Soldering 101: II: The Miracle of Soldering

Folks,

Pity Ötzi, The Iceman, circa 3500 BC. It is believed that he was involved in copper smelting as both copper particles and arsenic, a trace element in some copper ores, were found in his hair. Not only was he being slowly poisoned by the arsenic, but to smelt the copper he had to achieve a wood fire temperature of about 1085ºC (1985ºF), as discussed in my last post. The arsenic in the copper did have a benefit, as it gave the copper a little more strength than if it were pure.

Shortly after Otzi’s time, metal workers discovered that adding 10% tin to the copper produced bronze. Bronze is not only markedly harder than copper, but it melts at almost 100ºC lower than pure copper, making metal working much easier. The Bronze Age had begun. This period coincided with what scholars would recognize as the beginning of modern civilizations, such as those in Egypt and Greece.

Since it melts at a lower temperature, bronze also fills molds better. This improved mold filling is evident in Figure 1. This photo shows a copper and bronze hatchet that I had made. The copper hatchet on the left shows evidence of poor mold filling.

Figure 1. Copper, on the left, and bronze hatchets that were made for Dr. Ron’s Dartmouth College course ENGS 3: Materials: The Substance of Civilization. Note that the copper hatchet shows poor mold filling due to copper’s higher melting temperature..

In my opinion, it is almost certain that the Bronze Age is related to the development of soldering. The first evidence of soldering was about 3000º BC where, arguably the first civilization, the Sumerians assembled their swords with high temperature solders. Since the base metal for most copper-to-copper soldering is tin, the early metal workers almost certainly learned that tin could be used to join copper or bronze pieces together at much lower temperatures than smelting.

Until the European Union’s restrictions on lead in solders in 2006, most electronics solders were tin-lead eutectic. Eutectic is a Greek word that roughly translates into “easy melting.” Figure 2 shows the tin-lead phase diagram. Note that the melting point of tin is 232ºC and that of lead is 327ºC, yet at the eutectic concentration of 63% tin/37% lead, the melting temperature drops to 183ºC. This concentration and temperature is known as the eutectic point.

Figure 2. The tin-lead phase diagram. Note the eutectic point at 183ºC.

After the EU’s lead restriction went into effect, most electronics solders are based on a tin-silver-copper alloy that melts in the 217-225ºC range. The most common of these alloys being SAC305 (Sn96.5Ag3.0Cu0.5, where the numbers are weight percentages.)

Although the eutectic point is an interesting and usually beneficial phenomenon due to its lower melting point, the true miracle of soldering is that two pieces of copper that melt at 1085ºC can be bonded together with a tin-based solder at less than 232ºC. The value of this benefit cannot be overstated. Nature has allowed us to mechanically and electrically bond two pieces of copper together at a low enough temperature that we can do this bonding in the presence of electrically insulating polymer materials. Without this feature of solder, we would not have the electronics industry! An added benefit is that the bonding is reworkable, so that if a component fails, it can be replaced without scrapping the entire electronics printed circuit board.

It is natural to ask how this bonding takes place. The tin in the solder forms intermetallics with the copper. Typically Cu6Sn5 forms near the tin and Cu3Sn forms near the copper (Figure 3).

Figure 3. Copper tin intermetallics from Roubaud et al, “Impact of IM Growth on the Mech. Strength of Pb-Free Assemblies,” APEX 2001.

So next time you use your smartphone, laptop, tablet, or other electronics device, don’t forget that without the miracle of soldering it wouldn’t exist.

Cheers,

Dr. Ron

Soldering 101: The First Copper Smelting

Folks,

Soldering is an ancient technology. It is estimated that soldering was first discovered as long ago as 4000 BC. So soldering was much more ancient to Julius Caesar  (100 BC – 44 BC) than Caesar is to us today. Before considering soldering, let’s discuss early copper smelting, as copper is usually the metal soldered to.

