SPEAKER INTERVIEW
Andreas Griesinger, Head of the Center for Thermal Management (ZFW)
International technical associations IPC and ASTM have tried to establish test standards for insulated metal substrates (IMS) and thermal interface materials (TIM) to give electronics product developers a side-by-side comparison in material selection. Heatsinks lack similar standards, according to Professor Andreas Griesinger of the Center for Thermal Management (ZFW) in Stuttgart. He will discuss the issue in a panel on the second day of the Thermal Management Expo conference this December.
In a previous conversation you mentioned measurement of thermal data in heatsinks as a problem. Can you explain what you meant?
Heatsinks are a very important part in electronics cooling. You find lots of different data sheets with different values. The typical value given by the supplier is the thermal resistance. The big question is how to measure it.
Nobody knows how to measure a heatsink. Every company is doing in in their own way. This means the data sheet you get are useless, because nothing is defined. For it to make sense, there should be a clear definition of how to measure it. Up to now, there is no standard, national or international, how to measure it.
The typical setup for measurement of thermal resistance is you apply some heat and you measure temperature at the heatsink and also at the environment. The thermal resistance is nothing but difference of heat divided by applied heat.
In principle, it's easy, but the question is how to apply the heat. Do you heat the complete base plate of the heatsink or do you heat it by a small point source? The difference is very big. What about the air flow? If you have a force convection, is it a laminar flow or a turbulent flow?
The difference can be a factor of ten. Another point is at which temperature level you measure. The result is completely different if you measure at, let's say, 30 degrees or 130 degrees. Without any definition of all these parameters, it doesn't make sense to give any thermal resistance numbers to the customers.
Why is this a problem? What are the consequences?
The problem is that nobody can rely on data sheet values. All the suppliers give some resistance values, very precise, but they don't explain how they were measured, so the user of the data sheet, the user of the heatsinks, has no real benefit of it.
The consequence of this is that each customer has to do their own tests for their own applications. The manufacturer side could say, I give you this data sheet and we measure this data exactly according to this standard or according to this definition and if you have another application, compared to our lab setup, you will get different values. Nobody is to blame in this, but it requires serious scientific consideration.
Do you see the same problem with thermal materials? For example, do suppliers of thermal interface materials provide reliable, useful data?
A very good question. It is better nowadays. Let's say five years ago, it was not a very serious business. There were some suppliers, for example Henkel, that had really good, reliable data sheets. On the other hand, there were some other suppliers that had data sheets without any relationship to reality. It has been getting better every year.
For example, Audi in Germany defined a very clear limit for thermal conductivity, for gap fillers for batteries. That was a starting point. They said the limit of thermal conductivity must be 3.0 watts per meter Kelvin. If your gap filler has a lower thermal conductivity, there is no chance to get in to the market with Audi.
That started a big discussion how to measure it. A lot of suppliers had gone to Audi saying, we have a very good material with 5 or 6 watts per meter Kelvin. Now they had to start qualification and take serious measurements and learned they only had 1 or 2 watts per meter Kelvin. This was the starting point, some years ago, of much more serious consideration in this field.
Would we need the same thing for heatsinks; buyers setting and forcing standards?
The situation is a little bit different when we look at heatsinks, because the performance depends really strongly on the application. It's different from a thermal interface material. A thermal interface material has a thermal conductivity, a resistance, maybe you can discuss a little bit how to measure.
Heatsinks are different. You can get a thermal resistance if you measure it with one method, but if you measure it in a different way, you get another value and you cannot say the first method is correct and the second one is not correct. Both methods are correct, but you have to define it.