Led Solder

Folks,

Some time ago I wrote a post, “In Search of Tin Whiskers,”  Michael responds below.  He makes some good points.

Dr. Ron, I'm responding to your  blog regarding tin whiskers. I actually have a failure analysis report I did a couple of years ago in which failure of our product was due to this issue and occurred on a part that came into RoHS compliance only 3 months prior.
 

I'm not sure that your question of identifying whisker issues in product that proper steps have been taken to mitigate the problem is a constructive one. The fact is that many of the component manufacturers from overseas jumped into compliance without any thought or regard to this issue thereby flooding the industry with components such as plagued my company. We have not had this issue since we've specified an alternate finish.

These whiskers are so delicate that most problems disappear when the technician starts to work on the failed unit and the problem never re-appears so it is written off as an anomaly, loose/bad connection and not investigated any further. It was only my own curiosity as to the number of "no problem found" failures of our keypads we had suddenly encountered that caused me to dig deeper and when I looked into the connector I was amazed at the crystal city staring back at me. I couldn't believe what I was seeing after all of these years.

After seeing this problem first hand I became, and am, quite convinced that there were and are people who will be losing life, limb, and property because this forced compliance with its risk was not given proper worldwide attention.

Michael.

A popular topic Re my blog is solder density calculations. Rhonda writes……

Hi Dr. Lasky,
I am a precious metals recycler and would very much appreciate your verifying the validity of an equation that approximates the Karat Value of various alloys of gold based on S.G. which I will call density or "D," and the Karat Value is "K." The equation is seems to hold relatively true even when the exact composition of the alloy is unknown, although the percent of error obviously will increase as density decreases. I would also appreciate not only verification but also more specific information on percent of error for densities below about 14 or 15 g/cc. Here is the equation:

K = 0.0089D^3 - 0.550D^2 + 12.5299D - 77.06

Thank you so much for whatever assistance you can provide.

Rhonda

These types of equations can only work for one alloying metal with the gold.  This one is only for copper.  It is also calibrated in Rhonda’s favor as it reads the karat level about 10% low.   I was able to determine this by using the Excel Solder Density worksheet that I developed. If the alloy was gold and lead, a 50% by weight gold (12 karat) would show as 15.7 karat with this equation and Rhonda would lose her shirt.

In response to my blog post on copper as the precursor to civilization, Harvey writes about pollution from early mining operations…..

Also interesting, early copper mining and processing led to the first examples of human induced environmental damage. There are documented sites in the Alps where copper processing by prehistoric peoples has left areas treeless to this day, due to heavy metal contamination.

Harvey

Mining and smelting were very tough businesses in ancient days.  In addition to pollution, many workers died from toxic fumes.

Dr. Ron

Folks,

Let’s see how Patty is doing with the latest crisis……

Upon hearing Claire Perkins inform her that Rob was in the hospital, Patty froze and her face looked ashen. She quickly recovered and got her cell phone out to call Rob’s mother.

“Mom, what has happened to Rob?” Patty said, her voice quavering a little.

“He hurt his back at the gym, he can hardly walk. He collapsed under a heavy barbell. His head was injured too. He was unconscious for five minutes. I’m almost at the hospital now,” Rob’s mother, Hilde Gunther replied.

“I’ll see you there,” Patty said.

Both Sam and Mike insisted that someone take her to the hospital, but Patty refused. 

Patty looked at her watch, it was 9AM. Rob was working a “swing shift” for six weeks and didn’t have to go into work until 10AM, so he went to the gym from 7:30 to 9AM most days. Patty had been teasing Rob that his workouts were getting too vigorous. She knew he was trying to snatch over 250 pounds as he was in a friendly competition with one of his friends, Fred, to see who would be the first to accomplish this significant feat. She wondered if this goal led to his accident.

The drive seemed to take forever, but soon she was at his emergency room bed. Rob was awake but his face was black and blue.  Patty didn’t notice her mother-in-law, as she quickly ran to Rob's side.

“Rob, what happened?” Patty cried.

“The good news is, I snatched 250!” he chuckled, which caused him to grimace in pain. “It was 260 pounds that was my downfall, I collapsed under the weight,” Rob went on.

“How bad are your injures?” Patty asked, a little frustrated with Rob’s levity.

“My back hurts so much, I can hardly walk, my face just looks bad. I’m going for an MRI in a few minutes, they’re worried I might have a slipped disk,” Rob answered, becoming much more serious.

Just then an MRI tech came.

