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.
Sn96.5Ag3.5是221°C的共晶材料。这种银锡材料很早以前就被分层焊接(step soldering)和在汽车工业上使用(automotive industry ), 是最早的无铅材料之一。Dr. Ron Lasky的“Happy Birthday RoHS”博文中有更详细的介绍。
- 高热传导性：High thermal conductivity (33W/mK)
- 低的张应力（ tensile stress ）： 5800psi
- 良好的热循环性能thermal cycling：-55 到125 0 C
Pic: Google Image
Typical Tabbing Ribbon Solders
Only a few solder alloys have become common, industry-wide, among solar module assemblers, and those can be pared down into three categories:
- BiSn alloys (58Bi42Sn, 57Bi42Sn1Ag)
- SnPb alloys (63Sn37Pb, 62Sn36Pb2Ag)
- SnAg alloys (96Sn4Ag)
Tin-Silver Solder (SnAg)
SnAg has become the most widely used Pb-free solder alloy, particularly in tabbing ribbon designed for cell interconnection. Historically, its melting temperature (221°C) made it an obvious replacement for processes previously running SnPb solders.
In designs where step soldering is necessary (however uncommon in back end solar module assembly), SnAg can be used as the step previous to soldering with Sn63 or similar Pb-Free solder (albeit carefully since the second soldering temperature is quite near 221C).
While SnAg eutectic solder is a desirable composition for electronic component soldering, for instance, power semiconductors, recent studies using this alloy for stringing solar modules have indicated that the other common alloys listed for this application are easier to work with and better designed to meet the needs of this solar assembly application. SnAg does have a high melting temperature, and the preferred fluxes for module assembly are not yet optimized for this solder composition.
Regardless, SnAg has its benefits. When a solder that melts somewhat above the melting point of a “standard” solder alloy is needed, and it must be Pb-free, this is it!! Check it out!
- 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)
The solder can be applied in a number of different ways onto the substrate in Power Semiconductor applications:
- Preform (a specially-shaped solder piece) with TACflux® used to hold the preform and die in place
- Solder paste, which holds the die in place with no extra materials added
- 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.
Heat transfer through the baseplate and direct-bonded copper (DBC) makes 1/ and 2/ (above) the preferred method of attachment for IGBT modules. By using a vacuum reflow process, it is also possible to make even solder paste (which always seems to generate some voids, even in standard processes) almost void-free, which was demonstrated in our recent paper.
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 email@example.comCheers,
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!
There seems to be a growing trend to use a low-Ag or Ag-free solder alloy for Surface Mount Technology (SMT) electronics assembly, similar to what is commonly offered for bar solder, used in wave and selective soldering.
For through-hole performance, the strength and stability come from the entire barrel of solder, whereas it is usually the foot and heel fillets that give SMT solder joints their strength.
Lets talk about the other issue with using a eutectic solder alloy in SMT: tombstoning. One of the benefits of using the SAC (tin-silver-copper) alloy for SMT and solder paste, is that it has a built-in plastic range, similar to that of Sn62 (62Sn 36Pb 2Ag). It is this plastic range that prevents tombstoning, and takes into account the inconsistent heating of the solder across the part (which is the sole cause of tombstoning). Switching to a eutectic alloy eliminates the plastic range and opens the door for tombstoning.
Any powder manufacturing issues, such as the inconsistent distribution of dopants throughout the alloy and powder matrix, takes a back seat to the surface mount reliability concerns.
There are other alternatives, such as SAC0307 (99Sn 0.3Ag 0.7Cu)… But, with the price of Ag finally coming down, and a long history of SAC usage, we don’t think it’s going to be a major player.
Next time, we'll talk about the manufacturing and costs associated with low-Ag and Ag-free alloys.
I hope this helps. Contact me with any questions.
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). World 2008 bismuth refinery production was 15,000 tonnes, of which China produced 78%, Mexico 8% and Belgium 5%.
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.
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
PS: Read my follow-on posting about bismuth.
