  
 Thermal Interface
 Thermal Resistance
 Mounting
 Pressure/Time
 Electrical Properties
 Mechanical Strength
Table 1: Thermal Resistance vs.
Mounting Pressure
Click to expand
Table 2: Combined Effect of Timeclick to expand
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Under normal operating conditions, electronic devices can generate high levels of dangerous heat. If uncontrolled, such conditions can cause premature failure – ultimately a financial or customer loss.
For starters, heat spreaders may dissipate the heat effectively. The two mating surfaces are not perfectly smooth, however. In fact, their actual area of contact may be little more than 10% of the total surface area. The remaining 90% is air, a very poor thermal conductor (0.024 W/m°C). Compare this with Copper (385 W/m°C) and Aluminum (200 W/m°C).
Thermal management products, like Baukhausen’s heat sink bonding process, can be used as an interface between the heat generating device and the heat sink. The product minimizes thermal resistance, electrically insulating the device from the heat sink. The pliancy of these products allows them to conform to the micro and macro irregularities of the mating surfaces, eliminating air – and ultimately heat – from the contact area.
Thermal Resistance
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While thermal conductivity seems to be a good measure of thermal performance, this is not always the case. Thermal conductivity only measures a material's ability to conduct heat once heat is applied to it. Consequently, it does not guarantee optimal heat transfer from a component.
On the other hand, thermal resistance measures two important variables: how well a material conforms to the electronic component, and how much heat is transferred from the component to the heat sink. Thermal resistance provides a much more accurate measurement of total thermal performance.
Thermal resistance is affected by two external factors: mounting pressure and time.
Mounting Pressure and Time [back to top]
Table 1 demonstrates how thermal resistance is affected by mounting pressure. As mounting pressure increases, thermal resistance decreases to a point where additional pressure results in little or no improvement (inflection point "A"). The more pressure that is applied, the lower the thermal resistance. Gradually, the "Law of diminishing returns” appears (inflection point "B"). Ultimately, resistance will no longer be positively impacted by additional pressure.
Table 2 shows the combined effect of time and clamping force on thermal resistance. Note that after 1000 hours, the optimized solution gained a 22% improvement in thermal resistance over the original.
These tests are a testament to the real-world benefits of thermal interface solutions like the Baukhausen process.
Electrical Properties [back to top]
Insulators must withstand the voltage imposed on it during service.
Dielectric breakdown voltage is the amount of voltage a product will withstand before allowing the passage of current. Dielectric breakdown is determined by material thickness and chemistry. Environmental factors like temperature, humidity, time, and the wave form frequency of the applied voltage can further reduce breakdown voltage.
Frequently, the breakdown voltage of the product is a determining design factor.
Mechanical Strength [back to top]
In order to maintain thermal and electrical integrity, thermal interface products must exhibit enough physical strength to resist mechanical damage during assembly and in-service.
One of the most important mechanical properties of a thermal interface material is a resistance to cut-through to avoid electrical shorting from the device to the heat sink.
The ability to resist cut-through (thereby avoiding electrical shorting) is one of the most important mechanical properties of a thermal interface material. Cut through resistance is a measure of resistance from puncture wounds, metal burrs, crushing, or tearing from irregular mounting pressure. Consequently, irregularities in the mounting surfaces and variable mounting pressures cause the greatest potential for damage.
Note the diagram. The tested insulator is installed between a hardened steel base plate and a force probe. A 1000 VAC potential is applied between the base plate as the force load is increased at a rate of 15 pounds per second until voltage breakdown.
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