Tin Silver Solder

Folks,

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

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

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

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

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

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

Cheers,

Dr. Ron

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

Musings on Metals: Copper

It could be argued that civilization began with the smelting of copper.  Although thousands of years before, humans fired clay to make figurines and containers, smelting required several non-obvious steps.  After all, the firing of clay, at some level, can be accomplished by simply dropping clay into a fire.

To smelt copper, our ancestors had to:

1. Take malachite (see photo) or another copper ore, grind it up or break it into small pieces
2. Mix the ground malachite with carbon
3. Heat the mixture in a vessel to 1,085^oC. 

Malachite Ore

Achieving this temperature with a wood fire is, to me, astounding.  Think about those days when you are grilling some burgers.  You leave the grill on after the burgers are done, to burn off the grease.  You come back 20 minutes later and the grill is at 500^oF.  You can feel the heat.  Even touching the knob to turn the gas off is intimidating, as the heat drives you back.  This temperature, 500^oF, is only 260^oC!  The ancients reaching 1,085^oC with wood and bellows is, indeed, impressive. By the way, a good rule of thumb to convert degrees C to degrees F from 100^oC to 1,500⁰C is that 2XC=F, this fast approximation is accurate to about 10% in this range.

The confluence of the three procedures is not only non-intuitive, but think how many times the smelter of old could only reach 900^oC and failed.  I have argued that if copper melted at 1,200^oC or so, civilization would have never gotten started.  This temperature is perhaps a little too high to reach with a wood fire.  The smelting of copper encouraged investigations into other metals, eventually resulting in the discovery of the processing of iron, an even less intuitive process than smelting copper.  So, I believe that the success with copper was necessary to the production of steel. 

Copper smelting became an industry that encouraged permanent settlements and stimulated trade, which encouraged writing and ciphering.  An effective copper smelter would likely keep secret some of his craft as he wanted a competitive advantage.  He could make more by smelting copper than doing anything else, so he almost certainly was an early specialist.

Considering all of this, I believe that without the discovery of copper smelting, we might still be living in huts or teepees, using stone tools, and living a nomadic existence without commerce, writing, or mathematics.  Examples to support this thesis are the state of native peoples in the Americas in the 1400s.  These native peoples had never learned to smelt metals and hence also lacked the follow-on aspects of civilization mentioned above.

Today, copper is a foundation material for electronics, given its excellent electrical conductivity, second only to silver.  Copper’s ductility likely aids in the formation of PWB traces and plated through-holes in that it resists cracking.

Additionally, copper's ability to form an electrical and mechanical bond with solder is another trait that makes it a winner as an electrically-conductive assembly material in modern electronics.

Copper has been used for more than 10 millennia, but, as with most metals, 90 to 95% of it has been mined since 1900.  About 15,000,000 metric tons (MT) are used each year, third to aluminum’s   22,000,000 MT and steel’s unequaled 1,000,000,000 MT.

In the next installment, we will discuss tin and how it forms an intermetallic with copper during soldering.  Thus making solder paste, solder wire, and solder preforms critical components of electronics assembly.

 Cheers,

Dr. Ron

Folks,

Patty is getting ready for her meeting on "Copy Exactly" with Mike Madigan.......

It was after 6:30 PM and Patty was just arriving home.  Since Patty was working late, Rob had agreed to make his signature dish, crispy macaroni and cheese.  Patty and Pete had just finished their project to develop a copy exactly strategy for ACME.  They would present it tomorrow to CEO Mike Madigan.  The local GM, Sam Watkins, would be there too.  Technically Mike was her boss in her Senior VP position, but since she had an office at the ACME facility in Exeter, NH, she reported to Sam - “dotted line.”  Patty had been working late for weeks on this project and was glad that the greatest portion of the work was over.

As she opened the door to her house, her twin 2 year old boys ran up to her in their excitement to see their mom and nearly knocked her over.  She tussled with them for a few minutes and then went to give Rob a hug.  He had the dinner on the table and they all quickly sat down.  Rob and Patty had a "no technology" rule at meals…..no mobile phones, iPads etc.  Meal time was family time.  After discussing the events of the day, Rob’s face lit up.

“I found out today that there is something we look at more than anything else,” Rob stated.

“OK, OK, let me guess,” Patty replied.

After a number of tries, she hadn’t gotten it.