“Well Mr. Gunther, we are going to squeeze you in, so I need to put you ‘On Deck’ for an MRI that opens up. Realistically, it could be two or three hours,” the tech commented.

Both Patty and his mother kissed Rob on the part of his head that wasn’t black and blue as he left. After Rob was taken away, Patty chatted with her mother-in-law for about 30 minutes.

Even though to some people it would seem strange, Patty had a way of compartmentalizing things, she knew she could not help Rob, except to pray for him which she had already done. So, she decided to do some work on her laptop. Fortunately the hospital had WiFi.

Patty had some unfinished business from what she learned on her trip investigating NMAC/I/O. She wrote an email to the GMs of the sites that were using that cheaper solder paste that had the response to pause problems or that required kneading before being used, suggesting that they change to one of two corporate approved pastes that didn’t have these issues. She also wrote a note to the people that were using a full wavesoldering process for a PWB that had only two through-hole components, solder preforms should be used with the reflow process for a PWB like this she told them.

As Patty finished the emails she needed to send, she observed the activities of the MRI section of the hospital where she was waiting for Rob. It occurred to her that this was a process just like assembling electronics. Instead of stencil printers and component placement machines, there was an MRI machine. There were techs that ran the MRI machines just like there were operators. The nurses were like the process engineers, and there were some medical doctors that were like the mangers and execs at her company. Instead of producing electronics, the MRI section was producing MRI scans. There was really little difference.

Patty got curious and she decided to ask the scheduling assistant a few questions.

“Excuse me, my husband is getting an MRI and I have a few questions,” she asked Sara Carter the assistant.

“Sure,” Sarah said, “go ahead.”

“About how much does an MRI scan cost?” Patty asked.

“It varies depending on the extent of the scans needed, but $3,000 is a good estimate,” Sarah responded.

Patty asked more questions and learned that there were 5 MRI units and she assessed the headcount and floorspace needed to support the MRI unit. She also found out that each of the 5 MRI units averaged 9 scans per day. It then occurred to her that she could use ProfitPro to estimate the cost of a typical MRI scan. Under The Professor’s tutelage she has gotten quite good at estimating burden labor rates, etc, which would be needed for the calculation. She got her laptop out and using ProftiPro, in a few minutes estimated that the hospital’s cost of an MRI scan should be only $390!

“Why does it cost our insurance $3,000?” she thought.

It then occurred to her that her good friend from her days at Tech, Emily Chen, was a radiology resident at the hospital. She decided to send her a note and, in addition to telling her about Rob, ask about the MRI scan cost. 

After sending the email, she asked her mother-in-law if she would like to get a cup of coffee. In a short time, they were heading to the hospital cafeteria. Before they left, they found out that Rob was just starting his 45 minute MRI scan. 

Fifteen minutes later they returned, and Patty was surprised that she had already received an answer from Emily.

“Patty, I’m so sorry to hear about Rob. You probably won’t hear the official news on his MRI until tomorrow, but I will take a look at it and call you later today. BTW, my boyfriend works in the finance department here. I’ll find out about the cost. But, your numbers sound way off.”

Twenty minutes later Rob was finished. His doctor had given him some pain killers and muscle relaxers, so Rob was a little more comfortable, but the doctor wanted Rob to stay overnight for observation. Rob soon fell asleep from the medication. Patty decided to stay with Rob and by 4PM, she asked her mother-in-law if she could pick the boys up from day care.

At 4:30 PM another email arrived from Emily.

“Patty, good news. I looked at Rob’s MRI scan and it looks fine. He probably just severely strained a muscle. He’ll be as good as new in a month or so” Emily’s note began. Emily’s note went on, ”My boyfriend looked up the cost for the hospital to run an MRI scan. You were close, it costs $410. Neither of us can believe it. Where does the extra $2600 go?”

Dr. Ron note: I have done some investigations into MRI scan costs. As surprising as it sounds, these numbers are about right, the base cost for a hospital to perform an MRI scan is in the $400 range, but they have to charge $3,000 to break even. Considering that many hospitals are non profits and are losing money adds to the confusion.  At this point, I don’t claim to understand the cost structure of running a hospital, but one would think that one of the most critical questions in the current healthcare cost crisis in the United States, would be to understand why $3,000 must be charged for a $400 procedure to break even.  

The image is of Yegeny Chigishyov snatching about 450 pounds in the 2008 Olympics.

