Thermal Interface Material

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.

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

Cheers!  Andy

上週末看中央二台新聞“每週讀報”，有這樣一個話題引起了我的注意“今天，你低碳了嗎？”

從什麽時候開始，我也開始接觸“低碳經濟 Low-Carbon Economic” “低碳生活 Low-Carbon Life”“碳足跡Carbon Footprint” 等生活中和工作上的常聽見的詞語。甚至，《貨幣戰爭》的作者宋鴻兵還提出了以“碳排放量Carbon Emission”來作爲衡量一個國家GDP增長的指標之一。  就像“每週讀報”中號召的“節水，節電，節油，節氣，節能”，我也乘機來分享一下工作上和生活中的點滴響應這個號召的小事。

1.      LED燈---節電。 我們的生活中處處都有LED燈的身影。比如2008年北京奧運璀璨的鳥巢和水立方，就是大量使用了高亮度的LED燈 （High Bright LED---HB LED）。 現在各地政府給街道從白熾燈換到LED燈，據説如果全國1/3的城市都換上LED燈，大量節省的能源是無法估量的。 而LED組裝製造的一個重要瓶頸，就是解決散熱問題。 黨普通的化學材料無法對LED燈起到很好的熱傳導性能(thermal efficiency)，金屬散熱材料，從其物理特性來説，就是下一個首選的散熱材料了。 希望使用LED燈，能緩解各地方的能源緊缺，我們美麗的杭州西湖，也不用常常関燈停電了。 更多關於金屬散熱界面材料信息，請看這裡。 (Thermal Interface Materials---TIM)

2.      汽車---節油。 剛剛在美國底特律舉行完畢的汽車展，混合動力車(hybrid car)，無疑是一個最熱門的話題。 如果日後買車，除了關注你汽車的價錢，也請關注它是不是混合動力車，以及每升汽油的公里數哦。(在美國，是每加侖汽油的英里數，Miles Per Gallon--MPG)

3.      太陽能---節能。 我還想在以前的文章裏面，寫過許多關於太陽能光復電池製造，或是組裝加工的文章了。哈哈，我還是少賣一點“廣告”了。 一句話，Indium公司提供太陽能光復薄膜電池材料，以及太陽能電池板組裝焊接材料。

不積跬步，無以至千里。今天，你低碳了嗎？ 

PS:  前段時間我看2010 CES的新產品介紹，微軟公司居然說已經和好幾個汽車廠商在合作，不久的將來在汽車内部能安裝軟件系統，告訴你汽車每時每刻的carbon emission, 或是累積的排放！ WOW！！！我只知道，汽車内部的電腦系統用的電路板，組裝要求十分嚴格，通常要求100%的通過率。所以我們也必須提供十分高可靠性的焊接材料！　

At some point we have all had experiences which convinced us that metals have a high thermal conductivity. It may have been the hot spoon you left in your coffee after stirring in a little cream and sugar, or the hot door handle you grabbed on a simmering hot summer day when climbing into that now-vintage car of yours. In fact, the high thermal conductivity of metal can even account for the ability to get your tongue stuck to a metal pole in the cold of winter (or the metal screen door while waiting for the school bus as was my childhood experience). We generally understand the phenomena of metals to have a high thermal conductivity to be true, however what is the basic science behind the high thermal conductivity of metal?

In the July/August issue of Advancing Microelectronics, Dave Saums, Bob Jarrett, Andy Mackie, and Jordan Ross published an article titled, “Thermal Management Materials Choices for Power Semiconductors,” which begins to explain this.   


 

The article describes metal generically as positive ions within a “communal sea of their valence electrons”, together providing a net neutral charge.  The image above depicts this arrangement.   A metal is unique because unlike non-metallics which are viewed as highly organized lattices, valence electrons of metal atoms are not strongly held by the nucleus and are highly mobile. These mobile electrons transfer electric charge as well as heat across the metallic structure.  This freedom of the valence electrons accounts for the high thermal conductivity in metals.  At ambient temperatures, metals are attributed with high conductance, however an additional rise in thermal conductivity is found as environmental temperatures rise.  This activity can be explained using the principles explained in the Wiedemann-Franz Law.  