My Cornell colleague Steve Sass wrote a book, Materials: The Substance of Civilization, on which I based my course of the same name on. In his book, Sass points out that the importance of the firing of clay can’t be overstated as it is the first time humankind changed the nature of a material. Once clay is fired it forms ceramic, a material much stronger than dried clay. Artisans first performed this feat about 26,000 years ago in what is now the Czech Republic.

While I agree with Sass’s assessment, it could also be argued that the beginning of modern technology can be traced back to the first smelting of copper. The firing of clay is too simple a process to encourage much further experimentation, which is needed for technology growth. The process of smelting of copper, the first metal liberated from its ore, is quite complex and this complexity led to further experimentation that gave us iron and steel. Continued working with metals likely developed the scientific method, hence led up to all of the breakthroughs to this day.

Consider the novelty of the first smelting of copper. To smelt copper, our ancestors had to grind copper ore, malachite (Figure 1), into a powder, mix it with carbon, and heat it to greater than 1085ºC (1985ºF). By the way, you can estimate the Fahrenheit temperature by multiplying the Celsius temperature by a factor of two and will only be off by <10% from 100º-1700ºC.

Figure 1. Malachite (copper ore) is quite attractive. Perhaps this attractiveness brought it to our ancient ancestors’ attention as a candidate for smelting. (Copyright 2018, Ronald C. Lasky, Indium Corp.)

 

After I cook on my outdoor grill, I clean the grates by turning the propane to maximum to cook off the grease. Typically the grill’s thermometer will read about 600ºF during this process. The grill gives off so much heat that it is oppressive to approach it to turn it off. Needless to say, noting what 600ºF feels like suggested to me that it is very hard to achieve 1985ºF with a wood or charcoal fire.

Anyway, I recruited some graduate students to try and smelt copper as described above. They purchased many bags of charcoal, used a leaf blower to supply air and worked for two hours on two different attempts and failed both times. The next year some students built a tower with vents and put charcoal on the bottom with the copper ore and carbon in a crucible on top. Their tower was similar to a roman smelting furnace for iron (Figure 2). They were successful and produced a piece if copper about the size of a penny.

Figure 2. A Roman style furnace. Dr. Ron’s students built a similar tower from cinderblocks.

These two attempts demonstrate how amazing our distant ancestors were. How did they think to do it? There were certainly many failed attempts. How did they persevere? One thing is certain, they started the trend of discovery, in about 5000 BC, that led us to today. We owe them much.

Cheers,

Dr. Ron

 

 

Does Solder Paste’s “Five Ball Rule” Remain Valid in SMT Today?

Folks,

My good friends, Phil Zarrow and Jim Hall, in their audio series “Board Talk,” were recently asked about the “Five Ball Rule”. In the comments section for this session, one listener asked if this rule, created in the 1990s, was still valid. After all, the 1990s was the era of 0603 and 0402 passives; 01005 and even 008004 passives have arrived.

First, let’s consider what a “rule” is verses a “law.” As an example of a law, consider Newton’s Laws of Motion. At everyday speeds, these laws are shown to be accurate to within our capability to measure. As we will recall from Physics 101, these laws were superseded by Einstein’s Theory of Relativity, at speeds close to those of the speed of light. However, in our everyday world, Newton’s Laws are well … laws. They are, for practical purposes, exact.

What is a “rule” then? A rule is an expression that approximately fits some empirical data or the experience of experts. Moore’s Law is actually a rule, as it is not precise. The doubling of transistor density has varied from every 18 months to every two years. That’s why I call it a rule, a very useful rule indeed!

The “Five Ball Rule” is clearly a rule. It was likely developed a generation ago by some of the first SMT pioneers. It may be backed up by experiment, but I think it was likely more a consensus of SMT industry authorities from the 1980s and 1990s.

What is the “Five Ball Rule?” It states that the solder paste’s largest solder particle diameter should be such that at least five of these particle diameters would span the width of a rectangular stencil aperture (Figure 1).

Figure 1. The Five Ball Rule

 

 

 

 

 

 

 

When this rule was developed, stencil apertures were much coarser than today, and the finest solder powder was a Type 3, with Type 4 on the horizon. While it is true that stencil aperture widths are much finer today, solder pastes of Type 4.5, 5, and even 6 are now in use.

