Soldering 101: The Simplicity of Soldering – The Complexity of Solder Paste

Folks,

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.

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

Area Ratios for Elongated “D” Apertures

Folks,

Ismail writes: Dr. Ron, I know that the area ratio for circular and square stencil apertures is 4d/t.  What is it for an elongated “D” aperture?

 

The area ratio of a stencil aperture is the area of the aperture opening divided by the area of the side walls.  It is interesting, as Ismail points out, that the area ratio of a circular aperture is the same as that of a square aperture.  A little 10th-grade geometry will point this fact out.  It ends up that the area ratio of an elongated “D” is a little more complex.  All of these aperture shapes and that for a rectangle aperture are shown in Figure 1.   The area ratio formulas are at the bottom of the figure.

 

Figure 1. The area ratio for several shaped apertures. The elongated “D” aperture is third from the left.

 

 

 

 

 

 

 

 

 

A rule of thumb that still seems to hold is that the area ratio should be 0.66 or greater for the best printing result.  It is possible to do somewhat better (i.e with an area ratio less than 0.66) with a superior solder paste and/or some of the new stencil nano-coatings.

The derivation of the area ratio for the elongated “D” is in Figure 2.

Figure 2.  The derivation of the area ratio for an elongated D shaped aperture.

 

 

 

 

 

 

 

Cheers,

Dr. Ron

 

Alloy Metal Weight Fraction Calculation

Iasad writes,
“Dear Dr. Ron,

I see that you have developed software to calculate the density of an alloy if given the weight fractions of the constituent metals. Is it possible to find the weight fractions of the metals in an alloy given the alloy’s density? Thank you!”
Unfortunately, finding the weight fractions of the metals in an alloy from the alloy’s density can only be accomplished with a two metal alloy. First we must use the equation:

Equation 1

Where x is the weight fraction of metal A and the rhos are the associated densities.  All that has to be done is to solve for x.  The solution is worked out below in Figure 2, the final result is:

Equation 2

As an example, let’s say you have a gold-copper alloy with a density of 18.42 g/cc.  The density of gold (metal A) is 19.32 g/cc and that of copper (metal B) is 8.92 g/cc.   Substituting these values into equation 2 gives the weight fraction of gold as 0.958.  Hence the weight fraction of copper is 1-0.958 = 0.042.

I have developed an Excel-based software tool to perform these calculations. An image of it is shown in Figure 1.  If you would like a copy of this tool send me a note.

 

Figure 1.  A screen shot of the alloy metal weight fraction calculator.

 

Figure 2. The derivation of the weight fraction formula.

Cheers,

Dr. Ron

Self-Driving Vehicles Will Require Unprecedented Reliability

google_car

Google’s self-driving car

Autonomous (driverless or self-driving) cars will require unprecedented software and hardware reliability. This need may require double or triple redundancies in some critical systems. Those of us in electronics assembly think first of the reliability issues with hardware, but software concerns may be even greater.  Almost every day we have to reboot one of our electronic devices to get it working, due to software issues, yet seldom have a hardware fail. So the equivalent of the “blue screen of death,” may be the greatest concern for this future technology.

Still, hardware reliability will be a critical issue. Therefore we can expect our colleagues in automotive electronics assembly to be the most demanding in history regarding reliability.

Just how far in the future is the autonomous automobile? Some may think it is already here after reading about the auto accident death of a man while his Tesla was doing the driving.  However, this accident was caused by an auto with only the L2 capability of automation. In L2 automation only speed and lane changing is performed by the auto and only in special circumstances. The human is still in control.

The industry has defined 5 levels of automation, as shown in Figure 1 below. Only L4 or L5 is true automation. In L5, the auto would likely not have a steering wheel, as the human does not take part in driving at all. Figure 1 came from a recent article in Scientific American by Steven Shladover. Shladover argues that L4 and L5 vehicles are decades away, at the earliest 2045. Informal discussions I have had with a leader in the industry, who does not want to be quoted, agrees with this perspective.

 

selfdrivingcars-1

Figure 1. Many technologists suggest that only L4 or L5 automation is practical.

 

 

 

 

 

 

 

 

 

 

 

 

 

Many argue that it makes no sense to have L2 and L3 vehicles as the driver could lose focus while the auto is driven autonomously, and not be alert when needed. When the L4-/L5-era arises it will likely reduce the death toll from accidents significantly. When one considers that 100 people in the US are killed each day in auto accidents, this benefit will be welcome indeed.

