Intermetallic

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.

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

Solders, as a class, are "interesting" metals.  And the properties of indium-containing solders are exceptionally interesting.  Indium’s (and indium's alloys') physical and mechanical properties are unique when compared to the metallic elements and alloys typically examined.

To put this into context, a metallurgist from a customer company called me because, after looking over our table of solder alloy properties, he claimed our data couldn’t possibly be correct!  After a detailed conversation, I understood the nature of his concern.  His background was not in solder materials, and the shear strength data for indium (890PSI) is exceeded by its tensile strength (273PSI). This "interesting" situation prompted further questioning.  These numbers are, however, accurate.

The graph on right numerically depicts the shear nature of this material.  Over a test area of approximately 0.5 square inches, a soldered interface that was sheared at a rate of 1mm/minute to fracture extended 1.6mm before yielding. This extension is indicative of the putty-like nature of pure indium.  As expected, The load at yield roughly matched the shear strength cited above for the bulk material  because the yield location in this assembly was through the bulk material, rather than along the intermetallic edge.    

More extensive information on the physical constants of indium can be found in this application note.

Finally, click here to link to more information on indium metal properties and its uses.

As a sneak peak:

• Indium has a low vapor pressure when molten, rising quickly as the boiling point approaches (2080°C)
• Indium cold welds to itself
• Molten indium will wet glass and glazed ceramics
• Although the softest metal, indium will impart hardness, when added as an alloying agent to other metals such as gold. In fact, the gold indium alloys are used in dental crowns.

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

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

For instance, review this case scenario:

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

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

Why?

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

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

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

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,

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

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

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

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

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

Thanks Mike!

Cheers,

Dr Ron

The solder image is courtesy of Vahid Goudarzi of Motorola.

Folks,

I met Peter Borgesen back in the mid 1980s when he was a research scientist at Cornell working with Professor Che-Yu Li. Later we worked together at Universal Instruments. Currently Peter is a Professor a Binghamton University.  All during this time, Peter has been working on materials science-related topics in electronics packaging and assembly, most notably reliability. In addition to his many technical skills, he is a gifted linguist, speaking multiple European languages. Etched in my mind is Peter talking to several European graduate students in their native European languages in the space of 5 minutes, switching from one to the other effortlessly.

Few people know more about lead-free solder reliability than Peter. So I thought I would get his perspective on my recent post on lead-free field reliability data. His comments follow.

Hi Ron,

I agree that the sky is not falling. Also, we should be talking much more (only?) about life in service. I realize that we don't know enough about this (and our predictions based on test results are much more off than people want to recognize). The vast majority of practitioners focusing on 'engineering tests' are doing worse than wasting time and effort if comparisons of test results do not translate to relative performances in service. There is a lot of ‘sticking heads in the sand’ here.

I am not concerned about the long term life of cell phones, and not very worried about in which respect they do better or worse in service than with SnPb. Intermetallic bonds have generally gotten weaker and more prone to sporadic defects, and cratering is greatly enhanced for the devices Vahid Goudarzi mentions when discussing Motorola field data. I agree those are limited concerns, no sky falling indeed.

What still scares me (in the case of critical applications) or concerns me (in the case of expensive applications) is the naivete with which many seem to think we can learn much about sporadic disasters or long-term reliability of those from consumer electronics experiences.

I am not often interested in comparisons to actual life of SnPb either (any more). We face ever more applications (designs and service conditions) for which we don't have sufficiently accurate critical experience with SnPb either. The first challenge becomes not to be surprised by effects of long-term aging, combinations of loading, minor differences in pad finish, joint configuration, latent damage, process, .... and their interactions for the specific solder alloy used (!).

While I can't yet extrapolate test results to life in long-term service (I think we are close, but I really need an extra $1M to prove my hypothesis and turn it into a quantitative model) I can show how current models may easily be off by 2-3 orders of magnitude or more (worse, how they may screw up comparisons of alternatives). It obviously depends on the application whether this really matters (I side with companies who have cut drastically back on testing for many applications).

Keep up the good work.

Peter

I will keep in touch with Peter in the future for updates on his perspective.

Cheers,

Dr. Ron

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

As a tech guy, I couldn’t be more excited about testing these 8 different c-Si solar cell / metallization designs!

I just received a customer inquiry regarding a phenomenon that is little studied and even less quantified; “How many times can I reflow a solder alloy before damaging the solder joint?”

As you may already know, each time you bring a solder alloy above its liquidus temperature, it continues to dissolve the metallizations on the substrate to which you are soldering, as well as the metallizations on the leads of the component being attached. With modern processes, a 3-time “excursion” is common, especially with double-sided reflow and rework. Typically, the solder applied with paste is not reflowed again during the wave, because, through the use of pallets or selective soldering, it doesn't get quite hot enough to melt. That said, in such a case, the solder joint may become hot enough to receive some damage. To me, the most interesting thing with crystalline intermetallic layers, is that they don’t need to reach liquidus to form larger crystals. So a temperature excursion close to the liquidus may also increase the crystal structure size.

Another factor is surface metallizations, especially easy-to-solder surfaces such as gold or HASL. With gold, molten Sn/Pb solder at 200°C will dissolve at 35u-inch/s. So, a fine flash layer, such as 3-5u-inch, is gone within the first second, and the actual intermetallic is formed to the underlayment; most commonly nickel (Ni). This is similar with HASL, as the HASL layer is consumed into the solder joint at liquidus, and the intermetallic layer is formed with what is beneath the HASL.

The intermetallic layer will increase with time above liquidus (TAL) and also with temperature, with hotter dissolving more of the surface. This is why there are operating temperature limitations on the final solder joint, such as no more than 90% of the solidus of the alloy, in degrees Kelvin.

