Soldering enables modern electronics. Without solder, electronics would not exist. Copper melts at 1085°C, yet with solder, we can bond copper to copper at about 235°C or less with current lead-free solders. These lower temperatures are required, as electronic packages and PWBs are made of polymer materials that cannot survive temperatures much above 235°C.
Before the advent of RoHS, tin-lead solders melted at about 35°C less than lead-free solders. So today, soldering temperatures are at the highest in history. For some applications, it would be desirable to have solders that melted at closer to tin-lead temperatures. This desire has increased interest in low-melting point solders, such as tin-bismuth solders. Eutectic SnBi melts at 138°C, so reflow oven temperatures in the 170°C range can be used. These lower reflow temperatures are easier on some fragile components and PWBs and will reduce defects such as PWB popcorning and measling. However, the lower melting point of SnBi solders limits their application in many harsh environments, such as automobile and military applications. As a rule of thumb, a solder should not be used above 80 to 90% of its melting point on the Kelvin scale. For SnBi solder, this temperature range is 55.8 – 96.9°C. These temperatures are well below the use temperature of some harsh environments. In addition, SnBi solders can be brittle and thus perform poorly in drop shock testing.
So, the electronics world could use a solder that can reflow at a little over 200°C, but still have a high use temperature. This situation would appear to be an unsolvable conundrum. However, my colleagues at Indium, led by Dr. Ning-Cheng Lee, have solved it. They used an indium-containing solder powder, Powder A, that melts at <180°C and combined it with Powder B that melts at ~220°C. By reflowing at about 205°C, Powder A melts and Powder B is dissolved by the melted Powder A. To achieve this effect, the 205°C temperature must be held for approximately two minutes. The remelt temperature of the final solder joint is above 180°C. I discussed the phenomenon of a liquid metal dissolving another that melts at a higher temperature before. An extreme example of this effect is mercury dissolving gold at room temperature. So, don’t drop any gold or silver jewelry into a wave soldering pot and expect to fish it out an hour later!
Powder A would not be a candidate on its own as it displays some melting at 113°C and some at 140°C.
Using the criteria above, the use temperature of this new solder powder mix can be in the 89.4 – 134.7°C range, after reflow, as the remelt temperature is above 180°C. Tests performed by Dr. Lee and his team have shown the resulting solder joints also have good to excellent thermal cycling and drop shock performance.
Figures 1-3 show schematically how the melting of the two powders would melt at a peak reflow temperature of 205°C.
To me, this invention is one of the most significant in SMT in a generation. It could be argued that it is like finding the holy grail of soldering: melting at low-temperature with a service life at high-temperature.
PS. I developed an Excel spreadsheet to calculate the use temperatures. It converts degrees C into K. I used it to calculate the use temperatures above. If you would like a copy, send me a note at firstname.lastname@example.org.
To the SMT process engineer, the second most important thixotropic material in their lives is solder paste. If solder paste was not thixotropic, it would be difficult to print and would likely slump after printing the paste. What is a thixotropic material? It is a material that has a low viscosity when it is shear stressed and a high viscosity when it is not shear stressed. So, when the solder paste is forced through the stencil aperture by a squeegee, its viscosity plummets and allows it to fill the aperture. See Figure 1.
When the stencil is removed, the resulting solder paste deposit experiences no shear stress so the deposit maintains the shape of a “brick.” See Figure 2. So thixotropy is a very helpful property of solder pastes.
If solder paste was dilatant, it would be a disaster. These materials are the opposite of thixotropic materials. They have a low viscosity when not shear stressed and a high viscosity when shear stressed. So they could not be forced through the stencil aperture and, if they could, they would flow all over the board. Cornstarch and water is an example of a dilatant material.
Oh, yes, what is the most important thixotropic material to the SMT process engineer? Their blood. When getting up from lying down, our heart automatically makes a strong “pump” to rush the flow of blood to our head. Since blood is thixotropic, it shear thins and makes it easier for our heart to get the needed blood up to our head. If blood was not thixotropic, we might faint every time we rise from reclining!