Fully autonomous cars will be a major technology disruption. According to John Krafix, CEO of the Google Self-Driving Car Project, we use our cars only 4% of the time. In the era of driverless cars, why have the expense of owning one, when you can summon one for a much lower yearly cost?

It will be interesting to watch all of this unfold, and it will present new and rewarding challenges to those of us in electronics assembly. However, sadly, most of us working today will be well past retirement by the time it comes to full fruition.

 

Using the Coffin-Manson Equation to Calculate Thermal Cycles

Folks,

Let’s look in on Patty and friends ….

Patty, Rob and Pete were headed to their regular monthly meeting where they, along with the Professor, discussed a book they were all reading.  This month’s book was about General Leslie R. GrovesRacing for the Bomb

“This was one of the most interesting books we have read,” Pete said starting the meeting.  “I think most people are aware of the technical genius of the scientists involved in the Manhattan Project, such as J. Robert Oppenhiemer and Richard Feynman, but few appreciate the contributions of Gen. Groves,” Pete continued.

“I agree,” Rob said.  “Without Grove’s orchestrating of the overwhelming number of small and large details of the program, it would have taken three times as long,” he went on.

“Right!” the Professor chimed in. “He set up a $20 billion enterprise to produce the components of the bomb in less than three years.  Who else could have done that?”

“One of the things that I found almost comical was that he was so good at the secrecy of the project that his family had no idea he was working on the bomb until it came out in the newspapers,” Patty exclaimed.

The four book club members chatted about the book for about 20 more minutes.  Patty felt her cellphone vibrate.  It was a text from Mike Madigan.

“Rob, Pete, it looks like we may have another assignment from Mike. He wants us to call, so let’s go to my office,” Patty suggested.

Even though the three of them were all at the engineering school at Ivy U, Mike Madigan, the CEO of ACME, established a blank contract with them to do part-time consulting.  Part-time consulting is quite a common thing in the academic world as it helps the profs and technical staff keep current and also earn a little money.

Patty called Mike’s number and activated the speakerphone.

“We have a customer who we assemble TVs for.  Each TV goes through 10,000 on/off cycles in its field life.  The temperature change from these on/off cycles is from 20°C to 50°C.  We are performing thermal cycle testing of the PCBs from 0°C to 100°C.  How many thermal cycles will we need to perform to equal the 10,000 field cycles?” Madigan asked.

Patty chuckled to herself as she had just solved a problem like this for a reliability workshop that she was developing. So, the technique was fresh in her mind.

“You need to use the Coffin-Manson equation,” Patty explained.

“Whoa!” Mike chuckled, “Is the problem so serious that we need to worry about coffins?”

“Coffin-Manson is used to relate strain to temperature changes. It will help us to calculate the right number of cycles,” Rob chimed in.

Rob, Patty, and Pete all got calculators out to see who could get the answer first.  Pete won the contest.

“I get an acceleration factor (AF) of 25,” Pete announced victoriously.

“Agreed,” Patty and Rob sighed in unison.

“The equation is quite simple,” Patty shared.  See the figure below.

 

DrRon1

 

“The Coffin-Manson acceleration factor for lead-free solder, m, is about 2.7,” Patty finished.

“So, you need to perform about 400 (10,000/25) cycles in the test chamber,” Pete said.

“Wow! I’m really relieved,” Mike said, “I thought it might take 2,500 thermal cycles or more.”

“There is no way we had enough time for that number of cycles, but 400 is easily doable,” Mike concluded as he sighed a breath of relief.

The four of them chatted for a while more and then went their ways after having mastered another electronics assembly problem.

Cheers,

Dr. Ron

In SMT Assembly, Even 1 Second of Cycle Time Can Affect Profitability

Folks,

Patty had just returned from SMTAI 2015. It was a sentimental meeting with the retirement of longtime executive administrator JoAnn Stromberg. At one of the technical sessions, Patty was especially interested in epoxy flux being used as an underfill. She couldn’t wait to discuss it with The Professor.

As she drove up to Ivy University’s campus, she was struck by the many hundreds of students walking to class. No one was overweight and no one was smoking. She reminded herself to discuss this topic in her statistics class. Surely Ivy U did not represent the typical 18-22 year-olds in this regard.

Soon, she arrived at her office. After clearing her laptop of emails, she headed to The Professor’s office.

Patty had been working to improve her French. Since French was one of the 18 languages in The Professor’s repertoire, they often spoke it to improve (for Patty) and keep sharp (for The Professor). Patty chuckled to herself that her French was now good enough to hear The Professor’s Quebecois accent. He learned French as a pre-teen, as his parents were missionaries for Wycliffe Bible Translators and worked with some remote Indian tribes in northern Quebec.

