MNS Watlow vFinal DR 27/6/08 10:30 Page 21
MICRONANOSYSTEMS
21
Choosing the right material
through FEM
In a collabrative design with a die bonding
equipment manufacturer, Watlow employed a finite
element model (FEM) to tackle the stringent
thermal performance required of these heating
elements and to understand and optimise critical
material and performance variables. The model
was used to specify the power requirements for
given heating rates, predict the effect that power
densities have on thermal stress of different
materials, simulate the effect of thermal
conductivity on temperature uniformity, and
evaluate cooling behaviour under different
implementation schemes. The model not only helps
establish the material requirement but also helps
fine tune the heating element power distribution to
achieve a uniform temperature. presence of a heat sink and constraint of power A complete high
From a thermo mechanical point of view, input at the location. performance
thermal conductivity and the temperature ceramic heater
coefficient of thermal expansion (CTE) are the two Building the right heater module
most important properties that dictate Based on the results of the theoretical analysis for
performance of a candidate material for heaters in heater performance and design, Watlow developed
die bond machines. To establish a semi quantitative a manufacturing process and proprietary
relationship between power density and stress, a composition to realise an AlN heater that meets
model was created to predict the stress level under the aggressive requirements in semiconductor die
various power densities for two of the high bonding and IC testing applications.
performance materials, alumina (Al2O3) and The basic structure of the high performance
aluminium nitride (AlN). The maximum stress is ceramic heater consists of the AlN matrix, the
found to be about three times higher for a high heating element with distributed wattage based on
CTE and low thermal conductivity material such as FEM which ensures temperature uniformity, and a
alumina versus a high thermal material like AlN. high power input capability and terminal.
It was also found that stress increases much faster The basic structural units were assembled in a
with temperature in the case of alumina than that green state and then sintered in a nitrogen furnace
of AlN. It is clear that AlN is the preferred to allow densification to occur. The resultant AlN
material choice to meet the fast ramp-up heater is a nearly full density ceramic compact
requirement of this application. with little or no porosity which, combined with
AlN is especially suitable for applications uniformed grains, ensures high mechanical strength
requiring a clean, non contaminating heat source. and thermal conductivity. The mechanical strength The model not only
AlN heaters can operate up to 600°C. (ASTM type A configuration) of an AlN processed helps establish the
heater has a mean of 371 MPa and Weibull material requirement
Understanding thermal modulus of 11. Using a patent pending process, but also helps fine-
conductivity Watlow was able to integrate a thermocouple into tune the heating
The high thermal conductivity of AlN and an the assembly. This process ensures the reliability of element power
optimised circuit layout combine to produce superb the heater/sensor interface because the heater is distribution to
temperature uniformity across the heater surface. bonded into the assembly. This is an important achieve a uniform
Thermal conductivity also plays a key role for point as there is a need to ramp applications that temperature
achieving highly uniform temperature. It is possible require a high response rate.
to design a heater with extremely uniform surface
temperature when a distributed power input Using FEM to optimise
pattern is optimised using highly thermal performance
conductive heater matrix. Extremely high Custom designs can be rapidly accommodated
uniformity of surface temperature (steady state) including those with complex topographies such as
can be designed by properly distributing the power holes, notches and vacuum grooves. Using an FEM
within the heater. The cooler terminal side and non technique, the heater circuit is optimised and the
symmetrical temperature pattern is a result of the thermal performance is simulated prior to
July 2008 www.micronanosystems.info
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