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.

 

Patty and the Professor: What Homogeneous Means

Folks,

Let’s see how Patty and Pete are doing with their Medical Device RoHS Crisis ….

Patty and Pete sat in a plane on the runway of the Manchester, NH, airport. Patty was just calming down after Jeff Sparkel  told her that Hal Lindsay had performed an analysis to show that the flagship medical device that Jeff’s factory assembled was RoHS-compliant using tin-lead solder. Corporate RoHS compliance was under her responsibility and she was panicking that ACME’s St. Paul site would miss the July 22, 2014 compliance date for medical devices. She literally drove straight to the airport after chatting with Sparkel on the phone. Fortunately, she and Pete both had a three-day suitcase in their offices for such emergencies. Rob’s mom agreed to help with her twin boys. What a blessing to have a mother-in-law like Rob’s mom.

To add to the stress, she and Pete almost missed the plane. He insisted that he needed to stop at a drug store, though he was secretive about the reason.

As the plane lifted off, Patty had to find out about this drug store mystery.

“OK, Pete. Why the drug store?” Patty asked.

“I’m afraid that, if I tell you, you’ll lecture me,” Pete said sheepishly.

“Out with it! Out with it,” Patty commanded.

“I bought Vick’s VapoRub to put under my nose when we are with Mr. Lindsay.  Ain’t no way I’m gonna’ be with that stink bomb unprepared,”  Pete responded.

Patty was going to say something but she started chuckling uncontrollably.

“You are welcome to share with me,” Pete offered.

As Patty tried to catch her breath, she just shook her head no.

They arrived at their hotel room at 10PM, after a quick, late dinner.

Fortunately, the timing of events was favorable. Lindsay had planned to give his final presentation the next day. Sparkel was actually pleased that Patty asked to attend.

Patty met Pete for breakfast at 7AM. By then she had run 5 miles, worked out with weights at the hotel gym, and showered. They arrived at Sparkel’s office at 7:45 and headed directly to the conference room where Lindsay was preparing to present. Upon seeing Patty and Pete, Hal Lindsay seemed surprised and turned a little red in the face.  Pete checked the room for ventilation.

Patty and Pete agreed to listen to Lindsay’s complete presentation without interruption.

“I know everyone here except for that guy in the back. He looks like a lawyer,” Pete whispered into Patty’s ear.

“He looks like a lawyer because he is one,” Patty responded. “He is my special guest,” she said.

Lindsay began his presentation sharply at 8AM. Patty had to admit that she was impressed with Lindsay’s experimental procedure. He had taken three of ACME’s St. Paul site’s highest-volume products and carefully performed teardown analyses. He painstakingly extracted all of the solder from the PCBs. One product weighed 10.2 kg and contained 11.2 grams of tin-lead eutectic solder. Patty checked Lindsay’s calculations. The fraction of lead in the unit was 0.042%, less than 0.1% that RoHS requires. All three products were below 0.05% by weight lead.

Lindsay then discussed his plan to analyze enough units to give the data statistical confidence. His charge would be an additional $20,000. Jeff Sparkel then asked if there were questions.

Patty raised her hand.

“Mr. Lindsay, what about RoHS’s requirement that all concentrations of substances of concern by ‘per homogeneous material?’ ” Patty asked.

Lindsay looked confused. His face turned a little red. It appeared that he didn’t understand what she was asking.

“Patty, please explain what ‘per homogeneous material’ means?” Sparkel asked.

“It means that any part of the product that could be mechanically separated must be less than 0.1% lead. As an example, a soldered joint can be cut out of a medical device with an X-Acto knife. Accordingly, the small piece of solder must be RoHS-compliant, so the solder itself must have less than 0.1% lead,” Patty explained.

“Per hemorrhoidgenous material, don’t apply to no medical devices,” Lindsay grumbled.

Both Patty and Pete had trouble not chuckling at Lindsay’s mispronunciation of “homogenous.”

“I beg to differ. Dr. Coleman’s explanation of ‘per homogenous material’ is spot on,” said Patty’s special guest.

Patty chuckled to herself when she realized that her guest thought she had a Ph.D.

“Who are you?” asked Jeff Sparkel.