Soldering copper to copper with a tin-based solder, such as tin-lead eutectic solder or a common lead-free solder like SAC 305, requires only the liquid solder and copper to form the tin-copper intermetallic bond. This simplicity, with one small catch, was brought home to me by some colleagues at Speedline Technologies. They took a PWB with through-hole components mounted and ran it through a wave-soldering machine without using any flux. The result was comical. The PWB weighed about 10 pounds as it had huge solder ice cycles hanging off of it. Oxides that form on the copper created this mess. Running the board though again with a nitrogen blanket produced a beautifully wave-soldered board that could be ready to ship. So in reality, either a flux or nitrogen, preferably both are needed for successful wave soldering in addition to the solder and copper; however, it is still relatively simple.
Have sympathy for the solder scientists of the late 1970s and early 1980s. SMT was an emerging technology and the world wanted to buy solder paste; however, the only experience many solder scientists had was with wave soldering. In wave soldering, the flux’s main job is to remove the oxides from the PWB pads and components. The solder is in a molten state and its oxidation is not a main concern. In the soldering process, the solder only touches the board for a few seconds and the board only experiences the high temperatures during this brief period.
I imagine some early solder pastes consisted of solder powder with fluxes similar to those used in wave soldering. If so, they probably didn’t work too well. Consider the dramatic differences that solder paste experiences as compared to solder in wave soldering. The “flux” in solder paste has to remove oxides from the PWB pads, component leads, and solder particles, but it also has to protect all of these surfaces from re-oxidation for several minutes while in the reflow oven. To achieve this protection, the “flux” has to contain materials that act as an oxygen barrier. The most common materials used in no-clean solder pastes are rosins/resins. Rosins, or resins which are modified or synthetic rosins, are generally medium to high molecular weight organic compounds of 80-90% abietic acid. They are typically found in coniferous trees. Rosins/resins are tacky in nature, and provide some fluxing activity and oxidation resistance during the reflow process.
The reason I wrote “flux” in the above paragraph is that what most people call the flux in solder paste is a complex combination of materials. These “fluxes” will consist of:
Rosins/resins: for oxygen barrier and some fluxing activity
Rheological additives: to give the best printing properties, e.g., good response-to-pause, good transfer efficiency, excellent slump resistance, good tack, etc
Solvents: to dissolve the other materials
Activators: to perform the main fluxing action (removing oxides)
Figure. Solder pastes are one of the most highly engineered materials.
Modern solder pastes must have good oxygen barrier capability. In most reflow profiles, the solder paste is at temperatures above 150ºC for more than several minutes. During this time, an oxygen barrier is needed to protect both the solder particles and the surfaces of the pads and leads.
A common example of an insufficient solder barrier is the graping defect or its relative, the head-in-pillow defect. If you are experiencing one of these defects, a solder paste with better oxygen barrier properties is bound to help.
Before reflow, the solder paste must print well, possess good response-to-pause, not shear thin, resist cold slump, and have good “tack” to support the components after placement. During reflow, in addition to the oxygen barrier challenge, the solder paste must not exhibit hot slump, should “Avoid the Void,” not create the “head-in-pillow” (HIP) defect, work with all common PWP pad finishes, and produce reliable solder joints in thermal cycling, drop shock, and vibration environments. Whew! What a complex challenge.
As a result I would argue that solder paste is a candidate to be the most highly engineered material in the world… and it certainly is NOT a commodity.
If you think about it, to evaluate any process you typically want to know its precision and accuracy. Look at the dart players in the Figure 1 below. The yellow player has good precision, but his accuracy is off. The green player has such poor precision, it is hard to tell if his accuracy is good. The yellow player will typically be easier to correct, as she just needs to change her aiming point.
Figure 1. The yellow player has greater precision. She only needs to change her aiming point.
Recently I was asked to evaluate several solder pastes to determine which printed better. We used transfer efficiency (the volume of the stencil printed solder paste “brick” divided by the stencil aperture volume) as the evaluation metric, expressed in percent. So 100% would be the target. The lower specification limit we choose was 50% and the upper specification at 150%.