The particle sizes of different “Type” solder pastes are shown in Figure 2. Note that, for Type 4 powder, 80% by weight of the particle diameters are between 20 and 38 microns. 38 microns is considered the “largest particle.” So, from Figure 2, for Type 5 powder, the “largest particle” is 25 microns. For the sake of the Five Ball Rule, the “largest particle,” for each powder type are those shown in Figure 2.

Figure 2. Solder Powder Sizes.

 

 

 

 

 

 

 

 

 

So, is the Five Ball Rule still valid? It would be hard to argue that it is not. Hundreds of experiments have been performed using the Five Ball Rule, combined with the aperture ratio being >1.5 for rectangular apertures or the area ratio being > 0.66 for square or circular apertures, with successful results.

StencilCoach software now includes the newer (finer) solder powder sizes to 1) tell the user the fineness of solder paste powder for the Five Ball Rule, as well as 2) help with calculating aperture or area ratio. By the way, some have suggested that, for a square or circular aperture, an “Eight Ball Rule” is more appropriate. So, StencilCoach uses the Eight Ball Rule for such apertures.

Cheers,

Dr. Ron

 

Use an SMT Pre-Test Before Presenting a Soldering Workshop

Folks,

Let’s see how Patty is doing, it’s been a very, very long time …

Even though Patty and her husband Rob both worked at Ivy University, they seldom drove in together. It was just too difficult to organize their schedules so that it would work out. So, as Patty was driving in to Ivy U, she was listening to the last chapter of Ron Chernow’s biography of U. S. Grant. Her timing was excellent, since she, Rob, Pete, and the Professor were having their monthly book club meeting. Rob, Pete, and the Professor were always recommending books about World War II or the Civil War. Because of this trait, she groaned every time it was the three “boys” turn to suggest the next book. But, she had to admit that she always enjoyed the books much more than she thought she would. She especially liked a book Rob discovered, called A Simple Solder. Patty found this true story, about a young boy in the German army in World War II and how he survived to tell the tale, fascinating. She would never tell Rob, but she read it three times.

When it was Patty’s turn she made sure to avoid those military topics. Recently, she proposed another one of Chernow’s biographies on John D. Rockefeller. She also suggested  iGen: Why Today’s Super-Connected Kids Are Growing Up Less Rebellious, More Tolerant, Less Happy–and Completely Unprepared for Adulthood–and What That Means for the Rest of Us by Jean M. Twenge. This book convinced her and Rob to dramatically limit “screen time” for their 9-year-old twin boys.

As she approached her parking spot, the audio book on Grant finished. She was a bit sad, as she had enjoyed this book as much as any. Patty had the impression, from her high school history classes, that Grant led the Union to victory over Robert E. Lee only because he had superior forces, weapons, and supplies. Chernow’s book clearly dispelled that notion. Grant was a great general. In addition, he was an effective and honorable president, if a little too naïve and trusting to avoid numerous scandals among his subordinates.

In a few moments, they met in The Professor’s large office. After they finished their book club chat about Grant’s biography. Patty had a favor to ask.

“Mike Madigan asked me to give a three-day workshop on SMT 101 at one of ACME’s recently acquired facilities. He said he felt the technicians and engineers weren’t very knowledgeable. I’m having trouble deciding at what level to aim the workshop,” Patty began.

“You mean like for beginners, intermediate, or expert?” Pete asked.

“Yes,” Patty responded.

“Well, you should develop it in a logical sense, starting with what soldering is, discuss flux and solder paste, then stencil printing, component placement, reflow, test, etc,” Rob added.

“I agree with Rob’s outline, but you need to find out the current knowledge level of the students,” The Professor suggested.

“I once gave an eight-hour seminar on SMT Defect Modes and How to Fix Them. The workshop was advertised as for SMT engineers and technicians with intermediate experience. At the end of the workshop a person raised his hand and asked an unsettling question,” The Professor continued.

“And the question was?” Pete teased.