“I’m Aaron Toynbee, Esq, our company’s general counsel. My department has responsibility for interpreting corporate compliance with environmental laws like RoHS.  We have studied the RoHS law extensively and the requirement for medical device compliance. Almost all of the medical devices we manufacture must meet RoHS compliance by July 22, 2014. I was alarmed when Dr. Coleman pointed out that there was some lack of understanding here about this.” Toynbee said.

After Toynbee spoke, it was agreed that the St. Paul team would work with Patty and Pete to resurrect the RoHS initiative that had been developed some time ago. Patty let out a deep sigh of relief.

Just as it appeared that the meeting was over, one of the younger engineers asked, “Are we still going to have Mr. Lindsay perform the analysis he suggested. It seems to me that there may be some benefit in getting this type of data.”

There appeared to be some murmuring of agreement. Hal Lindsay brightened, as it appeared that his proposed work might still be accepted.

Patty sat by watching this with incredulity. She remembered the Professor telling her that sometimes people will be too polite and not say what needs to be said. This was not going to be one of those times.

“You have got to be kidding me!” she shouted.  “There is no way we are going to continue any of this useless work!” she said even louder.

At this, Hal Lindsay’s  face turned beet red and he charged over to where Patty and Pete were. Out of the corner of her eye, Patty could see the Vick’s VapoRub gleaming under Pete’s nose.

Patty was now standing up and Lindsay had advanced to within five feet of her.

All of the sudden Lindsay came up to within a foot of Patty.

“It’s tree-huggers like you that that allowed this RoHS crap to happen in the first place,” he screamed into her face.

Patty was not prepared for this olfactory assault. Worse yet, some of Lindsay’s spittle ended up on her face. A natural gag reflex took over and she started having trouble breathing. Those in the meeting were horrified as they watched Patty crumble and slump to the floor.

Pete jumped up and instinctively and firmly pushed Lindsay back away from Patty. His Vick’s VapoRub doing its job. Sparkel’s  second-in-command, Jennifer Halliday, gently escorted Lindsay from the building, before any fisticuffs ensued.

Sparkel  and one of the female engineers helped Patty as she tried to get up. Within a few moments Patty was herself again. Everyone knew what happened, but when Patty said she probably should have eaten more for breakfast, everyone murmured in agreement.  Sparkel asked if just he, Patty, and Pete could wrap things up. Patty agreed, but asked to go to the ladies room first.

When she returned, Patty again reiterated that medical devices have to obey the “per homogeneous material” requirement and that the only way this was possible was to change to a lead-free solder. Patty and Pete confirmed their agreement to stay on for a few days to work with Sparkel’s team, to resurrect the plan to be RoHS-compliant by June 2014, a month early.

With two days of hard work, the plan was redeveloped, and Patty and Pete were confident the St. Paul team was on the right track. Jeff Sparkel apologized to Patty about 10 times.

Within no time Patty and Pete were back on the plane, heading home.

“Hey kiddo! You should receive hazardous duty pay for this one,” Pete teased.

“No kidding,” Patty responded.

“When you said you needed to go to the ladies room, I was a little worried,”  Pete said. “I thought maybe some permanent damage was done,” he went on.

“It was worse than that. I had to wash Lindsay’s spit off my face,” Patty groaned.

“Definitely Purple Heart material,”  Pete teased.

They both chuckled.

 

Cheers,

Dr. Ron

 

The Limits of Mixing: A Chocolate Chip Example

Folks,

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.

Cheers,

Dr. Ron

 

Best Wishes,

Dr. Ron

Electronics Failure Analysis for Pb- and Pb-Free Solder Joints

Folks,

The Weibull distribution is arguably the most important distribution in failure analysis of leaded and lead-free solder joints. It is the first thought of someone trying to model thermal cycle, drop shock, or other failure modes associated with through-hole and SMT assembly.

 

Figure 1. The Likelihood of Getting Heads in 60 Coin Tosses is Described by The Binomial Distribution

The Weibull distribution was invented by Waloddi Weibull in 1931.  This invention fact was recounted by Dr. Robert Abernethy in his famous textbook on Weibull analysis, The New Weibull Handbook. This statement may not seem unusual, until we ponder that all common distributions in statistics were discovered, not invented.  The three most common statistical distributions are the Normal, Poisson and Binomial distributions. As an example of a discovered statistical distribution, let’s consider the Binomial distribution. This distribution describes, among other things, the odds in flipping a coin.  If you flip a fair coin 60 times, you are most likely to obtain 30 heads (H) and 30 tails (T), but getting 29 H and 31 T or 32 H and 28 T would not be all that uncommon. Mathematical analysis shows that the curve below results.  If a coin flipping experiment is performed many times, this curve will faithfully predict the results. The curve is not invented it is discovered from the deep theoretical underpinnings of the Binomial Distribution.