Figure 2. Data from Pastes A and B.
A good result would be an average of 100% with a “tight” distribution. The “tightness” of the distribution being determined by the standard deviation. Figure 2 shows data from two pastes. Note that Paste A has an average of 100% and a standard deviation of 16.67%, whereas Paste B has an average of 80% and a standard deviation of 30%. Clearly, Paste A is superior to Paste B in both accuracy and precision. But what is the best way to express this difference? Is there one metric that will do it? Cpk is the answer.
Cpk is one metric that is sensitive to both the accuracy and precision. Cpk is defined as:
Where x is the average and S is the standard deviation.
Using these equations, we see that the Cpk of Paste A is 1.0, whereas the Cpk of Paste B is 0.333. Note that Paste B has a significant number of data points (about 17%) outside of the specification limits, however, Paste A has almost no data points out of specification.
So when evaluating most processes, Cpk tells it all!
Patty was just dropped off at O’Hare airport after finishing a 3 day workshop on Lean Six Sigma statistics, design of experiments, and statistical process control. Interestingly, the students were lawyers. In recent years more and more service-based organizations were adopting lean Six Sigma and it was a long time since Patty had taught such a workshop to engineers. She noted that although the lawyer’s math skills were a bit rusty, they were very good listeners and picked up the math behind lean Six Sigma topics very quickly.
After paying the cab driver, she entered the terminal and went to see an agent. She was early enough to get an early flight home, so she had called the people at the online ticket agency during the cab ride. They said the change fee would be over $300, she felt that was just too much to pay. She was delighted to see that it was only $75 at the terminal.
She looked at her paper boarding pass and saw that she had more than two hours, just enough time for a relaxed lunch at Wolfgang Puck while she read USA Today. Patty was the only person her age that she knew who enjoyed reading a paper newspaper, she guessed that she picked the habit up from her dad.
The two hours went by quickly and she was standing in line waiting to board the flight to Boston’s Logan Airport. She had now been at Ivy U for a few years and traveled much less than when she worked at ACME. She had forgotten how stressful and unpleasant traveling was. As she stood in line, the man in front of her put his smartphone on the scanner and the scanner could not read the QC code. He and the agent fumbled for a while before they got it to work. This was another place where, in her opinion, paper was still king.
Patty got on board and settled into her middle row seat. She groaned a little bit at how uncomfortable and cramped it was. Patty was reminded of what her dad used to say in situations like this; “I know it is a bit uncomfortable, but just think what the 49ers went through to get to California,” he would tease.
After takeoff, she turned on her laptop. She absolutely had to send some emails, so she signed on to the onboard WiFi. She got sticker shock when she saw that it cost $18.95! Even though Ivy U would pay for it, the high price galled her.
After she finished the emails, a wave of fatigue swept over her and she needed a break. She chuckled to herself when she thought of a recent event. She had taken two of her best teaching assistants (TAs) to lunch and the conversation somehow came to discussing people who hid Jews from the Nazi’s in World War II. Patty mentioned to her two young protégés about an excellent book and movie she read and saw as a teenager, The Hiding Place. The story is about Corrie Ten Boom and her family and how they hid, and hence saved, many Jews from the Nazis in Holland during WWII. Although the movie was made before she was born, it was shown at Patty’s church every few years, for the new sets of youngsters who came along. Patty mentioned to her two superstar TAs that the film was produced by Billy Graham’s organization.
“Who is Billy Graham?” they both asked in unison.
Patty struggled to keep her composure as she explained who he was. How could they not know this? She decided to examine the situation a bit further.
Patty started humming a few bars of Orbison’s most popular song.
“Oh, Pretty Woman,” the boys said in unison.
Patty thought to herself, “Each of these young lads are the best student in every class that they take and yet they don’t know these ‘celebrities’?”