“Professor, you have used the word ‘SAC’ many times, what does ‘SAC’ stand for?” The Professor responded.

In unison, Patty, Rob and Pete groaned.

“That’s my concern! At which level do I aim the workshop? If I shoot too low, it might insult people. If I shoot to high it might go over their heads,” Patty responded.

“OK! So, how do I structure the workshop, not knowing the skill level of the students?” Patty asked a little frustrated.

“How about a pre-test?” The Professor suggested.

“OK! But how many questions?” Rob asked.

“It needs to be short, yet comprehensive,” The Professor suggested.

“Seems like a contradiction,” Pete grumbled.

“I think The Professor is right. Look at it this way, let’s say you want to assess if your 14 year old nephew knows much about The Civil War. Ask him three or at most five questions and you can determine if he does,” Patty suggested.

“How about some examples?” Pete asked a bit dubious.

“I’m getting it. How about when was the war fought, who was Robert E. Lee, what is the significance of Appomattox Court House?” Rob chimed in.

“OK, I see you point. If you know two or all three, you probably know a lot, one or less and you don’t know much,” Pete responded.

Patty then suggested, “OK let’s develop a list of ten SMT Pre-Test questions.”

After about 20 minutes of back and forth, our team of four converged on these 10 questions.

SMT Pre-Test

  1. What does the letter “S” in SAC stand for?
  2. How much silver is in SAC 305?
  3. PWBs are coming off of the final component placement machine at a rate of one every 20 seconds. The PWBs are 20cm long and should be placed with at least 4cm of space between them. What must the reflow oven belt speed be to accommodate this cycle time?
  4. The starting temperature is 25°C. It needs to be 145°C in one minute. What heating rate is needed, in °C/s, to achieve this temperature?
  5. About how much does silver cost per troy oz.? (+/- 30%)
  6. Which is a closest to typical stencil thickness?
    • 5 microns
    • 20 mils
    • 5 mils
    • 20 microns
  7. Which is closest to a typical lead spacing for a plastic quad flat pack (PQFP?)
    • 0.1mm
    • 0.1 mil
    • 0.4mm
    • 0.4 mils
  8. Which has finer solder particles, a Type 3 or 4 solder paste?
  9. What does OSP stand for?
  10. Place an arrow at the eutectic point of the tin-lead phase diagram below.

Would you like to try the pre-test? The answers have to be what you know without looking anything up. Send me your answers at rlasky@indium.com. The first person to get 100% will get an item of memorabilia signed by Patty, Rob, Pete, and The Professor.

Cheers,

Dr. Ron

On the Road at SMTA Pan Pac

Folks,

I am giving a paper, chairing a session and hosting a panel at SMTA Pan Pacific on Feb. 6 at the Hapuna Beach Prince Resort in Hawaii.

 

 

 

 

 

 

 

 

The paper is “Using Cpk and Cpk Confidence Intervals to Evaluate Stencil Printing,” with my coauthor Chris Nash of Indium Corporation. In this paper I will discuss how to calculate confidence intervals when using Cpk to evaluate the quality of stencil printing.

By comparing the confidence intervals of Cpks one can determine whether or not there is a statistically significant difference between different samples of stencil printing data.

The session I am chairing is on “Advanced Materials.” The papers in the session are:

  • “Oxygen Vacancy Migration in MLCCs” by Dock Brown, CRE, DfR Solutions
    “Update on Cu-Ni/Sn Alloy Composite Solder Paste for Harsh Environments” by Stephanie Choquette, Ph.D., Iowa State University, and Iver Anderson, Ames Laboratory (USDOE)
  • “Resistivity Stain Analysis of Graphene Coated Frabric for Wearable Electronics” by Martine Simard-Normandine, Ph.D., S. Ferguson, MuAnalysis, and K. Manga, Q.-B. Ho Grafoid.
  • The panel topic is “Solders for Harsh Environments.” Brief presentations will be given by some of the panelists with a question and answer period to follow. The panelists are Dwight Howard of Delphi Automotive, Iver Anderson of Ames Lab, John Evans of Auburn University, and Prabjit Singh of IBM.