Bonjour Professeur, comment allez-vous?Patty dit gaiement.

“Je suis bon Patty, comment étais SMTAI?” Le professeur a répondu.

The remainder of the discussion will be translated into English for our non-Francophone readers.

“It’s too bad that you couldn’t make it this year. The retirement dinner for JoAnn was touching,” Patty began.

“It will be hard to replace her, indeed. Her commitment was extraordinary,” The Professor responded.

After discussing this topic for a few minutes, The Professor changed the subject.

“Were there any interesting papers presented at the SMTAI tech sessions?” he asked.

“That’s why I’m here,” Patty replied. “There was a paper on epoxy flux as an underfill material. It was a great talk comparing epoxy fluxes to standard underfills. The speaker mentioned how using epoxy flux allows the operator to avoid using a separate dispensing process and curing oven that standard underfills require. His point was that the epoxy underfill approach would save a lot of money, as long as the epoxy process only added one second or less to the cycle time. This one second was the time it took to dip the flip chip or BGA into the flux.”

Patty immediately saw the troubled look on The Professor’s face.

“Professor, I sense you are thinking the same thing that I was,” Patty said.

“Yes, one second is a long time,” The Professor replied. “One second is 5% of a 20-second cycle time, so your production is reduced by 5%. Not a trivial amount.”

“My sense is that this one second would be a greater cost than paying for the dispenser and curing oven in a standard underfill process that keeps the cycle time at 20 seconds,” Patty said.

The Professor nodded his head in agreement and then went to his laptop. In just 3 or 4 minutes, he had calculated four different scenarios using ProfitPro software.

“Well, in most cases, the cost of that 1 second/cycle lost by the epoxy flux process costs the operator somewhere between a few hundreds of thousands of dollars to more than one million dollars per line per year,” The Professor explained. “This estimate even considers the fact that the standard process already needs a dispenser and curing oven.”

“You know what I always say.” The Professor started.

“It never pays to reduce productivity,” Patty chimed in, always the faithful student.

“Take a look at this one example. A large ESM manufactures a product with a 3-shift, 5-day/week operation on a state-of-the-art SMT line. The default, as shown in the figure, is the financial result for one year of production, using a typical underfill, assuming $200K for a dispenser and curing oven and a 28 second cycle time.

LaskyTable1

“The second run shows the financial results using an epoxy flux that requires a one second longer cycle time (29 seconds), but saves capital cost in that the line does not need a dispenser or reflow oven.”

“Wow, the company loses over $100,000 per year with the epoxy flux!” Patty exclaimed.

“Precisely,” The Professor responded.

“But, this doesn’t mean that people shouldn’t use epoxy flux as an underfill,” Patty stated.

“Right, they just need to avoid losing the one second.” The Professor agreed. “Where do you think the one second can be found?”

“Probably in line balancing,” Patty responded. “About the closest you can balance a line is within a second or two. It could be as simple as having the epoxy-fluxed part placed by the fastest placement machine.”

“And if there are many components that use epoxy flux?” The Professor asked.

“It would likely pay to get another placement machine,” Patty answered quickly.

“As always, there is never one right or wrong way to address a problem like this,” The Professor pointed out. “But, we should always perform the calculations to determine which approach makes the most sense.”

“Yes, and always remember that it never pays to reduce productivity,” Patty joked.

They both smiled as Patty left The Professor’s office.

Cheers,

Dr. Ron

The Rule of 3/N for Estimating Field Failure Rates

Folks,

It looks like Patty is a bit troubled….

When she was younger, Patty was always annoyed by cranky old people, and now she was worrying that she might become one. The trigger making her cranky was what students know and don’t know. It all started when a colleague showed her the “Texas Tech Politically Challenged Video.” 

“How could so many students not know who won the American Civil War, who the Vice President is, or who the United States won its freedom from?” she thought.

Some of her colleagues felt the video was staged, but the producers came up with a response video that strongly suggested that it was not. What was even more unsettling was the fact that all the students knew who Snooki was and who Brad Pitt’s wife was.

Some of Patty’s statistics students got wind of this video and decided to make a similar video at Ivy University. The results were mostly comforting: 49 out of 50 students knew who won the Civil War, and the one student who didn’t was from India. They also did well with some other questions, 85% knowing that Joseph Stalin was the leader of the Soviet Union in World War II, and a high number knew that Joe Biden was the VP.

