Electronic Packaging Based on Thermal Requirements

I.    INTRODUCTION

There must be a “Rule-of-Thumb” for selecting the power dissipation of a PWB in a CCA based on its size, housing, external boundary temperature, and hottest PWB temperature limit.  The hottest temperature limit is the mounting temperature for the electronic component.  The manufacturing company will provide the thermal resistance from the board (PWB) to the case, the thermal resistance from the case to the junction, and the joule heating of the device.  Reliability guide-lines shall provide the device hazard rate based on the maximum design junction temperature.  This information shall define the temperature rise from the board to the junction and thus define the hottest PWB temperature for the device to meet reliability requirements.  There is such a simple scheme.

The old schemes are difficult to manipulate.  They involve a computer aided design (CAD) package with a built-in thermal analyser solving the heat conduction equation.  The shapes in the CAD model are fixed during creation and hence there is no way to adjust the lengths after surface input.  If a new geometry is needed, a whole new CAD model must be created.  Thus, each new PWB size requires a new model with its associated housing.  A new approach is to program the heat conduction equation as a function of PWB and housing geometry.  Now all the geometry can be addressed as variables and quickly changed for new sizes of the PWB in a simple computer program.  Trades studies of PWB size are now easy to perform with the new scheme.

II.    TRADE STUDY

The trade study is focused on one particular housing, external boundary temperature, and maximum PWB limit.  Now we can vary the size of the PWB to determine the power flux.  As the PWB size increases, the power flux decreases.  Some where is the happy medium where the PWB has a reasonable size and the electronic components can be closely packed.  This concept can be checked by the various experts helping the Electrical Engineer design the PWB and the resulting CCA.  With all disciplines in agreement, the design process will easily move forward at a predictable pace.

A.    Formula for Success
a.    Pick the housing for the electronics.
b.    Pick the external boundary temperature, including cooling mechanism
c.    Pick the maximum PWB temperature for reliability requirements.
d.    Create a plot of various PWB sizes versus the maximum allowable power flux.
e.    Select the PWB size that matches the electronic component power flux.
B.    Example
a.    Let the housing be a normal box with CCAs input through the top and a lid screwed on.
b.    The boundary will be a conduction mount to a 71C base plate.
c.    Each component has a 10C rise above its mount and shall remain be low 100C.  Therefore, the maximum PWB temperature is 90C.
d.    The PWB has Flat-Pak components on the component side and a 0.062” aluminium frame on the circuit side.  The aluminium frame extends beyond the PWB to mount wedge-locks.
e.    The results are shown in Figure 1.

Figure 1   PWB Flux Requirements

These results were derived using CardTemp, a simple $400.00 software package for any Windows based platform.

III.    CONCLUSIONS

    Programming the heat conduction equation for a PWB in a particular housing allows easy access to various PWB temperatures based on PWB power dissipation.  Once the relationship between PWB size and power is known, the design has a robust concept.  It is now ready for detailed design by various experts.

REFERENCES

[1]    MIL-HDBK-217D, “Reliability Prediction of Electronic Equipment”, 15 Jan 1982.
[2]    MIL-HDBK-251, “Reliability/Design Thermal Applications”, 19 Jan 1978.
[3]    Cooling Techniques for Electronic Equipment, Dave S. Steinberg,
John Wiley & Sons, New York, 1980, ISBN 0-471-04403-2.
[4]    Principles of Heat Transfer, 2nd Ed., Frank Kreith, International Textbook Company,
Scranton, Pennsylvania, 1965, Library of Congress Catalog Card Number 65-16305.