The fact that the Weibull distribution was invented suggests that Weibull selected it because it fit many types of failure data.  He defined cumulative Weibull distribution is defined as:

 

 

where eta is the characteristic life or the scale function and beta is the slope, were as F(t) is the cumulative fraction of failures.  Weibull proposed this function because for beta less than 1, F(t) describes “infant” mortality fails.  In this situation the failure rate is decreasing with time. For beta greater than 1, it describes “wear out” failures, where the failure rate is increasing with time.  In electronics, we typically try to weed out infant mortality by using “burn in.” For beta equal to 1, the failure rate is constant.  These three scenarios are shown in the figure below.

So typically, in electronics failure analysis, we are plotting failure data versus time to determine beta and eta, typically with software like Minitab.

In the next posting we will analyze some failure data to determine eta and beta and discuss their significance.

Weibull himself was a curious character and much of the available information on him is chronicled by Abernethy.

For sure Weibull was a vigorous man.  His second wife was almost 50 years his junior and he fathered a daughter at about 80 years of age!

Cheers,

Dr. Ron

Talking Cleaning with Mike Bixenman

Folks,

There is a lot of interest in cleaning PCBs assembled with no-clean solder pastes. Recently I discussed the topic with my good friend Mike Bixenman of Kyzen.

Dr. Ron (DR): Mike, many of the best performing lead-free and lead containing solder pastes today are no-cleans. They have been designed to solve assembly problems like graping and the head-in-pillow defect. For the vast majority of applications, the small amount of residue left by a no-clean is not a problem. However, some assemblers want the performance of no-cleans, but need to clean the no-clean residue as they have extreme reliability or cosmetic requirements. Are there cleaning solutions for these situations?

Mike Bixenman (MB): Absolutely!

DR: Can you tell use a little bit about these cleaning solutions?
MB: Several factors come into consideration when engineering electronics assembly cleaning agents. Design factors include the soil make-up, heat exposure, Z-axis clearance under bottom termination components, material compatibility, and cleaning equipment. Typical process goals require that all flux be removed in one cleaning cycle, shiny solder joints (no chemical attack to the alloy), fast production speed, no material effect to labels and other materials of construction, long chemistry bath life, and low operating concentrations.

Cleaning solutions vary depending on the cleaning equipment. For solvent systems, a solvent cleaning agent is needed – with properties that allow for non-flammability, constant boiling mixture, and being environmentally-friendly to workers and the environment. For solvent cleaning agents that are rinsed with water, the cleaning agent requires a solvent mixture that can be rinsed with water while matching up to the soil and cleaning equipment. For aqueous cleaning agents, the cleaning agent is engineered with properties that provide solvency for the soil, polarity for inducing a dipole and/ or to oxidize and reduce the soil, low surface tension to reduce the wetting angle, buffers to stabilize pH, defoaming to reduce the tendency to foam at high pressures, and inhibitors to widen the passivation range on metallic alloys.

The most critical property is the nature of the soil. As soldering temperatures rise and the time exposed to higher temperatures increase, solder paste material supplies must improve the oxygen barrier and prevent flux burn out. This requires higher molecular weight compositions that may change the nature of the soil and the cleaning solution needed to remove the soil. Other factors such as processing conditions and how these conditions can change the soil’s cleaning properties must be considered. For example, excessive exposure to heat may polymerize the flux residue rending the soil uncleanable. To better understand and plan for these factors, solubility testing and matching the cleaning agent to the soil assist formulators in designing cleaning agents that are effective on a wide range of soldering material residues.

DR: What type of equipment is typically needed?
MB: Two key factors must be matched to clean:
1: Potential energy of the cleaning agent for the soil and
2: Kinetic energy of cleaning machine for delivering the cleaning agent to the soil necessary to create a flow channel needed to rapidly displace the soil.

