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.