In electronics packaging, there are many materials to choose from which will provide various thermal dissipation outcomes. Metallic materials are generally preferred for high power devices due to their high thermal conductivity, lending them for adoption in heat sinks, heat spreaders, baseplates, and even thermal interface materials as Indium is most familiar with.  

Generically, a phase change material is one which will store or release energy when it changes phase from solid to liquid or liquid to solid. According to this generic classification, there are 4 general categories of phase change materials.

• Salt Hydrates such as Sodium Sulfate, Calcium Chloride, Sodium Acetates,etc
• Eutectic Salts
• Paraffins
• Non-Paraffin Organics

These phase change material categories are not all-encompassing, however. Other materials such as metals, eutectic or not, are used as phase change materials for their thermal energy storage and removal abilities. 

Nearly all soft solders classify as phase change materials according to their melting temperature. According to Maurice J. Marongiu from MJM Engineering, who conducted a webinar on phase change materials, the melting temperature of a typical phase change material is between 0-250ºC. Solders officially may melt at higher temperatures, such as the AuGe eutectic alloy which melts at 356ºC, however the majority of solders used melt below 250ºC. 

Phase change materials are a common occurrence in the world of thermal interfaces for electronics. Here, tighter commonalities between phase change materials can be found. For instance, the phase change temperature for these thermal interface materials is within the range of a common TIM junction temperature, which is typically lower than 100ºC. For this reason, when we consider a metal interface to be a phase change material in this industry, it is an alloy or material which changes phase below 100ºC.

When implementing a metal or non-metal phase change material into a thermal interface, there are some design considerations to be made:

• Phase change materials are applied as solid pads. At room temperature they are firm and available with specific dimensions which make them easy to handle. Consistent application should be inherent.

• Phase change materials each change phase at a unique temperature. The appropriate phase change material engineered for an application will have a phase change temperature reached within the normal operating cycle of the device.

• Phase change materials are designed to turn liquid in operation. The liquid phase of these materials will have a distinct viscosity. Depending on the material, clamping pressure and assembly orientation, the molten material may leak. Proper precautions should be taken to prevent material leakage, especially toward active electrical components if the phase change material is electrically conductive.          

• When reservoirs are created to contain a phase change material, these reservoirs must accommodate the liquid phase of the phase change material as well as the solid phase. As a phase change material changes from solid to liquid there is an increase in the material volume. If the phase change material expands and fractures the reservoir, this will lead to leaks and the eventual failure of the electronic device as the thermal interface becomes backfilled with air.  

Materials to be used in packaging of high power semiconductor devices are often chosen by their coefficient of thermal expansion, or CTE. For instance, substrates such as AlSiC, Molybdenum, and Tungsten are chosen to mimic the coefficient of thermal expansion (CTE) values of the materials they will be attached to so as they expand and contract, the substrates do so in tandem, minimizing the mechanical stresses at the interfaces between these areas, or their CTE mismatch.

The coefficient of thermal expansion (CTE) of indium does not match many materials, yet it is chosen commonly as a solder thermal interface material between substrates with as dissimilar substrate properties as silicon and copper. 

How can indium bond together silicon with a CTE of 2.6PPM/ºC and copper with a CTE of approximately 17 PPM/ºC, then undergo years of thermal and power cycling, and not show degradation of thermal performance?

The answer is in the strength and malleability of indium. Indium is the softest metal which is stable in air. Although the CTE of indium is 29 PPM/ºC, the tensile strength of indium is 273PSI, which is very soft, and the shear strength of indium is 890PSI, which is significantly higher. In an application where indium is soldered to a back-side metallized die and a copper integrated heat spreader, there is significant CTE mismatch. 

However, assuming the interfaces of these solder joints is sound with minimal voids, the bulk indium will bend and stretch along with the contraction of substrates and will not crack.        

