Spreading thermal resistance and Thermal interface material

In electronics cooling, usually heat is transferred from chip to ambient. Heat sink is common to be attached at chip as cooling system. Surface area of heat sink usually larger than chip, this condition makes spreading heat transfer.

What is spreading Thermal resistance?

Thermal resistance is the ratio of temperature difference to the corresponding power. The difference temperature here refers to the place or point. At a processor with heat sink, heat dissipation is started from ship to heat sink. So obviously chip temperature is higher than heat sink. The temperature difference in this case is the difference of chip temperature and the heat sink, so we called chip to heat sink thermal resistance. It also can be determined, thermal resistance of other point. The total thermal resistance is from the source to the ambient.  

Spreading thermal resistance occurs when heat source is smaller than cooling system, such as heat sink. That geometry doesn’t distribute uniformly through the heat sink, but centralized on the middle where the chip is attached.

To analyzed and design cooling system in electronic, usually uses one dimension analysis. Therefore spreading thermal resistance is one of the interests of researcher and electronics cooling engineer.

Besides spreading thermal resistance, Thermal contact interface also influences heat transfer. When chip is attached to heat sink, there is contact between two surfaces.  To optimize heat transfer, there are several ways to be applied in thermal interface material. Those are: grease, elastomeric pads, thermal tapes, phase change materials, gels, thermally conductive epoxy, solder, and soft metal.

Thermal Resistance in Electronics

Thermal resistance is the ratio between temperature difference and power dissipated. In electronics, Thermal resistance is a great interest for engineer. It is due to every electronic equipment produce heat and need to be cooled. If they cannot be cooled properly, it will be harmed because of overheat problem.

Thermal resistance can be analogized by electrical resistance. Current represent heat flow, voltages represent temperature differences, and Resistor represent thermal resistance. It can be simplified: R_th=∆T/Q is analogized by R=V/I.

There are several kind of thermal resistance, such as conductive thermal resistance, convective thermal resistance, and spreading thermal resistance. Thermal resistance is basically needed to calculate heat transfer from one point/surface to another point or surface.

In electronic, Thermal resistance is the parameter that informs how effective heat dissipated can be transferred to the ambient. Moreover, it can be used to determine heat source temperature, usually chip junction temperature. In other words, heat source temperature can be known if thermal resistance is already known.

Let’s consider a simple electronic device with heat sink. Heat transfer is started from junction to casing and finished at ambient. This construction results several thermal resistances, those are junction to casing thermal resistance, casing to ambient thermal resistance through heat sink. Total thermal resistance of this system is R_total= (T_j-T_amb)/Q= R_JC+R_CA

Power dissipated from electronic device is assumed that all electricity is converted to heat. Therefore heat dissipation is : Q=VxI, where Q represent heat dissipation, V represent Voltage, and I represent Current. For LED heat dissipation, sometimes optical power from LED is considered, so heat dissipation becomes, Q= P_electrical-P_optical = (V*I)-P_optical.