Alright, I give up, Patty said with playful exasperation.

“Indium, or really Indium Tin Oxide (ITO), it is a transparent conductor of electricity.  We look through it when we look at our computer, tablet or mobile phone screens.  Think about it, for most of us we probably look through ITO for 8 to 10 hours a day.  It’s like we have a love affair with the stuff,” Rob explained.

Patty almost choked on some of the mac and cheese on the last comment.

“Why have you become such an expert on this stuff?” Patty asked.

“Well, you remember that ACME may go into component assembly? Sam asked me to look into indium thermal interface material (TIM)  for some of the component packages that need to dissipate a lot of heat,” Rob answered.

Patty knew a little bit about TIMs, but not about ITO.

“But why did you learn about ITO?” she asked.

“Sam is worried that Indium supplies may not be enough to satisfy TIM requirements, so he asked me to look into it,” Rob answered.

“What is the conclusion? Patty asked.

“Well, Indium is about as common in the earth’s crust as silver, but a little more difficult to extract.  This probably gives it the reputation of being rare.  Fortunately for me a recent analysis was performed that showed that the indium supply will be more than adequate for the next 75 years ,” Rob said.

Rob went on, “Indium is a very interesting material, it is one of the few materials that wets glass, so it enables metal sealing to glass.  It was only discovered in 1863 and it wasn’t until the 1930s that the first practical use for indium was discovered: aircraft bearing lubrication.  In a sense, it could be argued that it is one of the materials of the future, as we are just now learning about its potential.”

While he was talking, Rob reached into his backpack and took something out.

“Look at this, or rather listen,” Rob said.

With that, he took a thin bar of metal and bent it. A crackling sound came from the metal.  Patty was fascinated.

“What was that?" she asked.

“When a thin bar of indium is bent, it gives off a sound.  It is called “Indium Cry.”  The salesman for the TIMs we are using let me borrow it for a presentation I am giving to Sam Watkins next week,” Rob answered.

Dinner was soon finished and Patty had to get the boys to bed after playing with them for awhile.  Today was Spanish day and all of their discussions were in that language.  Another day was Mandarin Chinese day.  The boys already understood the three languages spoken at home.

A few hours later, Patty lay in bed - energized by the thought of her meeting tomorrow.

When she woke up the next day, she exercised at home, ate breakfast, and took the boys to day care.  See arrived at the office 30 minutes before the big meeting.  After checking emails, she went to the conference room where the meeting would be held, to set up her computer.  At precisely 8AM, Mike Madigan and Sam Watkins arrived.

“OK Coleman, let’s get this show on the road,” Madigan commanded.

“Since our last meeting we have analyzed assembly equipment and materials to determine which ones would be best for a copy exactly strategy,” Patty began.

She then showed her third slide and spoke to it.

“The winner for component placement equipment is Optoplace, as are their stencil printer and reflow ovens.  Exactotest makes the winning testers and ElectoMaterials the best solder paste and solder preforms,” Patty went on.

“Can you explain your methodology?” Sam asked.

“We looked at what The Professor calls ‘Profit Potential,’ simply the equipment and material that gives the most profit, assuming you are running a well tuned organization.  Fortunately, since ACME has 80 assembly lines we were able to get real process performance data on all of the major machines available, ” Patty answered.

“You answer seems a little evasive, why didn’t you use ‘Cost of Ownership?’” Madigan challenged.

“Some machines cost less to own, but they are down more for assists and when they need repair, we have to wait longer for the repair man.  From what The Professor taught us, uptime is very important. Anything that hurts uptime, like a late repairman or a machine that needs more assist time, will hurt profits.  The same is true for materials like solder paste.  If they cost less, but result in line downtime for response to pause issues or some other fault, they hurt profitability.” Patty responded.

Just then Sam’s administrative assistant, Clare Perkins opened the door.

“As you requested Mr. Madigan, your guest is joining the meeting,” Clare said.

“Well Torant, looks like Coleman said you lost,” Madigan said to the new arrival.

Upon seeing Rex Torant, Patty became a little unsettled and Pete turned his famous crimson red.  Patty and Pete called him “Rex the Torrent” as he spoke so rapidly when trying to sell them something.  Both found this manufacturer’s “rep” annoying.

“Everyone, I invited Rex to the meeting.  We met at the airport last night and started chatting.  He assured me that his Pinnacle equipment line and Ultima solder paste would be the winners today since they have the lowest cost of ownership,” Madigan explained.