Break down of a radio frequency

终于，终于发3G牌照了。基站设备厂商们拿到电信，移动，和新联通的订单后，也已经在紧锣密鼓地建设新基站或是给旧的基站更新换代了。

在3G基站的建设中，会用到Radio Frequency (RF)射频设备。一般来说，每一个RF中有两个功率放大器Power Amplifier(PA)。无论是PA的制造，还是RF的组装，Indium公司都提供了系列可靠性能强的精确焊接材料。比如说PA的制造，我们有AuSn预成型焊片(preform)或是焊带(ribbon)；RF的组装，我们可以提供有铅无铅的焊锡膏(solder paste)，含驻焊剂的焊片(flux coated preform)，还有散热界面材料Thermal Interface Material (TIM).

好友王说，移动在全国大概要建40万台3G基站；从家人朋友那里也听闻到电信，新联通的一些数据，粗略估计，分别建(或是硬件更新)30万台基站吧（如果我的估算偏差大了，欢迎随时联系azhang@indium.com ）。那么，全国就要建100万台3G基站了。平均每一个基站有6个RF，每一个RF有两个PA，每一个PA的制造/封装，可以用到4个含驻焊剂的焊片，每一个RF可以用到一个散热界面材料(TIM)，我们把这些乘的乘，加的加，也大概可以估算出中国的3G基站建设用到多少焊接材料了。

Cheers!

PS: 1. 好久没有回中国了，这次回来亲身感受到各方面的"冲击"，其中热门的3G应该是最大的冲击之一吧。 2. 好久没有正面接触曾经的老本行通信了；这些年来学习和工作的内容，与通信直接相联系的较少。这次当被一个同事问起3G的一些知识时，我哑口无言了…当时倍感对不起北邮，对不起亲戚朋友们。哈哈，看来我这"烂船"也要修修补补了，不然连"三斤铁"都没有了。

Pic: From Jordan Ross with Indium Corp.

Thermal Profile of Alabama. Source: Worldbook Encyclopedia

The program details are coming together for this event coming up in Huntsville, Alabama and the presentations are going to include some pertinent information on thermal interface materials for engineers or scientists working on thermal issues at all levels. There will be some discussions about the general principles behind thermal interface materials, a discussion about the characteristics of the various materials currently available, and a presentation of test data for the high performance material options.

Along with my fellow engineer, Eric Bastow, I will be hosting a technical symposium on TIM materials, discussing their purpose, commercial options, performance data of each, and high end applications where these materials are of critical importance. 

This will be hosted in Huntsville, AL on May 20. Please click to register. 

“Comparing Package Bond Thermal Performance” by John Parry (Mentor Graphics Corp.) is the final article of Advanced Packaging week.  Speaking for our thermal team, I’m sure they would love to see this system used to model CPUs and GPUs with and without Indium thermal interface materials.  If anyone out there is interested in Indium thermal materials, send us an email.

I don’t talk about thermal interface materials much anymore – we have a separate Indium Corporation blog  about that.  There was a time when promoting the use of indium as a thermal interface was my main goal.  It has so many advantages over other thermal interface materials, like the ability to be cold welded, 86W/mK thermal conductivity, it can be reused in some situations, and it self passivates itself with a thin oxide skin.  The important thing for you and I, is that it transfers heat away from the processors we work on.

From a different link in the thermal chain: I noticed an article dealing with fan design that you might be interested in (click here). It's nice to see everyone working together, from 1^st level thermal interface material designers all the way to the cooling fan designers. Heat dissipation is not an issue that can be fully solved by one part of the equation.

Much like this female G.I. Joe ninja, Amanda fits in with our predominantly male industry and she is definitely worthy of the title ֓Solder Ninjette!