November 21st is the birthday of one of the men who co-discovered indium metal back in 1863.  Hieronymus Theodor Richter, along with F. Reich, made the discovery but it wasn't until 1924 to 1933 when Daniel Grey created a process to extract and refine indium that the commercial possibilities began to be explored. Their work led to the founding of The Indium Corporation in 1934.

Dr. William S. Murray, the founder of Indium Corporation, received the first patent to process indium in 1926.  The first commercial quantities of indium were discovered in Kingman, AZ in the same year.

The importance of indium metal grew through the rest of the 20th century, in conjunction with each new technology discovered.  Whether it was its malleability (even at cryogenic temperatures), its low melting point, its electrical conductivity, or its thermal properties, indium has become a standard in almost every industry for unique as well as common place applications. 

Today indium is used in a variety of applications: as a low melting solder in electronics applications, as a coating for touch screens, LCDs, and solar panels, as well as a thermal interface in many of our heat-producing electronics.

Although indium has a relatively short history, particularly from a commercial standpoint, new discoveries and applications for this unique metal continue to be made.

For more information on indium, go to www.indiumsolders.com.

剛剛看見一篇挺有意思的文章，標題是“全球LED三大陣營比較及廠商排名”，不禁讓我想起最近確實有更多的客戶問起關於LED的封裝和散熱材料了。

除了傳統用的焊接材料(soldering material)外，最近的好些客戶都問到關於散熱方面的材料，主要是針對高功率，高亮度(High Power High Brightness HP HB)的LED應用。傳統的硅膠(thermal grease)因爲其化學特性，導熱率比較低，而且容易在界面中移動，不能使接觸面都均勻導熱。最重要的是，隨著時間的流逝，硅膠的導熱性能會越來越差，所以現在越來越多的客戶在尋求更好的解決方案。

Indium公司的散熱界面材料（Thermal Interface Materials TIM）， 都是使用導熱係數比較高的金屬/合金，加上特殊的表面處理后，能更完整均勻地接觸導熱界面。而且即使隨著時間流逝，金屬的TIM的導熱性能也能保持穩定。

Indium公司還特意推出的網上的購買，為各種學校/機構/小量使用購買者提供方便。

Cheers!

Image: Indium Corporation.

Those of you have been watching this blog for a while will know that I’ve been keeping tabs on the status of the European ELV (End-of-life vehicle) legislation on lead (Pb), mercury (Hg), cadmium (Cd) and hexavalent chromium (Cr^VI). It’s been both galling and heartening at the same time, to find that when I Google “elv legislation”, this (my) blog keeps coming up as one of the top 10 sources on the subject. OK: enough of the bloggy, solipsistic prevarication...

My friend, Geert Willems of IMEC late last week let me know that the EC (European Commission) had given its final decisions on Annex II ("the exceptions"), and pretty much adopted the recommendations of the Öko Institute from their 127 page report of September last year (2009). I have to say my hat is off to Dr. Otmar Deubzer of IZM and Stéphanie Zangl of Öko for the very thorough and logical background to this legislation.

The decisions that affect those of us in the semiconductor (flip-chip) and power semiconductor arena are primarily the ones on lead (Pb) in solders, that were formerly covered by section 8.a/ and 8.b/ of the old, outdated Annex II to Directive 2000/53/EC, and are now covered by this new legislation.

A quick visual summary of the legislation relevant to lead (Pb) in electrical interconnects is given below, and please consult the original document for confirmation, as I may have missed some subtlety of the legalese in my quest for brevity. Also, frankly, subsection 8 (b) led to some Transatlantic confusion over whether finishes on pin connectors and PWB's were covered(?), but I think the below is correct:




Refer to the table below for the timeline for of each subsection/exception:



Note that the last review of exemptions was carried out in 2009, with potential effect by 1/1/2011. This implies that the legislative hammer will potentially fall on each of those usages slated for future review on January 1st two years after the review year. Lead (Pb) for most electronics attach usages of interest to those of us in semiconductor and power semiconductor packaging may therefore be "legislated out" by 1/1/2016.

Basically, the use of Pb-containing solders in solder paste, die-attach paste, die-attach wire, solder preforms, and thermal interface materials (TIMs) in automotive electronics assembly is safe for now, and changes will not be forced on the automotive electronics assembly industry at a time when even current manufacturing practises may be leading to still-unresolved safety incidents.

