Heat Spring

Indium Corporation has won the Innova Award for Best Technology for its Heat-Spring^® metallic thermal interface material (TIM).

Heat-Spring is a clean, high-performance thermal solution for the increasing demands of high brightness LEDs. It is a compressible metal foil with proven performance in such demanding environments as electronics, aerospace, and power devices.

The compressible TIM provides low thermal resistance as a result of its high thermal conductivity (86W/m-K) and its ability to conform intimately to interface surfaces.

Unlike other thermal interface materials, such as thermal grease that bakes out, dries out, or pumps out during use, the thermal resistance of the Heat-Spring continues to improve with time and power cycling.

According to Jordan Ross, Market Manager for Thermal Materials, “Indium Corporation is honored to be recognized with the Innova Award for our patented Heat-Spring product. With its patented compressible interface design, Heat-Spring provides optimized surface contact, superior thermal conductivity, and enhanced heat flow.”

Sponsored by LED Journal, the Innova Awards feature leading companies within the LED market which have shown, through their products and services, the most innovative and advanced technology breakthroughs in LEDs. The award is designed to recognize companies each year for industry leadership, product development excellence, best new technology, and outstanding LED applications, which will eventually lead to the widespread adoption of LED technology in the marketplace.

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.

A typical LASER component before heat-sink mounting with a compressible Heat-Spring. Image Source: http://news.thomasnet.com/images/large/469/469336.jpg.

Case studies or product reviews typically provide a good indication that a product is worth purchasing. Recently, one customer (whose name I am purposely excluding) provided me some feedback worth sharing following their evaluation of pure indium Heat-Springs™.   The Heat-Spring™ was to be used in a LASER mounting application to a heat sink with limited workspace. Competitor materials had been evaluated already, but the resistance of these made the interface temperature too hot. Pure indium Heat-Springs™ were the next evaluation material.

In addition to the thermal benefits of this material, the customer realized some mechanical benefits as well. 

He said, "Handling the Heat-Spring™ material is 10-100 times easier from a fragility point of view [compared with previously evaluated materials], especially in the tight space of our configuration." The Heat-Spring™ can be handled as a small foil with tweezes or a suction tip and placed cleanly into location.

The Heat-Spring™ is then clamped down to make tight interfacial contact. The LASER customer noted, "Getting consistent results seemed much more straight forward when clamping the material down [compared with other thermal materials including regular indium foils].  I could actually feel the pieces compress as I tightened the screws."

For further information on using the Heat-Spring™ in applications similar to this LASER diode case study, please feel free to contact me.

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。 

At some point in our lives each of us has played the WhatӒs your favorite game. ԂWhats your favorite҅sport?  Whats your favorite҅.Ice cream?Ԃ Well, I want to play the, Whats your favorite TIM materialҔ game.  

I have worked with quite a few types of TIM materials in my lab testing, but surely not as many as my readers.  Therefore, Id like to know what materials you are using and why. ҂What makes them your favorite?



All Answers are appreciated. There is no wrong answer.



It is probably only fair if I begin this forum with a note on my favorite TIM. For many reasons, but most certainly its ease of application and removal, my favorite TIM material is the pure indium Heat Spring. Just like a piece of aluminum foil, an indium preform can be cleanly applied as a preform and cleanly peeled back just as easily. 

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.

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.