Another factor that affects grain structure, besides TAL and peak temperature, is cool-down rate. A faster cool-down rate will form a smaller crystalline grain structure, but keep in mind that a too-fast cool down rate may result in stresses being trapped in the grain structure from the CTE mismatch between the component and the substrate.

It has been our experience that 3 temperature excursions is the accepted limit (by most companies that I work). But, the only recommendation that we can offer is that you try “worst-case” scenarios, and have ALT testing and SEM cross-sections performed on real-world products in which 3, 4, and 5 excursions have taken place. Your particular case may be unique - it is well-worth determining your particular situation.

Since I couldn’t find a good beginners guide to c-Si metallization paste, (not even from Wikipedia) I thought I’d provide an explanation of this important module assembly material:

The silicon solar cell has a low-temperature glass-frit paste applied to the active surface. This combination of glass, Ag, and other binder materials is printed onto the solar cell and fired around 850-1000degC to form the solderable metallization on the cell. This glass-silver mixture recombines during the firing process to break through the passivation/antireflective coating layer on the cell and form a strong bond to the cell. During firing the glass and silver are suspended in a mixture with silver forming an electrically conductive path from the top to the bottom of the deposit – and ideally a silver-rich layer is formed on top. This silver is the surface that tabbing ribbon is soldered onto when interconnecting cells.

Because the structure of the glass-silver is formed in the firing process, the firing can impact the solderability of the final metallization. That is the reason it is so important to determine the bond strength and diffusion/intermetallic formation of the interface between the cell metallization and tabbing ribbon solder coating.

Now here’s my challenge to you:

If you know of another good description, post a link to the document in the comments field below!

Thanks,

          ~Jim H.

最近的客戶拜訪中，大家都對Indium公司的新焊錫棒(Solder Bar) Sn995十分感興趣。 Sn995是一種無鉛的焊錫材料，不含銀，主要成分是99.5%的Sn, 大約0.5%的Cu, 還有一些微量元素。Sn995的主要微量元素，是Cobalt 鈷 (Co)。

先在市面上的無鉛焊錫棒，除了SAC305， 也有很多SnCu+Ni的材料。在我們各種可靠性試驗中，都發現“Cobalt is a better grain refiner.”

²       Functional Test 整板功能性測試

²       Thermal Cycling Test 熱循環測試

²       Intermetallic Growth Test

²       Wetting Test 潤濕測試

²       Shear Test 剪切力測試 

²       Pull Test 拉力測試

²       Accelerated Aging Test 老化測試

²       Hole Fill test 填孔測試

²       Copper Loading Test

²       Dross Test

在以上的所有測試試驗中，Sn995 都呈現出相同或是更好的性能。進一步的詳細測試信息，歡迎隨時聯係我們: askus@indium.com china@indium.com

Cheers!

Picture: Jim Hevel with Indium Corporation

Reflow profiling can be broken down into several phases. I generally use the following;

Preheat

Pre-reflow

Reflow

Cooling

Preheat Phase preconditions the PCB assembly prior to actual reflow, removes flux volatiles, and reduces thermal shock to the PCB assembly. Because the preheat phase is often the longest of phases the ramp rate (rate/rise of time vs. temperature) is often established in this phase.

Pre-reflow Phase involves flux activation to remove surface oxides (on mating surfaces as well as the solder paste particles themselves), further pre-conditions the PCB assembly before reflow, and can be utilized for the soak portion of the profile, if needed. A soak profile may be suggested to diminish any delta T between components if there are both very small and very large components or the physical size of the PCB assembly is very large in and of itself. A soak profile is also often suggested to reduce voiding in area array type packages, though with Pb-free chemistries, this is often not as effective as with SnPb.

Reflow Phase is where the mechanical/electrical connection is made through the formation of intermetallics. Peak temperature and TAL (time above liquidus) help define the actual reflow portion of the profile. Peak temperature 20-40°C above liquidus and TAL of 30-90s is common.

Cooling Phase determines the grain structure when solidified and is defined as the solder cools from the peak temperature to solidus. A fast cooling rate is desired to create a fine grain structure (most mechanically sound) but is limited by the differences in CTE (coefficient of thermal expansion) of the joining surfaces. If excessive, stress can be exerted on the solder joint or component, fracturing or tearing can occur. Cooling rate of 4°C/s is commonly suggested.

Ramp to Peak profile depicted

For more please see “Best Practices Reflow Profiling For Pb-free SMT Assembly"

Ignoring the solder selection as part of your design process is risky business. 

As Terry Costlow, the IPC online editor of EMS Now noted in an article ‘Controlling the Explosion of Lead Free Solders’, the choice of the right solder alloy can affect the manufacturing process, the cost, and the field performance of the product.

Initially it was thought that the move to Pb free solders was just a matter of changing reflow profiles but major issues such as tin whiskers, brittle intermetallic layers and other concerns soon pushed solder selection into the forefront.

With over 200 published alloys and over 300 custom alloys shipped each year, we have seen the need for considering the solder design first.  Before you settle on a solder you have to consider:

·         Surface metallizations

·         Operational temperature of your product or device

·         Form of the solder you want to use (solder paste, solder preform, solder wire, etc.)

·         Temperature of subsequent soldering steps

·         Thermal coefficient of expansion

·         Tensile strength

And these are just a few of the considerations.  Let us help you make the right selection.  Contact us at: askus@indium.com.

Feel free to discuss solder selection with our industry professional, Dr. Lasky on November 11^th, IPC is having a materials conference: Engineering for Compliance in Irvine, CA.