Cheers!  Andy

I caught up with Alan Rae after a recent IWLPC committee meeting, where he jokingly asked me to, “Stop asking important questions” - LOL! He was kind enough to give me a few moments of his time to share his wit and wisdom, and answer some technology questions that, yes, I thought were kind of important…

[Andy Mackie] You’re increasingly being seen as “Dr Nano” by the electronics industry – how did you arrive as the focus of so much of this technology?

[Alan Rae] At the start of my career I was in the structural ceramics business. In the days of “ceramic fever” in the 1980’s the mantra was sub-micron and monosize (monodisperse) for lower temperature processing and better properties. It worked. Then at TAM Ceramics we made “sub-micron” barium titanate and other ceramic materials but we didn’t call it nano then. When I was at Cookson Electronics in the early 2000’s we started to see nanotechnology emerging from the woodwork with people saying the same about nanomaterials for the electronics industry. Then I joined NanoDynamics in 2004 and realized the scope and potential, ranging from semiconductors to touch screens to printable electronics, to LED lighting, to solar power, to materials such as nano solders, dielectrics, conductors…the list is growing but the leitmotiv is the same – small, monosize, tightly-controlled. 

[Andy Mackie] OK, so Nanotechnology has been a buzzword for quite a while – is there a clear definition yet, and what current uses are there for nanotechnologies that may not be immediately obvious?

[Alan Rae] Well, the definition has been really tough to derive – ISO TC 229 “Nanotechnologies” came up with a definition that one dimension of a particle, needle or plate should be less than 100nm but it’s really tough to define…should all particles be less than 100 nm? 50%? Any? And should it be exactly 100nm? There are a lot of opinions. The Woodrow Wilson Institute lists over 800 consumer products containing nanomaterials on the market now – industrially the products range from semiconductors, to fillers in packaging materials and underfills, to antimicrobial and self-cleaning coatings for phones. Solar panels, especially thin film ones, depend on nanomaterials in their manufacture.

[Andy Mackie] What is in the pipeline for nanotech electronics and semiconductor interconnect materials? I know that nanosolders are starting to gain ground in some areas – what else is upcoming?

[Alan Rae] Much of the work in nano metals is being done by universities and small companies – for example my small company is working with Purdue and the Air Force to develop a novel solder technology – but commercialization will come by partnering with established companies like Indium Corporation, who have the distribution and technical support so that customers will be comfortable with a new material. Cost and reliability are king. Indium is already in the reactive nano foil business; there are existing and near-term applications for silver, silver-coated copper, alumina coated boron nitride and their combinations in adhesives, shielding materials and thermal interface materials.

[Andy Mackie] Several years ago, quantum dots were being promulgated for tunable band-gap detectors and quantum computers. How close are quantum dots to seeing real uses, and what else is on the horizon?

[Alan Rae] Quantum dots are unique and have great potential in medical imaging and as frequency shifters for LEDs. The markets haven’t developed yet because of the cost and because some of the best dots are cadmium (toxic metal) based. I’m working with a group at University of Buffalo which has a silicon quantum dot process that looks like a promising alternative. Quantum dots will have their time…but not just yet. In terms of new developments – they range from core shell and modulated structures for thermoelectric to replacing indium tin oxide with carbon nanotubes or graphene. The US National Nanotechnology Initiative tracked $1.6 billion in Government spending (check out www.nano.gov) in the last year at Universities and small businesses and NSF has set up centers of excellence at Cornell and other great universities that are really working hard to translate science into technology so we can make practical products.

=======

Alan, many thanks for your time, and for sharing your insights with us.

Cheers!  Andy

Tommy: NanoFoil...it's kind of like aluminum foil in thickness and look, but a little stiffer and when you put energy into it, like a spark, it heats to 1500 degrees Celsius (which is hotter than lava in a volcano) for less than a millisecond!

My engineering and non-engineering friends display puzzled looks, you know the ones you get when you start explaining that your favorite sport is water polo and it has nothing to do with horses...ok the look you have right now

Engineering Friend: Umm so what can I blow up with it?

Non-Engineering Friend: So wait, if it's that fast and that hot, I bet you could make really fast grilled cheese. Have you called Healthy Choice yet?

Good Grief!

Al + Ni -> AlNi  (You didn't know there'd be chemistry involved did you?)

I just sat down to talk to Tommy Acchione (pronounced “akki-OWN”) Applications Engineer with Indium Corporation’s  new product line, Reactive NanoTechnologies’ (RNT) NanoFoil^®, about the technology, and its offerings into the semiconductor, power semiconductor assembly, LED and display assembly industries.