The image is of a bismuth crystal with an iridescent oxide surface from http://en.wikipedia.org/wiki/File:Wismut_Kristall_und_1cm3_Wuerfel.jpg
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,
I thought I would take a stab at listing the minuses, pluses, and “it’s a wash” aspects of assembling with lead-free (LF) solder. Here are my first thoughts. Please tell me what I missed or disagree.
1. Pb-Free requires higher reflow temperatures
The Tm for LF solders, in the 217-229C range, has created numerous challenges:
a. PWB warpage and damage
b. Component damage
c. New defect modes such as graping and head-in-pillow defects (although concurrent reduction in solder paste deposit sizes for 0201 and 01005 passives and 0.3 mm CSPs also exacerbate these defects)
d. Defects related to increased oxidation
e. Increases in voiding
f. Increases in tombstoning
2. The higher cost of LF solder, mostly for wave soldering
a. It’s not just the silver, tin is much more expensive than lead
3. Poorer wetting of LF solders, creating the most significant challenges in wave soldering
4. More rapid copper pad dissolution on PWBs in wave soldering
5. LF solder attack of wave solder machine components
6. LF reliability in harsh thermal cycle testing appears poorer than tin-lead solders
7. Tin Whiskers
It’s a Wash
1. Short-term reliability in consumer product-type environments
2. Protection of the environment if discarded products are improperly disposed of
a. Lead in electronics has never been shown to cause a problem in land fills
3. Since July 2006, about $3 trillion of products have been manufactured with LF solder, with no “the sky is falling”-type of problems
1. LF solder's poor wetting enables finer lead spacings (see photo Courtesy of Motorola)
a. It may be argued that some modern electronic products (e.g. smartphones) could not be made with tin-lead solder
2. It is safer to recycle LF solders, especially if performed in a non-controlled environment
OK - your turn. Please comment.
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.
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:
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.
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.”
To achieve enhanced properties, and fill niches that SAC fails to satisfy, research is being done on the addition of dopants to SAC solders. This is where I started to become intrigued with bismuth. The more I read, it seemed, the more I was running across this odd metal that I didn’t know too much about (remember, I’m new to the realm of solder). This prompted a small search that yielded some fascinating facts:
· Bismuth is a brittle metal, often displaying a pinkish hue due to its surface oxide
· Bismuth has a low melting temperature (271°C)
· Bismuth expands upon solidification, kind of like water
· Bismuth is the heaviest non-radioactive (perhaps considered slightly radioactive), naturally occurring metal on the periodic table
· Bismuth is not usually mined by itself, rather a bi-product of lead, tin, silver, and other metals
· Bismuth is the most diamagnetic of all metals
· Bismuth has the lowest thermal conductivity of all metals other than mercury
· Bismuth crystals exhibit a reflective rainbow of colors because of the varying thickness of oxide on the surface
· Bismuth subsalicylate, which is the active ingredient in Pepto Bismol, is outlawed in France (due to outdated concerns about it causing encephalopathy)
The most interesting attribute of bismuth, from an electronics materials perspective, is that, when alloyed with other metals, it creates low-melting temperature alloys. In particular, when alloyed with tin at the eutectic composition, the alloy melts at 138°C and displays properties comparable to the tin-lead eutectic. The brittleness of bismuth is the main concern when using it for soldering; however, this effect can be mitigated by the addition of more malleable metals to the alloy, such as silver. Many of the popular bismuth-containing alloys contain a high percentage of bismuth, but work is being done with lower concentrations, yielding different properties. Although the melting temperature will not be lowered as dramatically, bismuth at lower concentrations has the potential to enhance performance in drop testing and thermal shock.
Given rising concerns over the cost of metals, bismuth may become more of a contender in the search for alternative solder alloys. I’ve come to think of bismuth as my new friend, because I think that it offers a different avenue for exploration and seems promising for enhanced low-temperature alloys.
Answers to the quiz of a few weeks back......
Phil and Rob had agreed to ask the GM if it was OK to ask the tech and engineers at some of their subcontractors to take the test anonymously. Over a period of two months Phil and Rob got 52 people to agree, almost all of them after Phil or Rob agreed to take them to lunch. They asked Patty to grade the “exams.” Today Patty would reveal the results.
