| Advanced Ceramic Heaters Improve IC Packaging and System Performance |
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| Jan 01 2007 | |
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Investigating CeramicsTo tackle the stringent thermal performance required of these heating elements, the finite element model (FEM) was employed to understand and optimize critical material and performance variables. The model was used to simulate the effect of thermal conductivity on temperature uniformity, predict the effect that power densities have on the thermal stress of different materials, specify the power requirements for given heating rates. and lastly, evaluate cooling behavior under different implementation schemes. The model not only assists in establishing the material requirement, but also helps in fine-tuning the heating element power distribution to achieve a uniform temperature. From a thermomechanical point of view, thermal conductivity and the temperature coefficient of thermal expansion (CTE) are the two most important properties that dictate performance of a candidate material for heaters in die bond machines. To establish a semi-quantitative relationship between power density and stress, a model was created to predict the stress level under various power densities for two of the high-performance materials: alumina (Al2O3) and aluminum nitride (AlN), listed in Table 1. The maximum stress is found to be about three times higher for a high CTE and low thermal conductivity material such as alumina, versus a high thermal material like AlN. It was also found that stress increases much faster with temperature in the case of Al2O3 than that of AIN. It is clear that aluminum nitride is the preferred material choice to meet the fast ramp-up requirement. Thermal conductivity also plays a key role for achieving highly uniform temperature. It is possible to design a heater with extremely uniform surface temperature when a distributed power input pattern is optimized using highly thermal conductive heater matrix. Table 2 indicates that the ΔT (Tmax-Tmin) of 1.1ºC is achieved for AlN, while Si3N4 has a of 7.4ºC, which is about seven times larger. As shown in Figure 3, extremely high uniformity of surface temperature (steady state) can be designed by properly distributing the power within the heater properly. The cooler terminal side and non-symmetrical temperature pattern is a result of the presence of heat sink and constraint of power input at the location. |