[ACM] First of all: welcome to Indium Corporation! Can you tell us, in just a few words, what the basis of the RNT Technology is?

[Tommy Acchione] NanoFoil^® technology is a thin metal sheet (“foil”) made up from alternating ultrathin layers of aluminum and nickel (Al and Ni). The reaction between these two metals is stoichiometrically very simple:

            Al+2Ni -> AlNi₂

And extremely exothermic (heat-generating). This reaction (see picture) is started by a very localized heat or other high-energy source, such as a 9V battery or even a laser beam. For a fraction of a second, the alternating thousands of sandwiched layers reach temperatures as high as 2000degC, and this isotropic heatwave radiates away from the initial hot-spot through the foil at speeds of about 5-8meters/second.

Just banging two lumps of Ni and Al together will never initiate a reaction this intense, as the two large pieces of metal act as very effective heat sinks, but by layering the metals together, the heat-generating reaction propagates by allowing the adjacent layers of Ni and Al to rapidly interdiffuse, so giving out more heat, causing the nearby layers of Ni and Al to interdiffuse and so on.

[ACM] How are these materials manufactured?

[Tommy Acchione] First, we pull a high vacuum, equivalent to those vacuums found in outer space, then we sequentially deposit the alternating layers by a sputtering process onto a specially-made metal block.

For a bonding material, a layer of a specialized brazing material is initially deposited onto the metal block, then the Al and Ni are put down, then a final capping layer of braze is deposited. The initial brazing layer both enhances subsequent bonding and also helps with easy removal from the surface of the metal block.

[ACM] I understand that the uses of these materials are expanding all the time. Can you give some examples that you can talk about?

Well, as you know we have about 30 patents on this technology and 35 outstanding patent applications, but I still have to be careful talking about newer applications, which are emerging all the time.

The biggest uses are in sputtering target manufacture (which is a little ironic, since that is how they are made!); Component mounting; and what we can call “reaction initiation”, or “energetics” - things requiring an instantaneous heat-source.

Sputtering Targets: For sputtering targets of non-refractory metals, standard indium or diffusion may be the preferred method. For most refractory metals and ceramics, solder wetting and CTE mismatches make bonding with standard processes difficult. NanoFoil^® allows for these materials to be bonded at room temperature, thus removing any CTE mismatches during bonding or subsequent cooling processes.

However, as targets get larger for flat panel displays (and we are seeing needs for up to 3m x .4m targets with higher generation depositing), indium starts to become too weak to take the weight of the indium-tin oxide (ITO or InTO) target itself, and only the strength of a NanoBond^® is sufficient to hold the target in place. Another key factor is that a manual bond of a large target to its backing plate starts to become simply physically unwieldy for an operator, as its size and weight increase. NanoFoil^® becomes the elegant and simple solution here.

Component Bonding: One major market that we are seeing is in component bonding. I can’t talk too much about this, but for high-brightness LED’s (HB-LED’s) and photovoltaic concentrators (CPVs) there is a growing demand for a high-temperature stable, thermally-conductive flux-less bonding material able to provide low junction temperatures over the lifetime of the device.

Energetics: Here we are talking about fuses and timed devices, with specially-shaped initiators that take advantage of the ignition properties and the reaction rate and energy produced by the NanoFoil^®.

[ACM] Tommy: very interesting! Many thanks for your time.

Picture of a Used Car: Because you're Good People

Drilling into what's happening with European ELV legislation and how US stakeholders are reacting has been an interesting trip. Nancy Malo of the US-led Automotive Industry Action Group AIAG put me in touch with Lynn Smith of the US Council for Automotive Research USCAR who recently heard about the uses of indium in Pb-free solder for use on glasses (8b of Directive 2000/53/EC annex II). I hope to hear from Lynn soon.

Meanwhile, earlier this year, the European Union contracted Öko-Institut, who themselves contracted Kevin Kennedy and Associates Inc (KKAI), who themselves requested our old friend, renowned author and consultant Dr Jennie Hwang to report on Pb-free (lead-free) solder alternatives for electronics attach on glass.

I still haven't found anyone who can help me out with the latest on Annex II 8a (all non-glass attached electronics) and frankly keeping the above straight has given me a headache and I need a Stella before I broach the subject again. All help and corrections appreciated.

Cheers!   Andy

The junction temperature in an LED (the p-n junction temperature) is most critical to consider for LED cooling. If this temperature rises above the prescribed level recommended by the LED manufacturer, the lifetime of the LED as well as its intensity and color may be affected.   