The next day Patty arrived at her office early to meet with Rob and Pete to discuss how the presentations that they were making for Mike Madigan on voiding were coming. Patty had arrived so late the night before, that Rob was already asleep. She did not see him in the morning as it was her turn to get the boys ready for school and he was off early to get in his 90 minutes of exercising. So, they had no chance to discuss the progress of the presentation.
“Pete, your presentation of BGA voiding is terrific. How is my hubby doing on BTC voiding?” she chuckled as she looked at Rob.
“I feel like I’m going to get yelled at ’cause I didn’t do my homework,” Rob said sheepishly.
“Yikes! We only have a few days,” Patty responded. “And I have yet to do my part on using solder preforms to minimize voiding,” she went on.
“I’m only teasing. I have quite a bit of info,” Rob said.
“We have been out of the mainstream for a while and one thing is for sure, voiding is the number one issue among assemblers today. So many people are assembling QFNs and are struggling with voiding. Voiding with some solder pastes can be over 50% of the area,” Rob went on.
“Wow! With 50% voids, think of how poorly the heat is being transfer away for the BTCs,” she looked at Rob and chuckled. “Remember, ‘BTC’ not ‘QFN,’ Patty went on.
“Yes ma’am,” Rob jokingly replied.
“Can you imagine the effect on reliability and field issues with so little heat being removed? The ICs inside the BTCs must be frying” Pete added.
“Voiding at this level has got to be really costly,” Patty mused.
“One of the things that really helped me was that I found quite a few experiments on voiding,” Rob added.
“What were some of the key points?” Pete asked.
“Well, as you might expect, the solder paste is typically the most critical part of the process. Some pastes have voiding lower than 10% with others above 50%,” Rob replied.
“What about the process?” Patty asked.
“Well, the reflow profile can be very important, as is controlling the PWBs and components. But, with the best pastes, it has been found that you can control the voiding content even if you can’t change the reflow profile and the PWBs and components have some issues,” Rob responded.
“Look at the x-rays of poor and good voiding between two pastes,” Rob said.
“What a difference,” Patty and Pete said in unison.
“What about the stencil design and venting?” Pete asked.
“Chris said that stencil design for venting is not as critical as once thought, although a window pane design is usually used,” Rob replied.
Figure 1. The window pane design for the stencil is used to permit venting.
“So it sounds like starting with the best solder paste solves 90% of the problem and adjusting the process, say with the right reflow profile, helps refine the result,” Patty summed up.
With this Rob went off to put the finishing touches on his PowerPoint® slides for his part of the presentation, while Patty started working on her part of the presentation on using solder preforms to reduce voiding.
Two weeks later.
Patty’s mom and dad came for a visit on a Sunday. Her mom had graciously offered to bring a complete Sunday dinner. Patty, Rob and the boys were grateful for the delicious meal. As they began to eat, Patty shared the story of her best students not knowing Billy Graham, et al.
“But, what was even more surprising was that I ended up asking 10 or 20 more students and only one had ever heard of any of these four ‘famous’ people,” Patty sighed.
“It’s your age,” Patty’s mom replied.
Thirty years old was not that far in the rear view mirror for Patty and she really didn’t consider herself old.
“These youngsters were born in the late 1990s, a generation after these people were prominent,” her mom went on.
A while ago, I developed an Excel-based spreadsheet, StencilCoach, that calculates optimal stencil aperture parameters for several common SMT solder paste printing applications. These applications include standard apertures, passive apertures, pin-in-paste (PiP) apertures, and preforms with pin-in-paste (PiP+) apertures. These algorithms are now online at http://software.indium.com/. Over the next several posts, I will review the use of this software tool.
Let’s first look at standard apertures. After logging into to the website, click on “Stencil Coach” and then “Standard Apertures.” The page gives the definitions for “Aspect Ratio” for a rectangular aperture and “Area Ratio” for circular and square apertures. The aspect ratio, which is defined as the width of the rectangular aperture divided by the stencil thickness, should be greater than 1.5. Whereas the area ratio, for circular or square apertures, is given as the area of the opening divided by the area of the sidewalls. This formula simplifies to D/4t, where D is the diameter of a circular aperture or the width of the square aperture. The area ratio should be greater than 0.66. These recommendations are not standards, but are good rules of thumb.
