Pushing the Boundaries of Thermal Efficiency

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Optimizing Electronic Packaging

At Advanced Thermal Engineering, we specialize in optimizing electronic packaging based on thermal requirements. Our expertise in thermal management enables us to address critical factors such as the size of printed wiring boards (PWBs), circuit card assemblies (CCAs), housing, power dissipation, external boundary temperature, and hottest PWB temperature to ensure efficient thermal performance.

Ensuring Successful Mechanical Design

By integrating thermal guidelines into our design process, we pave the way for successful mechanical design. Through meticulous trade studies encompassing PWB size, housing, power, and external boundary temperature, we deliver circuit card assemblies that align with electronic component maximum junction temperature requirements.

Partnering for Cost Savings

Our commitment to precision and optimization translates into tangible benefits for electronic box manufacturers. By streamlining the thermal concept and design process, we help our partners save valuable time and resources, ultimately contributing to enhanced efficiency and cost savings.

Electronic Packaging Based on Thermal Requirements

Abstract—There is a connection between the size of a printed wiring board (PWB), its circuit card assembly (CCA), housing, power dissipation, external boundary temperature, and its hottest PWB temperature. A simple software package can solve this very quickly.
Knowing this general guideline allows the thermal concept to advance to a successful mechanical design. Trade studies of PWB size, housing, power, and external boundary temperature can produce a CCA that will satisfy the electronic component maximum junction temperature. This saves time and money for the electronic box manufacturer.

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 guidelines 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 analyzer to solve the heat conduction equation. The shapes in the CAD model are fixed during creation. Hence, there is no way to adjust the lengths after surface input. A new CAD model must be created if a new geometry is needed. 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 using 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. Somewhere is the happy medium where the PWB is reasonably sized, and the electronic components can be closely packed.
This concept can be checked by the various experts who are 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 the 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 below 100C. Therefore, the maximum PWB temperature is 90C.
d. The PWB has Flat-Pak components on the component side and a 0.062” aluminum frame on the circuit side. The aluminum frame extends beyond the PWB to mount wedge-locks.
e. The results are shown in Figure 1.

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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. Utilizing MILHDBK-251 methodology, CardTemp provides guidance similar to IEEE CPMT TC-9 and ASME K-16. Note that CardTemp is listed on the System Reliability Center website.

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.

About Advanced Thermal Engineering

CardTemp is designed for electrical engineers. It allows him to perform trade studies on the thermal concept as he decides how many circuits will fit on one PWB. Now, he is prepared to share his concept with other experts to evaluate a new design. This process builds a team spirit for success.
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