As with most electronic systems, the LED assembly location where the highest temperatures are reached is the junction temperature. Many thermal management materials may be used to control this temperature, such as heat pipes or metal core boards, but each of these carry their own thermal resistance. An optimal cooling design is one which includes the lowest sum of thermal resistances for the system.

Ideally, no one thermal management material will be a bottle neck for thermal dissipation, however the materials closest to the heat source are most critical. High performance thermal management materials should be considered here. If the highest resistance measured is at the interface junction, the junction temperature will be raised more than if the bottleneck in resistance were at any other location. 

There are various types of LED assemblies, but a typical high power LED is depicted here. In this type of assembly, implementation of high performance thermal management materials would be most critical in the die attach material, heat sink slug, and solder as these are closest to the heat source and will have the greatest impact on dissipating the heat away from the p-n junction.

Customer Visiting (Sales Calls)

上周，Amanda和我去波士顿(Boston)地区拜访了一些使用我们的热管理材料(Thermal Interface Materials---TIM) 在LED应用领域的客户 (LED Applications)。客户们都十分热情友善地接待了我们。 

•   了解客户最新的应用领域，能引导我们最新产品的研发
•    倾听客户对他们各自市场的描述，有助于我们把握各个应用领域市场的动态，并有助我们公司内部各种策略(strategy)的制定
•   留心客户提到的各个竞争对手的信息，哈哈，"知己知彼，百战百胜！"

 

最近在做我们Indium公司的产品在LED应用上的市场调研分析。哈哈，又一次大长见识了。  

对了，公司的同事Amanda有她个人关于TIM的blog: http://www.indium.com/blogs/TIM-Blog/index.php

Pic: Jordan Ross with Indium Corp.

With the Strategies in Light show just behind us, I thought it a good time to discuss a little further the use of metal thermal interface materials in high power LEDs.

I have not met a designer yet who doesn't worry about thermal issues in their LED packaging. Everyone has a thermal interface material solution, but not everyone has opted for metal interfaces – just yet.

I wondered if one reason for this might be simply a lack of understanding of the metal thermal interface materials or their design possibilities. From my experience, designers of high power LEDs often choose metal thermal interface materials which are stiff, and resistant to oxidation. The solder is supplied as preform discs or rectangles which the manufacturers use to solder their chip to a Heat Sink Slug. To minimize the thermal resistance of their LED package (and maximize the thermal power that can be dissipated), the preforms used in this application are very thin. They can be manufactured as thin as 0.0005" in thickness. At this point, the resistance of the heat sink which the chip is soldered to has a higher resistance than the metal thermal interface material, making the interface resistance a negligible concern if it has been soldered adequately.

One reason that the high temperature preforms are the material of choice and not other alloys in other forms such as solder paste is that the preforms can be soldered without using a flux. For assistance with choosing the best preform for your LED application, please e-mail me!

Image Courtesy of http://www.lumileds.com/products/line.cfm?lineId=18

Simply stated, YES.

In effort to stay on top of industry technology I spend as much time as I can afford attempting to learn about the current thermal solutions being used on various devices. LEDs have long been controversial regarding their requirement (or non-requirement as the case may be) for high efficiency thermal solutions. Although I have spoken to many LED designers in the past who were adamant about and enforced the use of metal thermal interface materials on their LEDs, after sitting through the recent Photonics West trade show, I have to admit now that these engineers are no longer the minority. LED packages are now commonly including Aluminum or Copper heat sink slugs which transfer heat directly from the chip to a printed circuit board. Taking this a step further, LED design engineers are now even considering the use of active cooling which could reduce their thermal resistances by another factor of ten. None of this makes sense though if the design skimps on the die bonding material. Low power LEDs regularly use silver-filled epoxies, but most high power devices are now using solders due to their low thermal resistances.

The word Creep is not something an engineer wants to hear when designing a metal TIM into their Laser Diode Stack up. But is creep really an issue, and do we really understand creep when talking about indium? The answer is relative. Attached you will find a stack up diagram I made of a typical Diode stack. This could be a laser application or an LED application. The Die Attach layer is usually a high temp solder or a silver filled epoxy. Can indium be used at this level? The answer is yes, if and only if the temperature of the junction is far away from the melting point of pure indium, which is 156C. Soft solders are just that, they are soft, and in die attach some customers have used indium at this level, but it is not nearly as common as other higher temp solders like AuSn, Sac alloys or SnAg. Silver filled epoxies are really getting better and better but there are some issues with their conductivity and their process. When using a solder as a die attach the die itself can float or move during reflow. In this case some kind of mechanism can be used to hold it in place during reflow to ensure that its alignment is perfect. So to answer the question, solder can be used as die attach in this application, but the alloy chosen will really determine how effective it is and how reliable it is.

