Solder Melting

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.

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

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

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

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

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

The two alloys:

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

What they have in common are:

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

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

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

Let us help!

Carol Gowans

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

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

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

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

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

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

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

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

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

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

 To learn more, please contact us.

 Cheers!  Andy

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

Reducing the surface oxides of Nitinol is just the first step in getting a good solder joint with this versatile medical assembly material.

Next you have to choose the right solder alloy.  You will probably want to stay away from anything containing lead, cadmium, or antimony, particularly in medical applications.  And you will want something with a high tensile strength.

The best choice is Indalloy #121 (96.5Sn 3.5Ag).  It has a tensile strength of 5,620 PSI and a melting temperature of 221C and is obviously lead-free.  It wets well to the cleaned Nitinol.

If you need a higher melting temperature solder (one that can withstand autoclave temperatures for example) you should consider Indalloy #182 (80Au 20Sn) which melts at 280C, has a tensile strength of 40,000 PSI, and has long been considered a highly reliable solder.  Additionally, this alloy is available in very fine diameter solder wires to minimize waste.

Soldering temperatures should be 25C to 50C above the liquidus temperature of whichever solder you use and proper cleaning should be always be performed afterwards.

Contact us at medical@indium.com for more information about soldering for medical devices or visit our web site at www.indium.com/medical

Carol

Back in 2005 a customer left a question on our website and it was answered by one of my solder heroes. Here is the Q&A:

Question: “With regard to Indium Corporation's indium sulfamate plating bath… …can it be deposited onto platinum-gold thick film inks? Namely DuPont solderable inks on 96% alumina?”

Answer: “Thick film inks often contain low melting glass frit particles which enhance bondability to the alumina substrate. Solderable thick film inks are designed so that the glass particles do not reside on the surface, thus allowing the solder to wet. As in solder wetting, having a glass frit-free particle surface will also allow electrodeposition of any metal. Therefore if the ink is solderable it should be plateable.”

I learned from this answer so I thought it would be good to share with you. Call me or email me to discuss your questions.

~Jim

We are frequently asked if it is possible to solder to aluminum. The answer is yes, if the following guidelines are followed: 

FLUXES:
Because it is difficult to solder to aluminum, Indium Corporation developed Indalloy Flux #3 (activation temperature is 96-343°C) to remove the tenacious oxides that prevent the solder from wetting to the surface. This flux is very corrosive and is not recommended for electronic applications because, if any of the post-reflow flux residue remains after a warm water rinse with mechanical scrubbing, the joint may be compromised. This flux is recommended for mechanical assembly joining applications only. 

Another alternate solution is to use a forming gas consisting of nitrogen and hydrogen. This method of oxide removal is generally used when the soldering temperature is greater than 350°C which is ideal for activating the hydrogen to reduce the oxides. With this method, there is no post-reflow flux residue to clean up.

METALLIZATIONS:
An alternate to corrosive fluxes is to nickel plate the aluminum so a weaker flux (RA, ROL1) can be used. These fluxes are less corrosive and can be easily removed with an appropriate solvent.   There are many solder alloys that will wet to nickel. Check out our solder alloy physical properties table.

SOLDER ALLOYS:
The solders that are normally recommended for joining aluminum are:

• Indalloy #201 (91Sn, 9Zn); 199°C E
• Indalloy #176 (95Zn, 5Al); 382°C E. 

Here is a question that was received and answered on our website almost a decade ago – but it is still quite relevant:

Question:
“I have an application where I need to solder to 0.5µm thick gold. What alternatives do I have? What alloys are likely to work?”

Answer:
“Being that your gold is relatively thin, you really do not have any limitations as far as [indium-based solder] materials go. You should consider the temperature that the solder will see and try to choose an alloy that melts at least 40°-50°C higher. You should also consider the sort of mechanical strength that you will need.”

Here is a list of solder alloys we offer, including indium based alloys: Indalloy Chart

Tombstoning

• the transition to Pb-Free (higher reflow temperatures, and related flux issues)
• miniaturization (0201s and 01005s)

• The reflow oven operator can slow down the ramp rate. A slower ramp rate allows for more uniform warming of the PCBA.
• Another technique is to employ a "soak" just below the melting temperature (solidus) of the alloy. For example, for a SAC305 profile (217°C solidus), one may implement a "soak" at 205 to 210°C for 30 to 120 seconds. This allows for the cooler parts of the PCBA to "catch up" to the warmer parts. After thermal equilibrium has been achieved, one can spike the temperature up to the appropriate peak temperature (i.e. 245°C). This technique (depicted in the reflow profile shown at the right) allows for all of the solder paste deposits to melt and wet the component terminations at roughly the same time; thereby, mitigating tombstoning.