Torant saw the slide announce Optoplace, Exactotest and ElectoMaterials as the winners.

“My products are just as reliable and cost 30% a year less to own,” Torant fumed at Patty.

Patty had not anticipated Torant’s attendance at the meeting but had prepared for this type of question.

“Mr. Torant is correct, however Pinnacle’s component placement machines have more downtime for machine assists and, when the equipment does malfunction, it is down for repairs on average for 28 hrs, whereas Optoplace is only down for 14 hrs.  All in all, Optoplace machines are up 6 hrs more a week in a two shift operation,” Patty calmly responded.

Will Patty’s arguments win the day?  Can a 30% more expensive machine really have more “Profit Potential?”  And what about the solder paste and materials?  Stay tuned.

Cheers,

Dr. Ron

image

Folks,

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

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

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

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

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

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

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

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

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

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

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

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

Cheers,

Dr. Ron

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

Folks,

Some time ago, I mentioned that I was working on a consensus of the status of lead-free/RoHS compliant assembly. My hope is to find data and facts that will support the consensus. I am making progress, but at this time I would like to share the subtopics in the consensus. Look them over and see what you think:

1.       Was/Is lead-free electronics/RoHS needed to protect the environment?

2.      Is lead-free solder easier and safer to recycle than lead-containing solder?

3.      How has the increased use of tin and silver affected their supply and price?

4.      How much did it cost to implement lead-free/RoHS compliant electronics?

a.      What is the cost adder to a typical lead-free product?

5.      What are the process challenges of lead-free assembly?

a.      Are these challenges being addressed?

b.      If so, how?

6.        What is the reliability of lead-free vs leaded electronics for commercial applications?

a.      E.g. 0C to 100C thermal cycle, drop shock

7.        What is the reliability of lead-free vs leaded electronics for harsh environment/military applications?

a.      E.g. -55C to 125C thermal cycle, other Mil stress tests

8.      What is the threat of tin whiskers, tin pest and other similar lead-free related reliability phenomena?

9.      What is the status and need for halogen-free assembly?

Help me by suggesting topics that I have left out.     
Contact info here.
Cheers,
Dr. Ron

Folks,

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

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

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

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

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

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

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

Figure 2. World Tin Production at Mines. 

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

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

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

最近有一家供应商突然中断提供了银锡焊接材料，所以有些客户朋友们马上向Indium公司资讯。还好，Indium一直为大家提供银锡材料。

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。

SnAg银锡合金有以下几种特点：

• 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
  

Cheers,

Acknowledge: Dr. Andy Mackie with Indium Corporation; Dr. Mackie's blog post "Tin/Silver Solder Paste in Die-Attach (Sn/Ag)"

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!

Happy Testing!!

Amanda

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

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

Folks,

I have 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

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.

Folks,

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

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

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

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

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

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

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

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

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

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

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

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

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

Folks,

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

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

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

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

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

Cheers,

Dr. Ron 

PS: Read my follow-on posting about bismuth.

从去年年中到现在，由于国际原油价格的连续上涨，带动了大宗商品包括金属价格的飙升。近期的利比亚动乱，使情况进一步“恶化”。

我们常有的锡铅SnPb63/37焊锡膏，和无铅锡银铜SAC305焊膏，都含有锡，SAC305还含有3%的银。根据下面的图片，不难看出锡的价格比去年年中涨了近两倍，银的价格更是涨了两倍多。这都使焊锡膏Solder Paste的金属成本上升了。随着原油和大宗商品的价格上升带来各国一定的“通货膨胀(inflation)”, 大家也应该发现，生活中各种商品的价格也有攀升。同理，焊接材料(solder materials)的运输成本，包装成本，劳动力成本，企业运营经费等都跟着上涨。





虽然Indium Corporation的主营焊接材料现在面临一定的成本挑战，但是我们还是采取积极的方法, 在保证品质和服务不变的情况下，给客户们提供物美价优的有竞争力的产品！

Cheers!

Pic:
1. www.lme.com
2&3. www.kitco.com

PS:国内的亲友们都“抱怨”物价的狂涨，美国这里又何尝不是呢。

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.”

Folks,

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.

Cheers,

Dr. Ron

Minuses

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

Pluses

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

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.