Around the office some of the guys have called me the Solder Ninja.  The term comes from the fact that I am pretty quiet, sometimes they don't even notice I'm in the office.  I try to get tasks done efficiently, much like a sneak attack…

Anyway, if you have ever spoken to or met Amanda Hartnett, you have met a "Solder Ninjette".  Amanda is an Application Engineer for engineered solder materials at Indium, and she has a way of surprising me with the projects she gets into.  Check out her two blogs for: Engineered Solders and Thermal Interface Materials.  You'll notice that for a young engineer, she has a lot of experience already.

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！"

Heat-Springs®

Thermal Conductivity Chart

最近我接受了公司一系列的散热管理材料培训(Thermal Interface Materials---TIM)。其中最有意思的要数Heat-Springs®了。 

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.

Liquid metals containing gallium are not simple to handle, but there are ways to do it, and anyone I know who has worked with these materials and handled them correctly, is pleased with the results they received.  As explained in my previous post, the thermal performance of liquid metals is superior to all other commercially viable alternatives.

As a liquid metal with low flow stress, the liquid metal thermal interface materials are required to be specially packaged into an application to prevent leakage.  Since gallium-containing liquid metals are corrosive to some materials and stick to others, the packaging materials must be carefully chosen. 

A set of materials that I have worked with which were compatible with all the liquid metals are the Dupont Krytox products.  There are a number of forms that these materials can be produced in, but the chemistry in any of these products makes them a good material fo r a dam surrounding the liquid metal.

These materials can be engineered for applications in both TIM1 and TIM2 applications.

A drop of liquid metal.

Gallium is corrosive to aluminum. This picture was taken after gallium reacted with a sheet of aluminum foil. Image source: http://sci-toys.com/scitoys/scitoys/thermo/liquid_metal/liquid_metal.html

Liquid thermal interface materials are available in two forms:

1. metals liquid at room temperature
2. phase change metals

The major difference between the two is in the temperature which these alloys become molten.

Liquid metals remain molten at room temperatures. The following three liquid metal alloys become liquid at temperatures below 30 °C.

– Indalloy 51 (62.5Ga, 21.5In, 16.0Sn)
– Indalloy 46L (61.0Ga, 25.0In, 13.0Sn, 1.0Zn)
– Pure Gallium

Phase Change Metals are applied in solid form and melt when exposed to heated junction temperatures. The most popular phase change alloy melts at 60°C.

– Indalloy 19 (51In, 32.5Bi, 16.5Sn)

The advantages to using these liquid metal thermal interface materials are many and include:

• Extremely Low Thermal Resistance
  • The resistance obtained with the Indalloy 51 was tested to be less than 0.015 cm2-°C/W.
  • Metal in its liquid form has virtually no contact resistance
• High Thermal Conductivity
  • As a completely metal thermal interface material, bulk thermal conductivity is premium
• Ability to Withstand dramatic Thermal Expansion Mismatch
  • The low flow stress of liquid metals allow them to maintain surface contact while substrates pump during power or temperature cycling
• Ultra Low Bondline Thicknesses
  • Liquid metals can be compressed to thicknesses below 0.001"

One difficulty encountered when using these alloys is the ability to contain them. All of the alloys which are liquid at room temperature contain gallium. Gallium is corrosive to various metals, especially when hot. As the temperature of the gallium is raised, it becomes increasingly corrosive, reacting through thicker layers in a short amount of time. One metal which gallium is very reactive with is aluminum. It will corrode through .002" thick aluminum foil within hours at room temperature, and at 500°- 1000°C, this reaction becomes much faster.

Gallium is non-reactive with other metals however such as molybdenum, tungsten, and nickel.

In addition to reacting with metals, gallium with also stick to non-metallic materials, making it difficult to package. Stay tuned for my next blog posting which will discuss packaging options for liquid metals.

image: cartoonstock.com

In some of my recent posts, I have proclaimed the benefits of indium and metal thermal interface materials.  I have tried to be as non-biased with my thermal interface materials testing and claims as any material vender can be, however after working with these materials for more than a year, I have become very proud of their performance, and feel that the benefits I see are worth sharing.
 
I know I am walking a fine line though because I don't want you to fall into the trap the fighters in this cartoon did when they saw a salesperson coming.