“Phil, this is one of the best bets I have ever made,” teased Rob.
Everyone at the lunch table chuckled, but the look on Phil’s face said he expected to lose. Rob has said that he thought the average score would be less than 70%, Phil insisted that it would be greater than 85%. In asking the different folks to take the test, invariably Phil started asking questions not on the test. He was surprised that no one knew what tin pest was. He even asked how to time balance a chip shooter and flexible placer, only one in twenty knew.
As Patty approached the lunch table, the ensemble held their breath.
“OK, Patty, tell us the bad news,” Phil said in a resigned tone.
“Rob wins, the average score was 58%,” Patty said getting to the point. “Here are the answers and percentages on each problem,” she went on:
1. What is the composition of SAC305?
96.5% tin, 3.0% silver, 0.5% copper. 60% got this right.
2. What are tin whiskers?
Tin whiskers are metal whiskers that can “grow” from tin plating on component leads. They are mitigated by 2% bismuth in the tin, a nickel overplate of the lead copper, a matte tin finish, and a few other mitigation approaches. 40%.
3. In a stencil aperture, what is the area ratio?
The ratio of the area of the aperture opening divided by the area of the side walls. This ratio is typically used for circular and square apertures. It is equal to D/4t, where D is the diameter of square side and t is the stencil thickness. 40%
4. What is an approximate peak temperature for a reflow oven in lead-free assembly?
Any answer 235 to 250C accepted. 90%
5. A board is inspected after wave soldering and one lead is not soldered to the board. The board is run through the wave solder machine again and has the same defect on the same lead. What is the most likely cause of the defect?
a. The solder temperature is too low.
b. The pad on the board is oxidized.
c. The preheat temperature is too high.
6. What are local fiducials on a PWB for?
Local fiducials are located near the pads of a component with fine lead spacings to assure accurate placement. 70%
7. What does "thixotropic" mean in regard to solder pastes?
The viscosity decreases with increasing shear stress. Hence, during printing the viscosity drops as the paste is forced through the aperture, aiding good aperture fill. It increases as the printed deposit rests, minimizing slump. 20%
8. A chip shooter places passives at a rate of 36,000 per hour. It is placing 300 passives on a PWB, how many seconds will the chipshooter take to place the passives on one board?
300/36000 = 1/120 hr = 30 seconds. 90%
9. A reflow oven belt speed is 100 cm/min. The PWB is 40 cm long. What is the minimum cycle time that the oven can support?
The amount of time that the belt needs to cover 40 cm is 40/100 = 0.4 minutes = 24 seconds. This is the minimum cycle time the oven can support. 40%
10. What is "tombstoning"?
Tombstoning is observed when a passive component's terminations experience unequal wetting forces which are strong enough to lift one end of the passive so that it looks like a tombstone. 60%
Overall average score 58%.
“Wait a minute Patty, your answers are too demanding,” Phil shouted.
“Calm down Phil, I gave full credit for anything close,” Patty responded.
In unison, almost everyone at the table sighed “Yikes.”
Patty interjected, “One person who received a 70% commented after completing problem 9, ‘I didn’t think I would need a PhD in math to do this quiz.’ “
Dr. Ron, Thanks for your helpful post. I get close (-1.1%) with your formulas for an alloy I am working with. I think the crystal lattice packing factor for some of the individual elements is throwing off the result since it is different than the alloy. I tried to take this into account, but I get an error on the opposite side (+1.6%) of the actual. Any thoughts on if your formula can be made more accurate by taking element and alloy crystal lattice packing factors into account?
The solder alloy calculation assumes that the metals mix with no interaction, much as miscible liquids, of different densities would. There are numerous phenomena that could cause errors, they include:
1. Metals can come form different crystal systems. Lead, silver and copper are face-centered-cubic, whereas tin, the base metal for most solders, is of the tetragonal system.
2. Some metals form intermetallics with tin, such as copper and silver. These intermetallics have different densities than the metals or the resulting alloy.