We expect to learn a lot. I hope to see you there!

Cheers,
Dr. Ron

 

Is Industry 4.0 around the Corner?

Folks,

I attended a technical session on Industry 4.0 at SMTAI in Rosemont, IL, in September. I admit to not knowing much about it, so I found the topic fascinating. Industry 4.0 begs the question as to what were Industry 1.0 to 3.0 are (were?) The image below explains the progression, Industry 1.0 was mechanization with water and steam power, Industry 2.0 is mass production with the assembly line using electricity. Industry 3.0 adds computers and automation. Whereas Industry 4.0 is the age of cyber physical systems, the internet of things, cloud computing, and cognitive computing.

Industries 1.0 to 4.0. Source: https://en.wikipedia.org/wiki/Industry_4.0#/media/File:Industry_4.0.png

One could imagine an Industry 4.0 (I4.0) workplace something like the following in an electronic assembly factory. A customer places an order in the cloud. It is received by the factory and after some analysis performed by a “Watson”-type AI, the order is accepted. The I4.0 system then goes to work scheduling the job and ordering the correct components, PWBs and hardware. It designs the stencil from a Gerber file and so on and so on. There is little human interaction and the factory runs at about a 95% uptime and is profoundly efficient and profitable.

As with self-driving cars, I am a bit of a skeptic of I4.0. To be sure there may be a few factories that exhibit some of the Industry 4.0 technology, but I don’t see this major technological shift becoming mainstream for a generation or so.

One of the reasons is that I don’t think most factories today are even at Industry 3.0 (I3.0), they are more like Industry 2.5 (or less?). Many colleagues that I chat with about these types of things, and I have toured more than 100 factories world-wide and still marvel at how inefficient they are. I was once asked to give an executive, new to our industry, a tour of an electronics assembly facility. The facility that graciously offered to let us tour had six assembly lines. In the 90 minutes we were there, not one line was running. The reasons were typical: for line 1 the team could not find the right stencil, line 2 needed a reel of components that no one could locate, line 3 had an equipment malfunction, etc., etc. These types of experiences are discussed in The Adventures of Patty and the Professor.

Another example of electronics assembly being a bit short of I3.0 was demonstrated by a student project that was recently commissioned to measure uptime on a simple assembly line. The line consisted of a stencil printer, component placement machines, and a reflow oven. The engineers that worked for the company that sold the assembly line were confident that the students would have no difficulty measuring uptime by sampling signals from the computers controlling each piece of equipment. After hundreds of hours of work by the engineers and the students, it was concluded that it was not possible to measure line uptime without adding some type of sensors on the assembly line to detect the flow of the PWBs. Industry 3.0 indeed!

At SMTAI I was asked to participate on a two-person panel on the topic, Will Virtual Reality Soon be Used in Electronic Assembly? Readers will likely guess that I was the skeptic. Watch the video and see what you think.

As with self-driving autos, I think Industry 4.0 is a great idea and encourage the many people working on it, but I believe it will be quite a while before it arrives in any meaningful way to typical factories. In the meantime, let’s all work to ensure that the factories we currently operate approach Industry 3.0 are run efficiently with high uptimes.

Cheers,

Dr. Ron

 

Intermetallic Growth Rate is Strongly Temperature Dependent

Folks,

In a previous post, I discussed that, contrary to popular belief, intermetallic compounds (IMCs) formed in soldering processes are not necessarily brittle. I reviewed some literature that indicated that failure modes are usually at interfaces between the IMCs themselves, the IMCs and the copper or solder and often in the bulk solder itself. The perspective that IMC growth may not significantly affect reliability is also supported by work performed by Lee, et al. Figure 1, from Lee’s paper, shows that aging for 250 hrs at 150°C does not significantly affect characteristic life in thermal cycle testing.

Figure 1. Aging for up to 250 hours at 150°C did not significantly affect characteristic life in thermal cycle testing in Lee’s referenced paper.