Phone: +1.315.853.4900
E-mail: ebastow@indium.com

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

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

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

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

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

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

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

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

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

~Jim

最近越来越多的客户都在询问Indium Flux #2，其中有不少是制造医疗产品的客户。我自己对Indium Flux #2其实也是一知半解的，就借此机会学习了解了一下。

Indium Flux #2是一种强酸性的液体状助焊剂，激发其活性的温度范围是100-371⁰C。 其最显著的特点是能够使用在不锈钢的焊接上(soldering to stainless steel)，而且和很多合金都兼容。因为这是强酸性的助焊剂，在焊接完毕后都应该有温水洗干净或是擦干净它的残留物，不然会造成腐蚀。

在医疗焊接上，可以用Indium Flux #2焊接Nitinol(用在可移植性器械上，implantable devices)。常用的和Nitinol焊接的合金有Indalloy 121 (Sn96.5Ag3.5, melts at 221C) 和 Indalloy 182 (80Au20Sn, melts at 280C);也常有客户用Indium Flux #2把Nitinol 和不锈钢焊接在一起。Indium Flux #2可以有效去除焊接表面的硬厚的氧化物，两个焊接面能更好的被清洁，焊接润湿性更好。

Cheers,

Hi there!

I am excited, this is my first blog post -ever. I am excited that it is a technical blog of Indium Corporation.

My story is very interesting; a common village boy has grown to become part of a BIG corporation in which everyone is obsessed with soldering! It was my passion to learn electronics assembly techniques 10 years ago. I strived and spent many sleepless nights on this – I would say on SMT.  When our Marcom Superstar Anita told me about the blogging opportunity I was really excited… how would I…? Anyway I am here!

So … soldering and solder paste is my passion. I have published two technical papers on solder paste and reflow. And you will see more thru this blog.

My two cents on soldering… although soldering process looks simple and any one can define with a single sentence; it is not a simple process. It is comprised of chemical, physical, and metallurgical process and deals with fluxing, melting of alloy, wetting, spreading, surface tension, coalescence, wicking, intermetallic growth/bonding, time above liquidus (TAL), cooling down for smooth grain structure etc.

We will have more discussions in upcoming post; stay hungry, stay foolish!

Best Regards
Liyakathali.K (Liya)
Sr.Technical Support Engineer - India
Based in Chennai, Tamil Nadu

Folks,

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

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

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

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

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

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

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

Figure 2. World Tin Production at Mines. 

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

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

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

Folks,

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

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

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

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

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

Cheers,

Dr. Ron

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

Folks,

It was five years ago today that RoHS was launched, amid concerns that the world of electronics would collapse due to the many challenges of lead-free (Pb-free) soldering. Well, we have five years of field data with no “the sky is falling” lead-free reliability events. But, has it been just five years?

No. As I mentioned in a recent post, Motorola implemented lead-free soldering around 2001 to take advantage of lead-free solder’s poorer spreading.  Hmmmm,  so it has been ten years! Not too bad!

Well it is actually better than that. SnAg3.5 solder has been used for decades in both:

1.     Step soldering:  with a eutectic temperature of 221C, SnAg3.5 can be used as the step previous to soldering with Sn63 or similar Pb-Free solder. The principle is to solder first with the SnAg3.5 and then with a lower melting temperature solder. The second soldering step is performed at a lower temperature, therefore not disturbing the SnAg3.5 solder joint or bond. 

2.      Mid-Temp Pb-Free alloy:  when a solder that melts somewhat above the melting point of a “standard” solder alloy is needed, and it must be Pb-free, SnAg3.5 is often the choice.  The automotive industry has used SnAg3.5 in these applications for decades.

While I still agree that lead-free solders need some time and experience, especially in harsh environments, to establish acceptable reliability for mission critical applications, the experience with SnAg3.5 is adding to lead-free solder’s reliability portfolio.

This information came to light with the recent announcement by a major solder materials supplier that they would no longer supply SnAg3.5. But take heart, Indium Corporation still supplies SnAg3.5.  

• 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.