But Patty was most troubled that almost 50% did not know who wrote A Christmas Carol. She had discussed the topic with Rob and was further annoyed that he didn’t seem as troubled as she was. Rob pointed out that some international students might not have had English literature in their studies, and being a story about Christmas, it could be a cultural thing. Patty was unconvinced by his arguments. It still seemed troubling to her.

Charles Dickens

Charles Dickens in 1867, 24 years after he authored “A Christmas Carol”

As she was mulling over these thoughts, the phone rang. It was Mike Madigan, CEO of her former employer, ACME.

“Hey, Patty, it’s Mike,” Madigan said cheerfully. “I need your help with a statistic problem. It might be good if Rob and Pete were involved, so could we do a teleconference?”

Patty scheduled the teleconference for later in the day. When the time came, Pete and Rob were in Patty’s office and she called Madigan. After exchanging pleasantries, Madigan got to the point.

“We have a demanding customer from the military,” Mike started. “They have a Zero Defects program and want to know how we can guarantee it after field exposure.”

“To clarify, you mean guarantee zero defects for units in the field?” Pete asked.

“Yeah,” Mike replied.

“The way I figure it, if we have 20 units in the field and none fail, we can say with 95% confidence that we have zero defects, because one unit is 5% of 20, and if none fail, that means we can be 100%-5% or 95% confident,” Mike said.

Patty instinctively reached for the mute button, as Rob and Pete went into hysterics. She glared at both of them.

“Hello, hello, are you there?” Mike asked as he heard no response.

Finally, with Patty continuing to glare, Pete and Rob had stopped laughing. So she unmuted the phone.

“Sorry Mike, the failure rate in the situation you described is that you can be 95% confident that is it less than or equal to 15%” Patty replied.

The other end of the conference call was quiet for a while and finally Mike answered,

“Yikes! OK, can you explain?”

“Patty and I have developed the math to explain how to calculate confidence limits on field failure rates,” Rob responded. “For 95% confidence we have developed what we call The Rule of 3/N.”

“How does it work?” Mike asked.

“If you have N samples in the field, and none have failed, you can say with 95% confidence that your failure rate is 3/N or less. As an example, let’s say you have 300 units in the field and none fail. You can then say with 95% confidence that the failure rate is less than or equal to 3/300 = 1/100 = 0.01 = 1%.”

“If we have 300 units with no fails, we can only have confidence in a 1% failure rate?” Mike groaned.

“One percent or less, with 95% confidence,” Patty chimed in.

Is demonstrating a 0% failure rate possible?  Will Patty and the team find a way to help Mike? Stay tuned for more details.

Cheers,

Dr. Ron

 

Phil Zarrow Weighs in on Productivity

Folks,

I ran into good friend Phil Zarrow the other day. Phil, Jim Hall, and I developed the SMTA Certification Program. We ended up chatting a bit about productivity, one of my favorite topics.

Ron: Phil, you have likely visited more assembly factories than anyone I know, hundreds for sure. What are some of your observations on how folks address or don’t address productivity?

Phil: Ron, there are so many bad practices that result in low productivity. More often than not, when we enter the manufacturing floor (for a process audit or other reason) we see a sea of red and/or orange light towers – rather than PCBAs in process. Most managers have no concept of the capacity they are operating at and usually feel that adding another line (with faster equipment) will increase capacity. However, there are three top “sins” that should be addressed – immediately!

The first is setup time. Unless you’re an OEM building the same PCBA day in and day out, this is something you have to master. And the higher the product mix, the more line changeovers prevail, and the more this impacts throughput. There are a number of things that can be done to “expedite” setup and they all add up. Any facility with more than one active line can benefit from a systematic approach toward setup. I tend to favor (and have had excellent luck with) the “Pit-Crew” approach. Note that the operators and setup crew are working together. Sequential changeover goes a long way: as soon as the last PCBA in a run passes through a machine center the crew commences changing over that machine (stencil, feeders, programs, etc.) rather than waiting for that last PCBA to clear the reflow oven.

Usually, hand-in-hand with this situation is a lack of adequate feeders for the different components that need to be changed over. Having a feeder already loaded with the component and “popping” it in rather than having to remove a reel and replace the component reel goes a long way. Feeder carts go even further. But this costs money and management usually doesn’t “get it.” In fact, we’ve encountered situations where there is such a shortage of extra feeders that, when the tech or engineer discovers that a feeder is malfunctioning, they don’t have a “spare” and are forced to continue using it, continuing to produce defects that have to be attended to (more time, expense, etc.).