However, it would be prudent to minimize the thickness of IMCs. So this raises the question: how quickly do IMCs grow at any given temperature? Work performed by Siewert, et al[i] holds the answer. In this paper, Siewert supported past work that the thickness of IMCs grows as X=(kt)0.5 and added new data to support modeling using this equation. In this equation, X is IMC growth distance, k is a constant dependent on temperature, and t is the time. One might expect that X is strongly dependent on temperature (T) and it is. Using data from Siewert’s paper, I was able to generate values of k as a function of T and plot them in an Arrhenius plot. See Figure 2.

Figure 2. An Arrhenius plot for k.

 

I next used Figure 2 to obtain a value of k at 70°C and plotted the IMC growth X in microns as a function of time in hours. The result is in Figure 3.

 

Figure 3. IMC grow as a function of time at 70°C.

Note that about 40 years are required to obtain a little over 10 microns of growth. Figure 4 shows the results for IMC growth at 200°C. In this case, only 100 hours are required to obtain about 10 microns of growth. So going from 70 to 200°C produces an acceleration factor of over 30,000 in the effective IMC growth rate to 10 microns.

Figure 4. IMC growth as a function of time at 200°C.

These are theoretical calculations from data collected at different temperatures. Let’s see if the formulas work in real life. In another paper [ii]by Ma, et al. his team aged some solder joints at 125°C for 120 hours. The equations used above would predict IMC growth of 2.2 microns under these conditions. From Figure 5, we see about 2 microns of growth consistent with the calculation estimate.

Figure 5. Images from Ma’s paper of IMC growth at 125°C for 120 hours.

So although IMCs are not that brittle, it is wise to limit their growth. Hence, limiting exposure to very high temperature aging is wise, but certainly minimizing solder rework is advisable, as the molten solder enables very fast IMC growth.

Cheers,

Dr. Ron

[i] Siewert, T. A., et al, Formation and Growth of IMs at the Interface Between Lead Free Solders and Copper Interfaces, IPC Apex, 1994.

[ii] X. Ma, et al Materials Letters 57 (2003) 3361-3365.

How to Land That Job in an Interview

Folks,

It’s a stressful time when you are interviewing for a job. As a former manager at IBM, Universal Instruments, and Cookson Electronics, I have interviewed more than 300 people for jobs. I’ve also interviewed for a few myself. As a result of these experiences, years ago some management friends and I collaborated on some of the best job interviewing tips.

Having taught about 3,000 students at Dartmouth over the last 15 years, I typically give a lecture on the last day of class entitled, “Tips for Success and How to Interview for a Job.” These Powerpoint slides have some of the ideas gleaned from what I learned above.

Some years ago I was interviewing for a job and the interviewing manager asked me, “Why should I give you this job?”

I answered without hesitation, “Because I led a team of engineers that contributed to the design and build this optoelectronic transceiver module.”

As I handed the optoelectronic transceiver module to the manager to look at, the non-verbal vibrations I received from him where very positive.

A few weeks after I got the job, he told me that handing him the hardware that I worked on was what sealed the deal.

Most of us recognize that an artist or writer should have a portfolio when they go on job interviews, but don’t appreciate that an engineer should have one too, even if like mine, it had only one item. I share this strategy with my current students and on a regular basis they tell me how this approach led to them getting a job.

Here is an email from a student who graduated more than 10 years ago, relating to a new job she just landed:

“I just wanted to thank you for a job interviewing tip from back when I was in school.  You suggested that we bring an example of something that we’ve worked on.  I’ve done that all these years, and I have to say, I’m pretty sure it’s gotten me several job offers!  I know at least one of my current employers’ interviewers definitely appreciated it.  Anyway, so thank you!”

So, hopefully you have a job you love and never have to interview again, but if you do, take something that you worked on as a “show and tell.” It also helps in that it focuses the interview on something you know about and will look good discussing.

BTW, if you would like a copy of my Powerpoint presentation mentioned above, send me a note (rlasky@indium.com) and I will send you a copy.