3. Grain boundaries can leave some (probably small) empty space.
So I think Ken's 1% accuracy is very good. The biggest mistake one can make however, is the most common......assuming that the density is simply given as the sum of the metal mass fractions times the metal densities. To many, it seems logical, but it is wrong.
My original posting on how to derive the formula for solder alloy density is below.
Wandering through the references to indium metal on the internet, I sometimes see it referred to as, "that 'rare' metal." But is it really so rare? I recently talked to my colleague, Claire Miko, Director, Metals and Chemicals for Indium Corporation and asked if the reports of the rarity of the metal (like the death of Mark Twain) were greatly exaggerated.
Question: The element indium is widely used today in many electronic (glass coating, low temperature solder, hermetic sealing and thermal interface material) and solar applications (CIG solar panels), but very little is known about it. Can you tell us where indium metal comes from?
Claire: Indium is a by-product of several base metals such as zinc, lead, copper, tin and other poly metallic ores. It is very abundant on the crust of the earth (much more than silver for example and the annual silver production is at least 40 times bigger than the annual indium production). Geographically indium is abundant in
Question: Does indium have to be refined after it is mined?
Claire: Indium is present in the base metal ores at ppm levels. It first needs to be separated from the base ore and concentrated. This is done at the base metal smelter (for example during the refining of zinc, lead, copper, tin etc). It is then further refined and purified at indium refineries.
Question: Indium Tin Oxide (ITO) is the one of largest indium-containing products today. How much of the indium mined goes to making ITO?
Claire: About 50% of the indium refined is used for making ITO. A larger percentage is needed to start the ITO target productions but the sputtering process used (when putting the ITO layer onto the glass) is inefficient and generates a large quantity of indium which is reclaimed and is then recycled and put back into circulation.
Question: Is there enough indium available to meet the current and future needs of the marketplace?
Claire: The indium production has always expanded to meet growing demand. Indium production grew from 70MT (metric tonnes/year to over 500MT/year over the last 20 years. At the moment only one-third of the indium mined yearly is being refined in indium metal, another third accumulates in residues that are more expensive to treat but they remain available for future processing, and the last third is currently lost because it does not reach a base metal smelter which has the equipment to separate it from the base metal ore. Investments at these smelters would enable the extraction and refining of these quantities if the need arose.
Question: Are there recycling programs in place to recover unused ITO from the targets used to deposit it onto the glass surfaces where it is used? What is the rate of recovery?
Claire: There is ample capacity to treat spent ITO targets (as per point 3) and the recovery process is now mature and very efficient. The cycle time of this process has also now become very short enabling a very quick return of the refined indium for new consumption.
Question: Are there any viable alternatives to ITO?
Claire: A far as we know ITO remains the best material for LCD and other flat panel displays applications. It offers the best performances in terms of optical transparency, electrical resistivity, uniformity of both transparency and resistivity, chemical and mechanical stability, resistance to corrosion, and, finally, uniformity of etching.
The cost of the ITO on 42” TV represents less than $2 and less than 1% of the display cost. It is a small cost to pay to ensure that the quality of the display is maintained. Alternative materials have shown significant process problems with resistivity, uniformity and chemical and mechanical stability.
After my recent post on the fact that there was no data linking tin whiskers to the Toyota sudden acceleration issues, there continue to be more posts saying things like “Tin Whiskers Implicated in Unintended Acceleration Problems.” Many of these posts link back to the earlier TechEye post. The basis for all of the posts, is a paper written by EurIng Keith Armstrong . Armstrong’s paper is titled: “Toyota ‘sticking pedals’ recall is a smokescreen, Their sudden unintended acceleration problem is caused by electronics either due to EMI, lead-free soldering or software ‘bugs.’” It does not appear that Armstrong’s paper was sponsored or refereed.
Since it appears that this entire wave of reporting implicating tin whiskers, in this important issue, emanates from Armstrong's paper, it is helpful to quote his entire comments on Tin Whiskers:
"9.0 Lead-free soldering:
In recent years, various countries and trade blocs (including the European Union) have banned the use of lead on electrical solder, on the basis that lead going into landfill when electrical and electronic products are disposed of is bad for the environment, and hence for people.
