管的应用中，就可以用金锡做成微细的像小弹簧形状的物体，放入心导管中，帮助心肌梗塞的病人…随着全球人群的老龄化，在医疗器械、仪器方面的金锡应用，应该会越来越广泛。(当然，我可不希望有一天自己要靠它来救命哦。)  

当然，金的价格不菲，特别是这几年的疯长。所以选用这种合金，出于经济的考虑因素，也是要很谨慎的。 如果几年前我也买了金存着，现在也是一笔增值可观的小财富了：-）

Cheers!

PS: 我们家的张“小兔”宝宝五月底就要出生了，他现在已经在我肚子了老是练功夫了，踢来滚去的，肯定是我怀孕以来突然多看了功夫片和武侠小说的缘故。如果今年真的是金兔年(Golden Rabbit Year)，只希望小兔以后能够健康和财运亨通（lots of gold!），并且有一颗金子般的心(a golden heart).

Pic:
1. Baidu image
2. www.indium.com Indium Corporation

 

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.