Cheers,

Dr. Ron

Intermetallics and Kirkendall Voids Continue to Grow at Room Temperature

Folks,

In my last post, I discussed intermetallic compounds (IMCs) and what I referred to as the “miracle of soldering.” I also mentioned that research focused on the brittle nature of IMCs suggests that failures in stress tests are more likely due to failures between the interfaces of the IMCs and the solder, the IMCs and the copper, or the IMCs (Cu6Sn5 with Cu3Sn) themselves and are not related to any perceived brittle nature of the IMCs.

Another weakening mechanism in soldering and thermal aging of solder joints is Kirkendall voids. Kirkendall voids form when one metal diffuses more rapidly into another metal than vice versa. A copper-tin interface displays such a mechanism. Copper diffuses into the tin more rapidly than the tin into the copper. This mechanism can result in actual voids in the copper at the metal interface. See the image below. In addition to causing a possible weakness at the interface, the excess copper that diffuses into the tin creates compressive stresses than can result in tin whiskers.

Kirkendall voids

(Source: http://www.jfe-tec.co.jp/en/electronic-component/case/img/case_solder_02.png)

IMCs and Kirkendall voids are formed quite quickly at soldering temperatures. However, even at room temperature IMCs and Kirkendall voids continue to grow, albeit at a much reduced rate. The reason for this continued growth is that on the absolute temperature or Kelvin scale, room temperature is a considerable fraction of the melting temperature of solders. As an example, the melting temperature of SAC is about 219°C, this temperature is equal to 492K (219+273), whereas room temperature is 295°K, so room temperature is 60% of the way to the melting point of SAC solder (295/492 = 0.60). Compare this situation to steel, which melts at about 1480°C. The steel would be red hot at 60% (780°C) of its melting point on the absolute scale. So, since room temperature is 60% of the way to melting, the IMC and Kirkendall forming processes don’t stop at room temperature. Hence, IMCs and Kirkendall voids continue to grow, as do related effects such as tin whiskers.

Stay tuned. Next time we will discuss IMC growth rates and resulting effects in stress testing as we wrap up this series on IMCs.

Cheers,

Dr. Ron

 

Copper-Tin Intermetallics: The Miracle of Soldering

Most articles discussing the copper-tin intermetallics that form during soldering refer to them as a necessary evil. The evil being the perception that intermetallics are brittle and can lead to failures in thermal cycling or drop shock.

I view the situation differently. From my perspective, the formation of copper-tin intermetallics is the miracle of soldering. Look at it this way, to assemble electronics, bonding copper to copper (the leads on the components to the pads on the PWB) in the presence of polymers (the PWB epoxies and the component cases) is required. These polymer materials can only take about 250°C for a few minutes. Copper melts at 1083°C, so bonding copper to copper in the presence of polymers would appear to be quite a challenge. Enter tin-based solder.

Lead-free (tin-based) solder, say SAC305, melts at about 219°C. So, with a peak temperature of about 245°C, in the reflow oven, solder can be melted and form an electrical and mechanical bond with the copper in the leads and pads. At 245°C, the many polymer materials are unharmed for the 90 seconds or so that soldering requires at this temperature.

But, what about the material properties of the intermetallics that are formed? Aren’t they too brittle? Lee et al* performed analyses that suggest that the intermetallics formed in soldering are not brittle. Their work also suggests that the failure modes are not in the intermetallics, but in the interfaces between the intermetallics and the solder, copper, or the different intermetallic compounds, Cu3Sn and Cu6Sn5. These two intermetallic compounds are shown in the figure below.

Copper tin intermetallics from Roubaud et al, “Impact of IM Growth on the Mech. Strength of Pb-Free Assemblies,” APEX 2001.

 

It has long been assumed that the thicker the intermetallics, the greater the risk of failure due to the intermetallic thickness. Lee’s work would appear to bring this concern into question.

Stay tuned for a continued discussion on intermetallics and their effect on reliability.

*Lee, C. C. et al, “Are Intermetallics Really Brittle,” IEEE Electronics Components and Technology Conference, 2007, pp. 648.

Cheers,
Dr. Ron