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Four major influential factors in thermal resistance


The most important way to achieve better heat dissipation is to decrease the thermal resistance of the semiconductor device. The factors that determine thermal resistance are detailed below.

Thermal resistance that can be calculated in the same manner as Ohm's law
Thermal design that considers heat dissipated from semiconductor devices uses the concept of “thermal resistance," based on the fact that heat transfer can be compared with the way electricity is transmitted.

In the case of electricity, the relationship among voltage (electric potential difference), current, and resistance is expressed by Ohm's law, as shown below. Heat resistance can be calculated using a law similar to Ohm's law by replacing voltage with temperature difference, current with heat flow, and resistance with thermal resistance.
Ohm's law
Voltage = Resistance × Current
↓   ↓   ↓
Temperature difference = Thermal resistance × Heat flow
Thermal resistance (unit: degrees/W) = Temperature difference ÷ Heat flow
As can be seen from the above expression, thermal resistance indicates how readily heat is dissipated.

The four factors that determine the thermal resistance of a package

The overall thermal resistance of a package is almost entirely determined by:
1. Package structure
2. Package size
3. Chip dimensions
4. Air flow rate
 Each of these elements influences the thermal resistance as follows.
1. Package structure
Packages come in various types, each with different thermal resistance characteristics (refer to figure). Packages such as ABGAs and FCBGAs, which feature a copper lid to which the chip is directly attached with thermally conductive paste, offer excellent thermal resistance characteristics. In the case of PBGAs, thermal resistance can be lowered by using a 4-layer substrate instead of a 2-layer substrate, and it can be further lowered by placing solder balls directly underneath the thermal via holes. 		  Thermal Resistance Differences According to Package Structure
Figure 3: Thermal Resistance Differences According to Package Structure

2. Package size
Generally, the larger the size of the package, the lower its thermal resistance. This is particularly true of ABGAs and TBGAs, which have a copper lid offering excellent thermal conductivity characteristics. In the case of packages with lower thermal conductivity such as FPBGAs, there is a weaker correlation between thermal resistance and package size, and there is little variation in thermal resistance among different size packages if the chip dimensions are the same.

3. Chip dimensions
The thermal conductivity of silicon, the material chips are made of, is about 100 times higher than that of mold resin and about 10 times higher than that of package substrates; therefore, the surface area of the chip itself greatly contributes to heat dissipation.

4. Air flow rate
The air flow rate is not directly related to the thermal conductivity of the package itself, but forced air cooling, such as through the use of a fan, efficiently transfers heat from the package surface or printed wiring board to the surrounding atmosphere and thus reduces thermal resistance.