The cleaning machine requires energy to deliver the cleaning fluid across a distance and create enough force to deflect fluids under the Z-axis. The capillary attraction for moving the cleaning fluid into an out of tight gaps is created by fluid flow, spray impingement pressure and surface tension effects. When cleaning under tight standoffs, cleaning agents that wet (form small droplets) improves capillary action, penetration and wetting of the residue. The solubility rate is dependent on the soil, temperature effects and concentration of the cleaning agent needed to dissolve the soil. Hard soils clean at a slower rate and remove the soil in a concentric (tunneling effect) manner. Soft soils clean at a fast rate and remove the soil in a channeling (multiple tunnels) effect.

The Z-Axis gap height has a direct correlation to the energy required to penetrate and remove the soil under components, time required to clean the soil and wash temperature. The irony is that lower Z-axis gaps increase capillary action of the flux for underfilling the bottom side of the component. When this occurs, flux residue dams up and closes any flow channels under the component. Research findings indicate that high pressure coherent spray jets are needed since energy drop is less and defective energy is higher. The wash time needed to clean under a 1 to 2 mil gap as compared to a 4 to 6 mil gap can range from 4t o 8 times longer. Higher wash temperatures increase the softening effect and aid in penetrating and removing the soil. The net effect is that, as components decrease in size, the Z-Axis gap height reduces and the cleaning factors needed to clean the soil increase. These effects favor spray-in-air cleaning equipment over immersion cleaning equipment.

DR: How are the results of cleaning assessed, so that we know that the boards are truly clean?

MB: The first level that we judge cleaning performance by is the visual presence of the residue post cleaning. Most cleaning processes have no problem with removing surface residue from the assembly. The issue is the residue under the bottom side of the component. This complicates the issue since the residue under a specific component is where most failures occur. These site-specific failures may reduce the confidence in existing IPC standards that correlate anion and cation ionic residues over the entire board surface area. So, when designing the cleaning process, we use test cards with bottom termination components and judge cleaning performance by the level of flux residue remaining under those components. To achieve this value, all components are removed and the surface area of the residue under components is graded and statistically analyzed.
Let me finish by adding that highly dense interconnects assembled onto circuit boards is advancing at a rapid pace. Traditional SMT component spacing between conductors was larger. No-clean post soldering residues posed minimal risks to reliability. The information age has spoiled us in expecting higher functionality in smaller spaces. As assembles reduce in size and increase the levels of functionality, cleaning becomes more important. I hope that the cleaning factors discussed in this interview provide insight into cleaning process design considerations that may be of help.

DR: Mike, thanks. Who should folks contact if they would like more information on cleaning boards assembled with no-clean solder pastes.
MB: Thanks for letting me share with your readers. I would be glad to help anyone with the cleaning challenges they face. Contact me at mikeb@kyzen.com.

Cheers,
Dr. Ron

Service Life

A reader writes:

My company makes an electronic product that requires a 40- year shelf life. We assemble with tin-lead solder on FR-4 PWBs. The product is to replace older technology (i.e. 1960-70s), but has some newer components such as BGAs, SOICs, and PQFPs. The product will be stored in dry nitrogen at 70F.  We take great care in manufacturing, by cleaning, inspecting, and testing the end product.

My question is, Do you know of any studies that would discuss the reliability of products stored or in use for 40 years?

My sense is that our reader will be successful, but his question is profound and hard to answer with confidence. The military would like their electronics to perform for that long, but realistically much of it is replaced every 10 years or so. If you look at something like the B-52 bomber, which debuted in 1952, the electronics have been upgraded regularly. So there isn’t as much 40-year electronics experience as one might think. An exception being the IBM AP-101 computer. This computer was kept in service for over 30 years, because it served its function and had survived the rigorous and expensive military qualification testing.

However, anecdotal data might support optimism for 40-year shelf life. In a class I teach at Dartmouth, The Technology of Everyday Things, I have sought out some old transistor radios from the late 1960s and early 70s to show the class how this old technology works. Anytime I have every found an old device like this, they always work, unless the batteries have leaked inside the radio.

This question raises an interesting thought. Although those of us in electronics assembly are concerned with tin-lead and lead-free solder joint life, what about the modern devices inside the components? Forty years is a long time. How will the 3D-22 nanometer copper circuit lines in a modern microprocessor hold up over this amount of time? These circuit lines lines are so fine that the 22nm width is only about 70 atoms.  In addition, copper integrated circuits are still a relatively new technology. I’m sure much accelerated life testing has been done on such circuits, but would such testing confirm 40 years of shelf or service life?

I would appreciate any thoughts that readers have on these questions.

