Thermal Interface Materials (TIM) Blog When Connections CountSM
The TIM Blog serves as a clearinghouse for information relating to thermal interface materials including products, technologies, news, and events. We encourage comment and feedback on anything thermal interface material related.
The heatsink can be mounted uniformly using the Heat-Spring as a TIM because the surface alteration in this compressible metal squishes with surface irregularities of the heatsink. Image Courtesy of www.ocztechnologyforum.com
I received the following question from a customer today regarding heatsink attachment using the Heat-Spring as a TIM:
“My perception is that the Heat-Spring does not wet out the interface
surfaces.While there is clamping pressure on the heatsink, is it possible to rock the heatsink, releasing the clamping pressure?What will keep the Heat-Spring in place in this situation?”
I thought that this was a good question and I would pose the answer to it because it points out a unique attribute of the Heat-Spring™ and a question that others have probably considered.
My response:
The Heat-Spring does not flow.It is designed to compress into an interface, however.This is an engineered product, unique to each application, so the thickness and surface alteration are designed to compress enough that uniform contact is made along the entire interface, and no single location acts as a shim which will lead to rocking of the heatsink.When engineered correctly for an application, the heatsink will clamp tightly with the Heat-spring filling in all surface irregularities.
Indium TIMs are not susceptible to this type of metallic corrosion. Image courtesy of www.moonraker.com.au/techni/news2.htm
An area of concern for implementing indium as a thermal interface material has been its susceptibility to corrosion attack in humid conditions. Little information has been available regarding the corrosion of indium when used as a compressible thermal interface material or as a solder thermal interface material. The only published studies done on the corrosion of indium (which I am aware of) pertained to indium through-hole joints on gold.
An evaluation of pure indium showed that when used as a compressible TIM, the indium thermal interface material passed 1000 hours exposed to 85ºC and 85% relative humidity (HAST). This was posted previously on my posting titled Indium Bake and HAST Test Results. It is hypothesized that the source of this improved life over prior tests on indium solder joints is the ability of indium to form compressible hermetic seals. Traditionally, the solder joints studied were fully exposed with a great amount of surface area. The indium in a TIM application has limited exposed surface area. Only the edge perimeter is revealed and exposed to the halogen ions, water, and air required to cause this corrosion, slowing down the rate of corrosion significantly. During the 1000 hour test, there was no measurable change in the material.
Additionally, it has also been revealed that indium alloys are less prone to galvanic corrosion than pure indium.
In applications where this corrosion is still a concern due to a need for extended life reliability in extreme conditions, the thermal interface material can be sealed around the perimeter from ionic contaminants or extremely humid conditions which might progress the oxidative process.
Various assemblies utilize solders, including Au/Sn and tin solder, for thermal attachments. These solders perform multiple functions:
1)As a metal interface, they are thermally conductive and will spread heat quickly away from a heat source.
2)As a soldered attachment, they will form intermetallics with base metals. This bond is strong and will remain mechanically reliable for the lifetime of the device.
3)This solder bond will be electrically conductive.
If the primary reason for implementing a solder bond is purpose 1 (thermal attachment), a void-free solder bond is highly desirable. This will maximize the thermal dissipation through the high K solder.
Void-free soldering is typically achieved on gold-plated substrates. There are particular tactics for soldering to gold. Some of these involve the solder choice. Popular solder alloys include Au/Sn, tin alloys, or indium solders. Details regarding these material choices are outlined in our application note on soldering to gold.
For more information on assembly techniques when soldering to gold, check out my previous posting on soldering to gold.
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.
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.
I am proud to be an Indium Corporation employee for many reasons. The best of these is that I am able work with other experienced, innovative engineers and technologists who push the envelope to develop new materials which impact the industry.
Dr. Ning-Cheng Lee is one of those individuals and I am happy to say that I (and we) at indium are not the only one’s recognizing his skill and authority……
Ning-Cheng Lee: Distinguished Author and Distinguished Lecturer
Dr. Ning-Cheng Lee was recognized recently by two highly respected industry organizations as both a Distinguished Author and Distinguished Lecturer.
SMTA International has selected Dr. Lee as a Distinguished Author by “Special Invitation from the SMTA International Technical Committee”. Ning-Cheng was selected from authors of exceptional papers and “Best of Conference” award recipients from past events. This honor is part of the SMTA’s 25th Anniversary celebration.
In addition, the IEEE’s Components, Packaging, and Manufacturing Technology Society (CPMT) approved Dr. Lee to be a CPMT Distinguished Lecturer. According to the director for the CPMT Distinguished Lecturer Program, Dr. Lee was nominated and endorsed by several colleagues from the industry. The CPMT Distinguished Lecturer program includes “Fellow Caliber” technologists from all over the world who are available for any CPMT-sponsored venue. The CPMT is the leading international forum for scientists and engineers engaged in the research, design and development of revolutionary advances in microsystems packaging and manufacture.
I assure you, the following is not some strange alien test of foreign materials. Even though it resembles a substance from the spaceship in E.T. – this is definitely earthly matter. This video shows the following 3 low-temperature liquid metal alloys wetting to glass:
These are variations of Gallium, Indium, and Tin (with Zinc in #46L) that have amazing abilities – including incredibly low thermal resistance in heat transfer applications. These alloys also wet to other non-metallic substrates like silicon, quartz, ceramics, and diamond.
Call us @ (315) 853-4900, or email us if you'd like to learn more about metals that are liquid at room temperature.
Last week was Semi-Therm and it was a great time. With the economy suffering, I wasn't sure what to expect, but many of you made it out to get your fill of the technical session and exhibitions for the year.
Thank you to all of you who made it out and either attended one of my presentations or came by and said, "Hello" at the booth.
There were a couple of highlights for me at this show.
1) We had actual samples of liquid metal at the booth. It's such a unique material, that it was fun to let you get a hands-on feel for the nature of the material.
2) At my presentation on the various types of metal TIMs, I recieved numerous questions on how to process a solder TIM in an Integrated circuit TIM1 application.
In light of these two areas of interest, I will be writing over the next few days to elaborate more than I have in the past on these two classes of material and how they can be used.
The clock is ticking before the extension of Pb-free legislation for high Pb-containing solders expire. Luckily, when RoHS was passed, those on the committe knew enough about solder alloys to know that there wasn't a non-gold contained solder replacement material for these high-Pb alloys, so they made them exempt from the RoHS restriced materials.
Time has passed however, and this exemption won't last forever. Engineers want to be prepared and begin testing the replacement materials which have been developed over the past few years. Unfortunately, there still isn't an industry accepted material available.
Creating new solder alloys is not easy and Indium's Dr. Andy Mackie explains why in his recent blog posting titled, "Elementary, My Dear Watson..."
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