Enhancing reliability with thermal transient testing
Introduction
The reduction of device sizes in microelectronics seems to be a never-ending tendency. The channel-length of the MOS transistors is today in the deep submicron range. This tendency is resulting in an enormous increase in the number of gates integrated in the same chip. The dissipated power density is continuously growing. The 1200–1500 MHz processors appeared last year dissipate in the range of 100–125 W. As a result of this the thermal design of the IC elements becomes more and more crucial. The serious problems of heat-removal claim sophisticated measurement equipment, capable to analyze the heat flow properties in fine details.
The thermal transient testing, if accompanied with special fine evaluation methods, is capable to provide a thermal map of the investigated chip+package structure. The method can be categorized as a kind of thermal echography. The evaluation of the measured thermal transients is a particularly interesting field, in which a number of research groups have obtained valuable results [1], [2], [3], [4].
Reliability of the ICs is strongly affected by thermal effects. Overheating accelerates the various migration effects, excessive temperature cycling activates thermo-mechanical failure mechanisms. The thermal issues have to be taken into account in the design in order to reach better reliability.
Our research group has more than 15 years experience in the field of thermal transient measurement and evaluation. Three subsequent designs of thermal transient testers have been built in this period and a number of evaluation methods have been introduced and tested [5], [6], [7], [8]. Certain important parts of the acquired knowledge will be presented in this paper, together with considerations about the role of the thermal transient measurements in reliability enhancement.
Section snippets
Thermal transient measurement: the principle
The dynamic thermal properties of an IC chip or package are usually measured in the time-domain. The principle of the measurement is presented in Fig. 1. In the t=0 time instant a P0 dissipation-step is applied to the package. Beginning with this moment the ΔT(t) temperature rise of the chip is continuously recorded. This function is called the heating curve. It is obvious that the recorded function is the thermal unit-step response multiplied by P0. The unit-step response can be determined as
The thermal transient measurement in practice
The simplified block diagram of the thermal transient measurement equipment is shown in Fig. 3. The two main parts of the equipment are the power-switch unit and the measurement unit. The former unit provides the step-function power load and the latter one records the thermal transient. The digital control unit organizes the run of the measurement process. A personal computer is used to act as soft instrument, to control the measurement, to visualize the results and to execute the algorithms of
Assuring constant dissipation
The principle of the thermal transient measurement evaluation supposes step function excitation. In measurement realisation this means to stabilise the input powering on a constant value after switching-on. If we intend to meet this requirement, practical difficulties arise. The problem is the most explicit if a diode is measured, where the same device is used at the same time both as dissipating and sensor element.
This is usually the case e.g. when an IC is tested and the only way to power the
Evaluation of the heating/cooling curves
The thermal transient measurement equipment provides thermal step function responses (heating or cooling curves). These curves serve as the base of the subsequent evaluation. The essential step of this evaluation is to extract the apparent time-constant values and the related magnitudes, either in the form of a few discrete values, or in the form of the continuous time-constant spectrum.
The response functions typically look like the example of Fig. 2. For these responses it is highly
Identification of the different regions on the structure function
The above-presented structure functions are originated from the investigation of a ceramic pin-grid array package shown in Fig. 13 with its main dimensions. The chip in the package was a commercial 32-bit processor. During the thermal transient measurement the substrate diode was used both as powering and sensing element. A large-size finned cooling mount was applied on the package top surface, with thermal grease between the contacting surfaces. The applied dissipation step was
Testing for die attach failure
One of the most frequently encountered physical weakness of the packaging influencing the reliability is the die attach failure. Die attach imperfections cause excess temperature rise which in turn stimulate the further degradation of the die attach. Although the steady-state thermal resistance measurements may indicate excess thermal resistance in such cases, the origin of this excess thermal resistance can be localised and identified as die attach imperfection only by using dynamic methods.
As
Conclusions
In this paper the methodology for the thermal transient measurement of IC packages has been presented. The evaluation and interpretation of the measured results are discussed in details. In the last part of the paper the possible role of the thermal transient measurements in reliability investigations is discussed. Two subject were focused on, these are the detection of die attach imperfections and the prediction of transient thermo-mechanical strains. The thermal transient method has been
Acknowledgements
This work was supported by the PROFIT IST-1999-12529 Project (Prediction of Temperature Gradients Influencing the Quality of Electronic Products) of the EU.
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