Implementation of an on-line test for the monitoring of the thermal interface between semiconductor and heatsink

Author(s):
M. Ferro - GREEN POWER SOLUTION
S. Facelli - GREEN POWER SOLUTION

Industry:
Semiconductor

Products:
LabVIEW, M Series Devices

The Challenge:
-To develop a test capable to check the thermal interface between power semiconductor devices and heatsink in less than 5 seconds. -To design a test equipment for the screening of the production of voltage regulators for automotive applications.

The Solution:
The test consists of two time critical tasks (heating phase and test phase) and one voltage control loop. In both cases the FPGA-based NI PXI-7831R Series RIO is the chosen solution. For the other data acquisition and equipment control tasks the NI PCI- 6220 and NI PCI- 6514 have been adopted. In order to maximise hardware flexibility reliability and compactness, the application uses a PXI-based solution. NI LabVIEW has been used for the programming of the whole application.

The process of assembling power semiconductor devices (e.g. Power MOSFETs, IGBTs, diodes, etc.) on the heatsink using any kind of thermal interface material represents an example of one of the so called “special processes”. In fact, the thermal interface can hide problems introduced by the assembly process itself, or already present on the contact surfaces.
The methods for checking the thermal interface are not always easy to implement, especially when a “finished” product with several semiconductor chips in parallel need to be tested. Another important constraint – especially in case of high volume production – is the time required to perform the test.
The goal of this project was to develop a new method to test the interface between the power semiconductor device and the heat sink. The main characteristics that the test had to comply to were the followings:

- possibility to be performed on finished products;
- maximum duration of less than five seconds;
- capability to test parallel connected devices.

The developed test has been applied on the 100% of the production of voltage regulators for automotive applications but could be applied in the same fashion to almost any circuit topology and device type. The test is based on the measurement of the temperature increase due to a constant thermal load applied to the semiconductor device. The body diode forward voltage drop at low injection level (VDTH) is used as Temperature Sensitive Parameter (TSP) thank to its inverse proportionality to the die temperature.
The test process consists of four phases:

1. MOSFET body diode VDTH measurement, at ambient temperature;
2. device heating at controlled energy;
3. measurement of the diode VDTH voltage;
4. calculation of the temperature increase using the first and third phase measured VDTH and comparison with limits.
The main functional blocks of which the tester is made of are:
- two current sources (one low current source - CS1 - used to apply the proper current stimulus for the VDTH reading and one high current source - CS2 - used for heating the device at controlled energy)
- a power multiplexer used to connect the proper current source to the D.U.T.
- the PXI data acquisition system
- the user interface (both PC based and hardware based)
In the first phase, a 10mA current stimulus is applied to the body diode and the VDTH is acquired. During the second phase CS2 is connected to the D.U.T. and used to apply constant power dissipation to the MOS. The power is kept constant by controlling the voltage drop across the D.U.T by means of a P.I.D. regulator. After the heating phase the current is turned off and the VDTH value, after a fixed delay time of 50ms, is acquired and used to compute the temperature increase by comparison with the initial value. The test “core” consists of phase 2 and 3. In fact, most of the accuracy of the test results rely on the precision of the applied thermal power and on the exact timing of the VDTH acquisition after the heating phase. In fact, at the power removal, the D.U.T. starts cooling down following an exponential curve. Thus, it is extremely critical to measure the VDTH (which is proportional to the device temperature) always in the same point of the cooling curve.
To ensure an optimum performance for both the power control algorithm and the temporisation of the final VDTH acquisition, the second and the third phase have been implemented on the NI PXI-7831R Series RIO which, thanks to the use of the FPGA chip, allows the needed resources for the high speed control loops and the accurate timing. The program controls the execution of the test performing data acquisition, controlling the FPGA card, processing and storing data on datalog files. During the processing phase the two VDTH voltage are converted in temperature and their difference is compared with the limits.

The operator can start the test and check the results on the main front panel of the program (see Fig. 1). The test parameters can be set for a specific device through a special window called configuration panel (see Fig. 2).
Thanks to the NI LabVIEW language structure we have been able to quickly develop a modular program, easy to improve, change or extended with new functionalities.
Through the use of NI hardware and NI LabVIEW we have been able to develop our application (see Fig. 3) with the minimum time to market, with all the features needed to meet the initial design goals (including the software and hardware robustness necessary for the test of thousands devices per day). and with the lowest overall costs.

Author Information:
For more information on this Case Study, contact:
M. Ferro
GREEN POWER SOLUTION

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