But many accuse them of being shortsighted – lead has been added to solder in quite large amounts for many decades because it made the other main constituent, tin, behave much better, considerably improving reliability.
Now that lead has been removed from solder, which is now mainly tin (with a little silver and copper added) all sorts of new possibilities arise for short-circuits and open-circuits, and intermittent shorts and opens, mainly on printed circuit boards (PCBs) and mainly associated with small-footprint integrated circuits (ICs), especially ball-grid arrays (BGAs).
Its really just another cause of intermittent or fixed short-or-open circuits in electronic PCBs and modules - but one that would not have been any problem until a few years ago, and so could have caught Toyota by surprise.
John R Barnes has created a monumentally huge library of references to the problems of lead-free soldering, especially tin whiskering, see www.dbicorporation.com/rohsbib.htm. Prepare to be totally overwhelmed!
Removing lead from solder has the following effects:
9.1 Tin whiskers
These will grow out of soldered joints and can contact other conductors, causing short-circuits between PCB copper traces and the pins of connectors. They are often no longer than 0.5mm (about 1/50th of an inch) but can grow to 1mm (about 1/24th of an inch) or longer, especially in damp conditions.
Even at 1/50th of an inch they can short between the pins on a modern integrated circuit (IC). And the process of removing the PCB for inspection can brush them off, so you never find them.
And if you didn't accidentally brush them off, they are so thin they are very hard to see - you need a powerful microscope. They are as fine as the finest spider-web threads, yet can carry sufficient current to short-out the electronics. You won’t see them unless you are looking for them.
Being so thin, they can wave around in the breeze and/or due to shocks, vibration and acceleration, causing intermittent short-circuits.
The iNEMI organisation has published guidelines (www.inemi.org) on how to ensure that tin whiskers don’t grow too long, but I don’t know to what extent these are followed by suppliers of electronics to the car industry in general, or Toyota in particular."
Note that, in this paper, there is no data or any evidence re: tin whiskers discussed from investigating any of the vehicles in question. All of this paper is an opinion. In addition, the title of Armstrong’s paper leaves no room for any other cause, it has to be electronics or software. This position is very strong indeed for having no supporting data.
More recently Bob Landman added these comments to the tin whisker discussion:
“the increased use of electronics in automobiles when mixed with RoHS can make for a deadly cocktail. We don’t know what the causative agent [in regard to the Toyota recalls] was, but I have heard recently of new autos showing up at dealers that will not start. That cause has been linked to tin whiskers.”
Bob heard this. There is no report and no data. Until Bob gives us a reference for some analysis and data, his comments are little more than hearsay. I searched the web in vain to find information related to Bob’s quote. In addition this comment is a little surprising, tin whiskers are usually associated with a certain amount of aging, hence not usually found in new products.
That tin whiskers exist and cause failures is irrefutable. NASA has an excellent website related to tin whiskers and failures caused by them. However, the total number of tin whisker fails reported is less than 100. Many other types of electronic failure modes would appear to be much more common.
My purpose of writing this post is not to suggest that tin whiskers are not a concern in lead-free electronics. However, it is a fundamental principle in engineering and science to only make pronouncements on how something failed, when they can be supported with data. No data supports implicating tin whiskers in the Toyota incidents. It is also troubling how readily many people referenced the work of Armstrong without apparently reading what he said and checking his sources and lack of data.
The image is from: http://nepp.nasa.gov/WHISKER/photos/ziff/ZIFF-whisker-3.JPG
This blog has been in existence for a little over two years now, and we would like to thank our readers for the feedback and inquiries you have provided. I welcome your comments on what you would like from us. Leave a comment below, or email me at firstname.lastname@example.org.
And now a look back on past topics of interest:
If you look for indium on the periodic chart, you will see that it located right by tin (Sn) and lead (Pb) but it is a world away in terms of its properties.
Indium and indium alloys have some unique characteristics that make them ideal for a variety of usages including: soldering to non-metals, low temperature alloys, RoHS compliance, thermal management, battery manufacturing, cryogenic or hermetic sealing and many, many more.