Cheers,

Dr. Ron

Revelations at ACI

Folks,

I’m taking a few moments from Wassail Weekend, held annually in my village, Woodstock, VT (“The prettiest small town in America”), to write a post about the recent workshops at ACI.

Indium colleague Ed Briggs and I gave a three-hour presentation on “Lead-Free Assembly for High Yields and Reliability.” I think Ed’s analyses of “graping” and the “head-in-pillow” defect are the best around.

There was quite a bit of discussion on the challenges faced by solder paste flux in the new world of lead-free solder paste and miniaturized components (i.e., very small solder paste deposits.) One of the hottest topics was nitrogen and lead-free SMT assembly. There seemed to be uniform agreement that solder paste users should be able to demand that their lead-free solder paste perform well with any PWB pad finish (e.g., OSP, immersion silver, electroless nickel-gold, etc.) without the use of nitrogen. Not only does using nitrogen cost money, but it will usually make tombstoning worse. However, in the opinion of most people, nitrogen is a must for wave soldering and, since it minimizes dross development, it likely pays for itself.

After Ed and I finished, Fred Dimock, of BTU, gave one of the best talks I have ever experienced on reflow soldering. He discussed thermal profiling in detail, including the importance of assuring that thermocouples are not oxidized (when oxidized they lose accuracy). He also discussed a reflow oven design that minimizes temperature overshoot during heating, and undershoot when the heater is off. Understanding these topics is critical with the tight temperature control that many lead-free assemblers face.

Fred Verdi of ACI finished the meeting with an excellent presentation on “Pb-free Electronics for Aerospace and Defense.” Fred’s talk discussed the work that went into the “Manhattan Project.” A free download of the entire project report is available.

There appears to be agreement that acceptable lead-free reliability has been established for consumer products with lifetimes of five years or so, but not for military/aerospace electronics where lifetimes can be up to 40 years and under harsh service conditions. These vast product lifetime and consequences of failure differences are depicted in Fred’s chart (see the pdf link). Commercial products are in quadrant A and military/aerospace products in quadrant D.

One of the greatest risks faced by quadrant D products is tin whiskers. Fred spent quite a bit of time discussing this interesting phenomenon. One of the challenges of this risk is that there is no way to accelerate it, so you can’t do an equivalent test to accelerated thermal cycling or drop shock. Fred mentioned that there have now been verified tin whisker fails, the Toyota accelerator mechanism being one.

In addition to tin whiskers, lead-free reliability for quadrant D products (with a service life of up to 40 years) in thermal cycle and other areas remains a concern.  I mention that tin pest was not on the list of issues for this quadrant.

Fred and the Manhattan Project Team have identified many “gaps” that need to be addressed to determine and mitigate the risk of lead-free assembly for quadrant D products.  They plan to start this approximately $100 million program in 2013.

For those that missed this free workshop, another is planned in about six months.

Cheers,

Dr. Ron

Is a Pb-Free Consensus Achievable?

Folks,

Recently I posted a note about a flurry of Technet posts in which I was misquoted regarding the status of lead-free electronics assembly.  Harvey Miller then weighed in.  I responded. And this in turn raised more comments.

All of this caused me to wonder, is it possible to achieve a consensus on the state of Pb-free assembly? I think it might be and am going to try. The main thing that I think is important in this quest is that any points for the consensus, or lack thereof, be supported by data and analysis, not emotion.

If you have a point to add, that is backed by data and analysis, please share it with me.  One of the things I hope to accomplish is to develop a list of references, that can be referred to to support the consensus.

Stay tuned for more info on this effort.

Cheers,

Dr. Ron

Electricity Use in Pb-Free

Folks,

An obvious disadvantage of lead-free electronics soldering assembly is that the oven must be hotter and therefore will use more electricity (versus SnPb37 soldering). But is the extra amount of electricity significant?

KIC’s Brian O’Leary claims that a typical SMT oven uses $7,000 worth of electricity a year at $0.072/Kilowatt hour (Kwh) or about 100,000 Kwh. That number strikes me as about right, as a household uses about 5-20,000 Kwh per year.

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

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

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

Best Wishes,

Dr. Ron

All Wet

Folks,

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

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

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

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

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

Thanks Mike!

Cheers,

Dr Ron

P.S. The solder image is courtesy of Vahid Goudarzi of Motorola.