Perform thermal analysis of PCB mounted SOP(5)

Then, we placed the PSOP assembly into a still-air chamber using automated instruments and power supplies and allowed it to soak overnight without any power applied. The ADA4870-1 IC and temperature sensor were then both turned on and measurements of the PSOP temperature-monitor voltage and sensor-trimmed PTAT (power sub-threshold proportional to absolute temperature) current were made immediately. The temperature-monitor voltage measurement was related to the absolute temperature indicated by the temperature sensor. We repeated this process at several temperatures to develop a calibration of the temperature-monitor voltage to absolute temperature (figure 9).

Figure 9: Temperature monitor (TM) volts versus sensor temperature.

Using a linear fit to the curve (T [°C] = TM [V] – 1.93/0.003), we converted the voltage to temperature. We conducted additional steady-state tests to reveal the practical limits of power dissipation (maximum power) as a function of the applied heatsink. As shown in table 6, large heatsinks are necessary when operating at the limits of power dissipation for the tested IC. We calculated the junction-to-ambient thermal resistance (θ_ja) from the measured data by the following relations ships at steady state: θ_ja = ΔTM(V) − 1.93 (V) − 0.003 V/°C Δ Power (W) =°C/W.

Table 6: Thermal testing versus simulation results. θja: junction-to-ambient thermal resistance, Pmax: maximum power.

The results showed the CFD simulation to be in good agreement with the lab test results with a heatsink mounted, where the dominant heat-transfer path is from the die into the heatsink. There is a higher difference for simulations with no heatsink, where an appreciable fraction of the total heat travels through bond wires and leads into the top layer of the PCB. This difference can be attributed to assumptions in simulation we made in modelling the leads and bond wires in the simulation.

A complimentary tool
With these experiments, we found that CFD software is a complimentary tool to laboratory testing, enabling quick parametric and design optimisation studies in the thermal design. Such data is useful for studying electronics in harsh environments with increasing demands on power. The next step would be to analyse the transient behaviour of the package and thermal characterisation using structure functions generated by hardware testing, such as the Mentor Graphics T3Ster. A transient thermal simulation validated by test data would go a long way in simulating the transient response of a package for various powering conditions and reduce the number of laboratory tests needed.

References
1. Analog Devices, High Speed, High Voltage, 1 A Output Drive Amplifier ADA4870, http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4870.pdf
2. Blackmore, Byron, "Validation and sensitivity analysis of an image processing technique to derive thermal conductivity variation within a printed circuit board," Semiconductor Thermal Measurement and Management Symposium, 25th Annual IEEE SEMI-THERM Symposium, San Jose, CA, March 2009, pp.76-86.
3. Bornoff, Robin, Blackmore, Byron, Parry, John, "Heat Sink Design Optimisation using the Thermal Bottleneck Concept," Proceedings of 28th IEEE SEMI-THERM Symposium, San Jose, CA, March 2011, pp.76-80.
4. Li R.S., "Optimization of thermal via design parameters based on an analytical thermal resistance model," Thermal and Thermomechanical Phenomena in Electronic Systems, 1998. ITHERM 1998, pp 475-480.
5. Incropera, F., Dewitt, D., et al., Fundamentals of Heat and Mass Transfer, John Wiley and Sons (New York, 1993), pp. 65-67.

About the author
Robert Day is with Analog Devices.

Prasad Tota is with Mentor Graphics.