Does your application require you to bond to glass, quartz or ceramic? Then you know that traditional solders will not work. But if you choose Indalloy #4 (pure indium) or Indalloy #1E (52In 48Sn) you will get excellent wetting. If you choose an alloy that includes silver (like Indalloy #290 which is 97In 3Ag or Indalloy #3 which is 90In 10Ag) you will get slightly less wetting but a much stronger solder joint.
Surface preparation along with the proper tools and the proper process are key to acheiving the proper bond. You can get all the details with our PDS, Bonding Non-Metallic Materials Using Indium and High Indium Alloys.
I caught up with Alan Rae after a recent IWLPC committee meeting, where he jokingly asked me to, “Stop asking important questions” - LOL! He was kind enough to give me a few moments of his time to share his wit and wisdom, and answer some technology questions that, yes, I thought were kind of important…
[Andy Mackie] You’re increasingly being seen as “Dr Nano” by the electronics industry – how did you arrive as the focus of so much of this technology?
[Alan Rae] At the start of my career I was in the structural ceramics business. In the days of “ceramic fever” in the 1980’s the mantra was sub-micron and monosize (monodisperse) for lower temperature processing and better properties. It worked. Then at TAM Ceramics we made “sub-micron” barium titanate and other ceramic materials but we didn’t call it nano then. When I was at Cookson Electronics in the early 2000’s we started to see nanotechnology emerging from the woodwork with people saying the same about nanomaterials for the electronics industry. Then I joined NanoDynamics in 2004 and realized the scope and potential, ranging from semiconductors to touch screens to printable electronics, to LED lighting, to solar power, to materials such as nano solders, dielectrics, conductors…the list is growing but the leitmotiv is the same – small, monosize, tightly-controlled.
[Andy Mackie] OK, so Nanotechnology has been a buzzword for quite a while – is there a clear definition yet, and what current uses are there for nanotechnologies that may not be immediately obvious?
[Alan Rae] Well, the definition has been really tough to derive – ISO TC 229 “Nanotechnologies” came up with a definition that one dimension of a particle, needle or plate should be less than 100nm but it’s really tough to define…should all particles be less than 100 nm? 50%? Any? And should it be exactly 100nm? There are a lot of opinions. The Woodrow Wilson Institute lists over 800 consumer products containing nanomaterials on the market now – industrially the products range from semiconductors, to fillers in packaging materials and underfills, to antimicrobial and self-cleaning coatings for phones. Solar panels, especially thin film ones, depend on nanomaterials in their manufacture.
[Andy Mackie] What is in the pipeline for nanotech electronics and semiconductor interconnect materials? I know that nanosolders are starting to gain ground in some areas – what else is upcoming?
[Alan Rae] Much of the work in nano metals is being done by universities and small companies – for example my small company is working with Purdue and the Air Force to develop a novel solder technology – but commercialization will come by partnering with established companies like Indium Corporation, who have the distribution and technical support so that customers will be comfortable with a new material. Cost and reliability are king. Indium is already in the reactive nano foil business; there are existing and near-term applications for silver, silver-coated copper, alumina coated boron nitride and their combinations in adhesives, shielding materials and thermal interface materials.
[Andy Mackie] Several years ago, quantum dots were being promulgated for tunable band-gap detectors and quantum computers. How close are quantum dots to seeing real uses, and what else is on the horizon?
[Alan Rae] Quantum dots are unique and have great potential in medical imaging and as frequency shifters for LEDs. The markets haven’t developed yet because of the cost and because some of the best dots are cadmium (toxic metal) based. I’m working with a group at University of Buffalo which has a silicon quantum dot process that looks like a promising alternative. Quantum dots will have their time…but not just yet. In terms of new developments – they range from core shell and modulated structures for thermoelectric to replacing indium tin oxide with carbon nanotubes or graphene. The US National Nanotechnology Initiative tracked $1.6 billion in Government spending (check out www.nano.gov) in the last year at Universities and small businesses and NSF has set up centers of excellence at Cornell and other great universities that are really working hard to translate science into technology so we can make practical products.
Alan, many thanks for your time, and for sharing your insights with us.
From One Engineer to Another®
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