Let’s consider a situation where a PWB has rectangular apertures with a pitch of 35 mils and circular and square apertures with pitches of 40 mils each. The stencil thickness is 6 mils. See if you can develop the pad and aperture sizes and reproduce the figure below. Hopefully this tool will help you design your stencils.
PS: If you need a hand, feel free to contact me at rlasky@ indium com.
We tend to think of mixing as something that can completely even out those things being mixed. As an example, let’s assume you are making chocolate chip cookies and would like to have 10 chocolate chips in each large cookie. You make enough batter for 100 cookies and then mix in 1,000 chocolate chips. After mixing for a long time you put 100 dollops of the batter on the baking pan and bake up the cookies. Upon inspecting the cookies, to your dismay, you find that you have only 13 cookies with 10 chocolate chips. More than 40 cookies have 30 percent more or 30 percent less than 10 chips. Worse yet, 3 cookies have 4 or less chocolate chips and 7 have 16 or more. See the graph below. You decide that you did not mix them enough, so you make another batch and mix for 4 hours. The results are the same.
Statistics tells us why the above scenario is so. In a case like this one, the number of chips in a cookie is described by the Poisson distribution. The mean will be 10 chips, since we are using the Poisson distribution, the standard deviation will be the square root of the mean or 100.5=3.16, or about 3 chips. One way to ensure a more even distribution of chocolate would be to divide each chip into 10, so we would have 10,000 smaller chips in a batch. On average each cookie would now have 100 chips and the standard deviation would be 10. Plus and minus one standard deviation is about two thirds of the data, so two thirds of the cookies would have +/- 10% of the desired amount of chocolate, a much better result. If we divided the chips into even smaller sizes, we would further tighten the distribution.
How does any of this relate to solder preforms or solder paste? In the new world of lead-free solder pastes, where it is common to have 3 or 4 alloying elements, some in very small concentrations, it can be difficult to control the concentration of the alloying elements throughout a sample of the alloy. The limits of mixing are just part of several processes that are required to assure that a modern lead-free solder has a consistent formulation. These are some of the topics you should discuss with your solder supplier to ensure consistency in any solder alloy you purchase. Asking to see assay analysis of a solder alloy is often a good idea, too.
Our discussion of Weibull Analysis continues…. Let’s say you have worked hard and assembled some SMT lead-free PCBs for thermal cycle testing. You used the best lead-free solder paste and some lead-free solder preforms as you assembled several through-hole components with the pin-in paste process. You were a little concerned with the assembly process as the board was thermally and physically massive and the reflow process needed to be a bit above the recommended temperature and time.
The results of the thermal cycle testing are shown in Figure 1 below. You dutifully report the characteristic life (or scale) as 2,387 cycles and the first fail at 300 cycles. You were quite disappointed, as in the past similar, but slightly smaller boards, had a slightly higher scale, but more importantly, the first fail was about 1,000 cycles. Anyway, you write up your report and file it away.
Figure 1. A Weibull plot of the thermal cycle data.
Hold on! The data are screaming at you the something is going on. Look at the same data in Figure 2. Note two distinct lines shown in green. These two separate lines suggest very strongly that there are multiple failure modes. The line furthest to the right is likely the typical failure mode observed in the past. The line to the left is a new early failure mode. It could be due to something like oxidized pads or some other phenomena not seen when testing similar but smaller boards. Root cause failure analysis should be performed to try and understand to new failure mode.
Figure 2. A Weibull plot of the thermal cycle data with multiple failure modes noted.
Now for a human interest note: One of the rewarding aspects of being a professor at Dartmouth is the outstanding nature of many of the students. They are not just good academically, but often are talented artistically, athletically, etc. This point was brought home to me recently. In a class I teach, ENGS 1: The Technology of Everyday Things, we were recently discussing the conservation of angular momentum (CoAM). One of the most striking ways to demonstrate CoAM is an ice skater’s spin. I went on the internet and could not find a good video of a spin. I then remembered that one of my former students, Julia Zaskorski, was on Dartmouth’s figure skating team. I asked her if she had a video she could share. It appears here. She is a materials science and physics major. Who knows, maybe we will see her at APEX or SMTAI in a few years.