Ron: Phil, I have observed similar practices as, noted in my book “The Adventures of Patty and the Professor.” What is the second sin?

Phil: Another common situation is a lack of balance in the line. Particularly predominant in the placement machines, if one machine is waiting a disproportionate time for another machine, the line is unbalanced. Components can and should be shifted from one machine to the other. While most of the placement machines come with software for calculating this, it is very simple math – single variable algebra (like we learned in 8th grade). But the “math phobia” we seem to suffer from is a subject for a different day….

Ron: I agree. The engineers will tell me that the line is balanced, but when I go out to the shop floor and check with my watch, the lines are almost never balanced, even though, in theory, the placement machines will easily handle it.

Now, we are holding our breath, what is number 3?

Phil: I’d finally like to comment on, to use a term you originated, “floundering time.” This is where the operator or tech comes across a problem or situation and has no idea what to do. She is not sure of the reporting system or “who to call.” It could be a machine problem, a tooling problem, a component outage, or a variety of other things. But, they all result in unscheduled downtime and severely impact productivity.

That’s just the tip of the iceberg, Ron. But just addressing these areas can improve productivity and cost a lot less than adding another line.

By the way Ron, I know you have thoughts on how materials can affect productivity. What’s a top example?

Ron: Obviously the main consideration for materials is that they perform their material function well. As an example, you would want your solder paste to form a reliable solder joint. However, solder pastes can affect productivity. I have seen cases where the poor response to pause of a solder paste was so bad that, if the line was idle for more than 20 minutes, the paste would stiffen up and have to be wiped off the stencil and replaced with fresh paste. These types of issues are discussed in “The Adventures of Patty and the Professor” in Chapters 9, 10 and 21 and can affect productivity and profitability more than you might expect.

Phil, thanks for the nice chat!

Cheers,
Dr. Ron

Lead Free 2015

It is hard to believe that in July we will celebrate the 9th anniversary of the advent of RoHS. So the timing seemed right when I was recently asked to speak at the Boston SMTA Chapter on The Status of Lead-Free 2015: A Perspective.

An overview of the entire 75-minute presentation would be a bit long, so I am going to discuss three of the “questions” that I covered.

  1. Q: We are now almost nine years into RoHS’s ban on lead in solder. How has lead-free assembly worked out?

A: Something over $7 trillion of electronics have been produced since RoHS came into force, with no major reliability problems. One senior person, whose company has sold hundreds of millions of lead-free devices since 2001, reports no change in field reliability. The challenge that implementing lead-free assembly placed on the industry should not be minimized, however. Tens of billions of dollars were spent in the conversion. In addition, failure modes have occurred that were not common in tin-lead assembly, such as the head-in-pillow and graping defects. But assemblers have worked hard with their suppliers to make lead-free assembly close to a non-issue. Some people ask how I can say that lead-free assembly is close to a non-issue. My office is across the hall from some folks that purchase millions of dollars of electronics a year for Dartmouth. Several years ago, I asked them how they feel that electronics perform since the switch to lead-free. They answered by saying “What is lead-free?” If people that buy millions of dollars of electronics have not even heard of lead-free it can’t be a big issue.

  1. Q: In light of sourcing difficulties, is there an industry consensus regarding lead-free conversion for military, medical, aerospace etc. assemblers that will continue to be exempt?

A: The main issue is getting components with tin-lead leads, especially BGA balls. Many assemblers are reballing BGAs, which has become a mature technology, although with an added cost. As years go by and there becomes more confidence in medium to long term lead-free reliability, some exemptees may switch to lead-free. However, I think mission critical applications with 40-year reliability requirements must be extremely cautious to make the switch. There may be subtle reliability issues that may show up in 40 years, that are not found in accelerated testing. One concern is aging. Even at room temperature, solders are at over 50% of their melting temperature on the absolute scale (300K/573K = 0.52). So aging can occur at room temperature. Some research suggests that lead-free alloys may be more affected by aging than tin-lead alloys.

  1. Q: It has been said that you claim that lead-free assembly has some advantages. Can this be true?

A: Guilty as charged. Lead-free solder does not flow and spread as well as tin-lead solder. This property can result in poor hole fill in wave soldering and some other assembly challenges. However, this poor wetting and spreading means that pads can be spaced closer on a PWB without the concern of shorting as seen in the image below. Your mobile phone would likely be bigger if assembled with tin-lead solder.

image001

Lead-free solder does not flow as well as tin-lead solder. Hence, closer pad spacings are possible.

 

Cheers,

Dr. Ron

Photo courtesy of Vahid Goudarzi.