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Continuing with the topic of thermal interface material options, a popular material choice is a clad metal thermal interface preform including multiple layers of various metals, or a metal preform clad with an adhesive synthetic material. For specific applications where an interface is sandwiched between two drastically different substrates, clad metal preforms are great. Using clad preforms containing multiple metals, it is possible to clad a stiff material on one side which prevents deforming and a soft, conductive material on the other.

While there are applications that these materials cater to, it is important to realize the full impact of using such a TIM material.

Bob Jarrett recently wrote a summary of the resistance impact of adding an adhesive layer to an existing metal interface material.

Bob wrote:

The adhesive on the interface will act as another interface layer, increasing the thermal resistance, additively. The typical acrylic polymer adhesive has a thermal conductivity of ~0.1 W/m-K. If this adhesive is applied as a typical ½ mil (13 μm) film, the resistance increase is:

This is added to the 0.06-0.08 cm²-°C/W for the Heat-Spring a total resistance of about 0.2 cm²-°C/W. The thin layer of adhesive more than doubles the resistance.

This posting is not meant to deter you from using cladded interface materials, just to help you realize the potential effects of using such a material. Every layer added to the interface also adds some amount of resistance.

Bob Jarrett gave an excellent explanation of the thermal conductivity of metal alloys and their relationship with electrical conductivity. It is as follows:

The high thermal conductivity of metals is a result of energy transfer by free (or valence) electrons. These mobile electrons conduct electricity and thermal energy. For metals, the ratio of the thermal conductivity to the electrical conductivity is the Lorenz constant or the Weidemann-Franz ratio (L = κ/σT ~ 2.45E-8 WΩ/K²). When metals are mixed to form an alloy, the energy states of the free electrons are modified as the different elements have differing affinities for the wandering electrons.

The electrical and thermal conductivity of the mixture often deviates (negatively) from a linear rule of mixing. For example, Sn-Cu forms two intermetallic compounds (Cu3Sn and Cu6Sn5). The conductivity relationship still holds, but both the thermal and electrical conductivity are less than either constituent. Similarly in the Sn-Ag system, the conductivity of the 96.5Sn-3.5Ag eutectic composition is lower than either element.

Calculating the thermal conductivity of an alloy is not done by taking an average (or weighted average) of its elemental constituents. When two metals are mixed, their properties dramatically change, often in ways which would be difficult to speculate.

For example, the thermal conductivity of silver metal is approximately 429W•m−1•K−1 (according to Wikipedia) and tin metal is 73W•m−1•K−1. Both of these values are very high. Surprisingly, the conductivity of Indalloy #121 (96.5Sn 3.5Ag) is a considerably lower 33W•m−1•K−1.

This may not seem logical, but is correct. The thermal conductivity of an alloy is impacted more by the chemical bonding structure between the inclusive elements than by the measured conductivity each element exhibits alone. The bonding orientation of the metals affect the rate which heat can pass through the material.

Thermal conductivity of common metal alloys can be found in Indium’s Solder and Alloy Directory.

The typical extreme device needs an advanced thermal interface material to keep the interface cool while the device generates extreme amounts of heat. As discussed in my previous posting, the interface material chosen for extreme conditions often melts at a very high temperature so that it remains solid and stable during high temperature operation.

Other types of extreme devices have an opposite issue, a low reflow temperature requirement. A significant consideration when choosing a solder TIM is whether the entire package will withstand soldering temperatures. As indium supplies TIM materials with high temperatures, we have also studied and understand low temperature metals.

At the very lowest temperatures, there are metal alloys which are liquid at room temperature. These alloys contain indium, gallium, and tin. These materials can be deposited and remain molten during the entire life of the product. They do not require any elevated temperatures for their application.

For those thermal designers who are not ready to take the liquid metal leap, there are also alloys available for use in thermal interfaces which melt at approximately 60C. These alloys will wet a substrate, forming an interface with very low resistance.

Unfortunately, without a flux, these low temperature alloys are unlikely to form any intermetallic with the heat sink or spreader. If the interface material alone is to provide mechanical stability, an intermetallic is needed. The flux is used to remove oxides from the substrates, allowing for intermatallic formation. Most fluxes do not activate until temperatures or 125C. There are some fluxes available which activate as low as 100C however. For more information on these, please read Jim Hisert’s solder blog on this topic.

The most common metal thermal interface materials are made from indium alloys which melt between 100C to 200C.

Some power devices I have come across desperately want to make the step toward metal interface materials, but cannot use them in the traditional sense because the operating temperatures of their devices is hotter than 200C. This would melt the interface material as the device ran which could lead to reliability issues.

As this image depicts, we can’t cool down an electrical device by serving cold drinks. We can keep a device cool when the heat is just too much however by inserting an appropriate material at the thermal interface.

There are multiple alternative alloys for these hot applications and fortunately, these materials are well understood since they have been used in electronics soldering applications for years. Today, they have been adopted by many companies for their TIM properties, which are similar to indium. High temperature solder alloys such as 80Au/20Sn are very common, but it is also possible to use high temperature tin alloys (99Sn1Cu, 99Sn1Sb or 95Sn5Ag) or for the most extreme cases, pure gold at the interface. These materials all melt above 200C and also have high conductivities, which will minimize the resistance at the interface when the solder is compressed or reflowed.

Image courtesy of distractible.org

As many other companies are “going green” so can you with the right thermal interface material. Are you using indium materials now and collecting wadded up balls of scrap indium in your top desk drawer (As I’ve seen many customers doing already)? Worse yet, are you discarding your indium waste with other scrap metal?

If so, I have to say, STOP!

By using an all-metal thermal interface material, you have the added ability to sell back your metal waste. Indium will buy back your indium when you are done with it. Not only will this save the earth by lessening the landfill additions, but may also make you some money!

Image Courtesy of http://www.annaheldaudette.com

You may have noticed that the appearance of Indium’s thermal website has changed and now includes a list of thermal certified Indium employees.

If you have seen this, it is likely that you have wondered what it means. As members of Indium’s world-wide thermal team, engineers, salesmen and women, as well as members of our marketing support team are encouraged to undergo an extensive training course on the materials which Indium manufactures, how they are used, and how to support applications which may use these thermal materials.

The thermal training class concludes with a test, and those which pass the test are presented to you as “thermal certified!”

What does that mean to you? Having a thermal-certified support team dispersed around the world is to your advantage when you have questions about a device with thermal issues or simply questions about material options.

Metal thermal interface materials are not simple one-size-fits-all products. They are customized to fit your needs. For this reason, we have a support team avilable to evaluate your needs and suggest the best material available to solve your greatest issues.

If you have a device in need of a thermal solution, please contact the thermal-certified support member nearest you or e-mail me and I’ll direct you to the appropriate team member.

A recent topic which has come up in the soldering world has been the push for halogen-free or halide-free fluxes. There are many reasons customers are asking for these and for the past months, developing halogen-free fluxes has received a lot of attention from Indium.

We understand the impact halides can potentially have on solders, such as their corrosiveness on metals in the presence of moisture. As a result, Indium has developed halide-free versions of fluxes in each form of our preform fluxes. For more information on these fluxes, please e-mail me or to read more on halogen-free fluxes in soldering, check out Mario’s halide-free blog series.