In the case of TIM2 (thermal interface material level 2), there are a few more considerations here. Let us first assume that we are going to reflow at this level. Copper at the heat-spreader level will not be a problem, but Nickel at the spreader/sink level will be a problem. Aluminum will also be a problem here. The problem is that these materials are hard to solder to, but it can be done. A high activity flux such as Indiums RSA or Flux number 3 can be used to break the oxide layer that will be present. However a layer of gold on the surface will help assure soldering will be effective. Indium recommends not to exceed 50 micro inches of gold, and recommends that the thinner the better, usually 10 micro inches will do it. Indium the element will actually dissolve the gold or other wise known that the gold will diffuse into the indium. During soldering an Indium/Gold inter-metallic will form. This is a brittle layer and if too much gold is used can induce reliability issues and cracking of the joint. So back to our original question; why would you use indium here and can you use indium here? This is the most common area where creep of indium can be an issue. Creep can be acceptable however. Indium will not creep to the degree that it will come out like pump out or like play-doe. The degree of creep is related to the pressure that is put on it, ie: CTE movement or direct pressure from clamping, as well as the temperature that the interface sees. If the junction temp is less that 20 degrees of the melting point and some movement is allowable, such as in an LED application, this is acceptable. However in a laser application, Indium Corporation usually advises that we do not go with pure indium at this level and rather choose an alloy such as Indium Silver or Tin Silver. The more Silver you add to the alloy the harder the material will become. Consider this. Pure Indium has a conductivity of 86W/mk, mp=156C Eutectic and a tinsel strength of 273 psi. Add 3% silver and the MP goes to 143C Eutectic, the conductivity goes down to 73 but the tinsel goes up to 800 psi. Better yet, add 10% silver and the MP is now plastic from 143 to 237, conductivity goes down to 67 but the tensil goes up to 1650 psi. Then consider SnAg which has a melt point of 221C a conductivity of 33W/mk and a tensile of 5800 psi. What is the best for your application? The answer lies in what is acceptable to you, if CTE is an issue go with less silver, if temp is an issue look at no indium, if conductivity is an issue look at high indium.

When considering a compressible metal at the TIM2 level, the issue now at had is how much pressure you have, the temp of the junction, and the planarity of the surface. Obviously with a compressible interface you no longer need any gold, and there will be no issues with indium in direct contact with nickel. However, copper and indium can form an inter-metallic over time; however the oxide layer on the copper usually keeps this from happening. In fact in our thermal lab we only saw this happening when we actually baked the modules for over 1000 hours at 125C+. Even then the phenomena was nominal. If you do not believe you will rework the interface 4-5 years after its construction I would say that this will not be an issue, actually it will improve the thermal performance and reliability. Rework within 1-2 years will not be a problem. Many of our customers already use indium at this level as a compressible interface but few are aware that they can actually improve the performance if they convert to a Heat-Spring ™. The Heat-Spring is a patented process that allows us to decrease the contact resistance of the metal if the pressure is at least 50 psi. This allows the stack up to use a thinner bond line thickness and improve the thermal performance of the Metal Thermal Interface. So what about creep at this interface? Once again altering the alloy can eliminate the chance of this happening, but converting from a standard indium flat foil to a Heat-Spring will further decrease these chances because your bond line is usually significantly less if you use a Heat-Spring. (On average about .003").

In Summary, the things to consider when using a Metal Thermal Interface or Die Attach Solder are as follows:
• What is the working temp of the interface?
• Is this temperature too close to the melting point of the Metal Thermal Interface?
• Can the device handle the reflow temperature of a higher temp solder such as Gold Tin or Tin Silver
• Is the thermal performance of the interface an issue so if you change the conductivity of the metal thermal interface by decreasing the indium content, you detriment the conductivity of the entire stack up? For example going from 86w/mK to 67w/mK.
• Is creep really an issue? If your device can accept a small degree of movement there is no issue to use indium. If even a slight issue will cause a problem, suggest an Indium Alloy and not pure indium.

In the end, Indium Corporation is here to help you. Please see our web site for additional information including our e-list of alloys to help you choose the best Metal Thermal Interface Material.