Here is a little bio in her own words:
My name is Julia Zaskorski, and I’m a junior from Wellesley College taking part in the 12 College Exchange Program at Dartmouth. At Wellesley I am majoring in physics with the intent to pursue mechanical engineering. Despite Wellesley’s relationship with nearby MIT, Wellesley does not have its own engineering program, so I sought out the more self-contained curriculum and atmosphere at the Thayer School of Engineering. In addition to the draw of the Thayer School, the Dartmouth Figure Skating team was also a hugely motivating factor for my exchange, as Wellesley does not have a team, let alone a rink. I have known the coach of the Dartmouth team for several years now, and to finally see my name on the roster for the team is a dream come true. The engineers, as well as the winter activities here in Hanover, pulled my heart to Dartmouth long before I’d ever set foot on campus.
In the last posting we saw how Weibull analysis helped us to determine that SACM lead-free solder (SAC 105 with about 0.1% manganese) has comparable (actually better) thermal cycle performance versus SAC 305 solder. Software like Minitab will give us even more detailed information about the performance of the solder joints in stress testing as we see in Figure 1.
In addition to the Weibull plot, we also have the Probability Density Function (PDF), the Survival Function and the Hazard Function. The PDF tells us when it is most likely that a test board will fail in a test population, as shown by the inserted red line. We see that it is a little less than 2,000 cycles. The Survival Function shows the percent of surviving test boards. We observe that the expected life (the 50% point) is quite close to the maximum of the PDF. The Hazard Function tells us the rate at which the test boards are dropping out. It increases with time, but there are few boars left so the PDF drops down at the end of the test, even though the fallout rate is the highest.
It is interesting (and perhaps appropriate in the wake of Halloween) to consider if human mortality follows a Weibull distribution. I used some data for the Centers for Disease Control that are a little over 10 years old for males in the US. So, the mean life expectancy is a little low at 72 years. (I was a little lazy: the old data were a little easier to work with than new data, some conversions are needed to make it work.) The data appear in Figure 2.
As you can see, just like a solder joint, your life expectancy can be modeled quite well by the Weibull distribution.
1. The company will remain public, and the shareholders are the same. (Under the proposal, Cookson shareholders get one share in each of the two new companies.) Had this been an MBO or private equity group, I would expect slash and burn. But the transition as planned should bring much-desired stability to the new organization.
2. The upper management isn’t changing. Had Cookson Performance Materials group CEO Steve Corbett left, I might think differently. But Corbett, who joined Cookson in 1990 and has run Enthone since 2002 and both companies since 2004, is highly responsible for the existing management and operational structure. He knows what he is doing, knows the markets and understands the brands.
3. The debt is manageable. Alent (the new name of the former Cookson Performance Materials) will “get” about one-third of Cookson’s £451 million ($727 million) worth of debt. Given the new company’s sales of £418 million ($675 million) and profitability, it should be able to swallow that meatball.
4. The brands are intact. The Alpha Metals and Enthone brands are well-recognized and respected worldwide. Indeed, after spending some time trying to beef up the somewhat unwieldy Cookson Performance Materials name, the company reversed gears and has been working over the past year to rebuild those individual brand names. Perhaps this was in anticipation of the demerger, but either way, the strategy was well-timed.
In fact, the only casualty I see in all this is the Cookson name, which is, believe it or not, more than 300 years old. One wonders whether the Cookson name was seen as a negative by either of the spinoff companies.
And so goes Cookson. From its founding by Isaac Cookson in 1704 as a collection of metal and glass businesses to its aggregation of a herd of electronics assembly equipment and materials companies in the 1980s and 1990s to the respective divestitures of Speedline, then Polyclad and its Precious Metals business, Cookson has always been in a transition of some sort. It’s hard to believe, though, that this is its final move.