Folks,

If there is anything that I hope my blog does is to generate useful debate about the happenings in the electronics industry to help us all prosper. I will also be the first to admit that I am still learning and appreciate all of your constructive feedback. So I encourage all to read the comments generated by this posting, but I would also like to respond to a few.

Comments:

John B writes:

Dr Lasky,

Thank you for the kind reference to our site. I believe you missed the point. Our site does not promise the "truth about RoHS" as you state it promises the "Truth about RoHS and lead free soldering" and uses the EPA environmental report as evidence that the replacements for leaded solders are a far higher environmental burden than the currently used product.

As far as recycling is concerned, yes recycling of any product is an onerous task leaded or unleaded - I believe the industry needs to address this with investments aimed at virtually "pollution free" recycling plants instead of the current practices of dumping container loads of electronics waste onto third world populations.

I will let the EPA report speak for itself.

The European Commissioner has publicly stated on 11 November 05 that "The Commission is aware that the substitution of lead in electrical and electronic equipment may have led companies to invest in new technologies with greater energy use. However, if companies can prove that the elimination or substitution of the banned substances via design changes or the use of alternative materials and components causes negative environmental, health and or consumer safety impacts, and that these outweigh the environmental benefits of ceasing to use the banned substances, an exemption can be granted. So far, only one company has asked for an exemption on these grounds. All the other exemption requests are based on either the lack of suitable substitutes or for cost reasons, which cannot be considered under the RoHS Directive."

RoHSUSA has provided support documentation for 9 of the current exemption requests based on the fact that the replacements are environmentally more damaging. It has submitted evidence of the EPA August 2005 report to suport this.

I agree, it will be an interesting year,

Kind regards,

John B

PS Give my best regards to Dr Lee

John, I need to study the EPA report you describe. Thanks for the insight! Dr. Ron

John M writes:

Thanks for the laugh! I can just see the TAC advisors looking over the exemption request and actually having to seriously debate allowing Swatch watches complete exemption. Even if the request came with a complimentary Swatch watch it is doubtful that this will be debated longer than it takes the TAC to read the request.

Cutting edge electronics technology companies has proven the ability, in general, to comply with the requirements and yet the "technological leaders" of watch designs can't? Would be curious to nkow the average lifespan of a Swatch watch to begin with... would hardly have time for a tin whisker to appear.

Too funny.

Cheers
John M

John M, I am with you, but John B asks us to look at the support he sent for the Swatch Exemption Application See below. Dr. Ron

John B writes:

Glad you are laughing Mr Mitchel,

I submitted further support for the SWATCH application to the EU yesterday. It is posted on their web site

You will note that JES00337.pdf in support of the SWATCH application is dealing specifically with tin whiskers. Hope you can stop giggling long enough to read it. Which I am sure the TAC will indeed do.

And thank you Doctor Ron for this excellent discussion forum.

John B

Kurt Z writes:

Dr. Lasky,
You stated that Motorolla has over 120 million lead-free cell phones. Have you really determined the truth of this? My investigation has found most cell-phone companies stating RoHS compliance BUT, when you press for details, you find that they are claiming compliance by EXEMPTION not by exclusion of the banned material.

Kurt, this is one where I have a lot of first hand knowledge. Motorola has been shipping at first lead-free (and sometimes even mixed) technology cellphones to the field since September 2001. They are now fully RoHS compliant. Something like 130 million cellphones are out there with equal or better quality and reliability than leaded product. The reason I am so familiar with this story is that Motorola has used Indium lead-free solder paste from the begining. Dr. Ron

Oliver writes:

Dr. Ron,

are we really living in a world where 5 year operation is "very long term reliability"?

I don't think so. Most EEE I'm using at home or in the lab is older than 5 jears. Even the PC I'm using to write this is nearly that old. And I expect my (old, SnPb soldered) Omega watch to last much more than 20 years.

I'm designing electronics for industrial control where our customers expect more than 10 years service life. Even if we continue using SnPb solder, tin whiskers growing out of Sn plated leadframes might make this harder with RoHS.

So shall RoHS force us to arrive at the "don't last longer than 5 year" level?

Oliver

Oliver, I am in my office surrounded by electronics that are all < 3 years old. But in sympathy to your point, my belief is that with proper processing and correct design, that longterm reliability with leadfree assembly will be achieved. However, even for products where such care is not taken, I think there will be few cases of significant reliability issues. Dr. Ron