The Embedded usage model is characterized as one that is operated at the extremes of available performance. Knowing that the targets of the thermal design power usage mode are only partially applicable, embedded designers must satisfy the customers’ innate desires for maximum throughput and level of service at the edges of a constrained thermo- mechanical environment.

Figure 1. Package styles, left to right: ThermallyEnhanced CSP SO-8, SOT-23 and SO-8
The combined dimensions of operating temperature range, shock, vibration and mechanical constraints define the thermo-mechanical envelope. In the case of a circuit card, this operating temperature may run from -40°C to +85°C, which represents the free-air ambient temperature within an enclosure. Outside the enclosure, it is not uncommon to share the -40°C operational temperature with the inner circuitry, however, the upper limits are consistently in the +55°C to +75°C range with a larger number of deployments trending toward the top of this range and beyond. Storage and shipping temperature ranges also need to be considered, with - 55°C at the bottom end and +85°C as the upper limit.

While the mechanical envelope must handle the physical form factor and the necessary interconnects, mechanical stress must also be considered. From the MIL-STD-801F guidelines, four operational tests present challenges that exceed the shipping and handling stresses imposed by Method 514.5 of the same standard. These requirements, when superimposed upon the thermal aspects, create a formidable challenge for any SBC and peripheral mix.

The Computation Power Function

The process of electronic computation pushes electrons from place to place over resistive connections and internal devices that consume electrical potential, converting it to thermal energy as a byproduct of operation at a conversion rate that approaches 100%. Interconnects can transmit some energy out of the system and to a receiver, but the transmitted energy of an RS-422 or 232 interlink is infinitesimal compared to the thermal output of a CPU or GPU. Ethernet and optical interfaces are less efficient, as they consume significant power in the physical interface (PHY) before producing a waveform with mere milliwatts of energy. Such bi-directional communications dissipate received energy, often offsetting transmitted power. The exception to this rule is powered USB, which can consume and transmit ≥2.5W per port to an external peripheral with low loss, and therefore must be considered in design of the input power channel. Only the energy transmission losses will add to the thermal profile.

Practical power dissipation methodologies consider the total system input power as the total thermal output of the system, regardless of transmitted waveform energies. It is for this reason that efforts at CPU power reduction are at the center of low-power designs. Such an approach also includes the inefficiencies that exist in the numerous DC/DC converters ubiquitous on modern designs that regularly produce and regulate half a dozen voltage rails, all derived from a traditional 5V supply. High current CPU and GPU power subsystems may require input power at a higher potential to combat delivery and distribution losses, at the expense of converter efficiency.

Design Strategies and Constraints

Table 1.
Balancing physical size, ruggedness and power consumption is a three-dimensional tightrope. While low power consumption is commonly the primary driving factor, package choice or increased circuit area may yield significant benefits in operational power consumption. Silicon devices in small plastic packages may consume more power than a larger device featuring thermal attachment that keeps junction temperature comparatively lower. Thermally enhanced SO-8 packages have become the norm for external power-FET pairs at the core of synchronous-Buck converters (Figure 1). Increased copper area may be deployed to reduce resistance (Figure 2) enabling operation at elevated temperature.

Keeping semiconductor devices cool is a key to managing the negative temperature coefficient of silicon, and helps meet performance constraints. Compared to an onboard passive/radiative thermal solution, a small, lightweight heatsink and fan combination usually reduces system power consumption by more than the electrical load of the fan itself. Conduction cooling methods trade increased system mass for power reduction, improved service interval and greater overall reliability by elimination of frictional wear and contamination.

High-power CPU and support chips make holistic board cooling impractical, requiring an external thermal pathway. The method of attachment for such a thermal pathway should be considered at the same time that mechanical mounting holes and board topology are defined. A thermal attachment point may be problematic because it interferes with high-speed chip-to-chip interconnects, yet may be mitigated by judicious routing. Incorporation of thermal attachment points after layout and routing can be highly problematic unless only adhesive solutions are under consideration.

Internal thermal structures within the circuit card, normally designed to enhance device-to-board thermal transfer, can be exploited. Any conceivable structure, from a multilayer internal heat spreader to repetitive stitching of thermal attach points to ground planes can be leveraged (Figure 3). Excess capacity in the cooling pathway of high power devices helps to reduce board temperature through reversal of the thermal transfer of any enhanced device-to-board thermal pathway, allowing energy from nearby components to be dissipated through the cooled device.

When the environmental envelope precludes the use of mechanical forced convection, a thermal pathway from the source to the outer surface of the enclosure is necessary to satisfy the thermal aspects of the operational envelope. Minimizing the temperature differential across each thermal interface is highly desirable, to the point that rugged design solutions will incorporate mechanical and thermal mounting into a single metal structure that may also function as the chassis wall. While this eliminates a thermal interface, it may not be truly necessary or desired. Large output areas and attachment pressure greatly reduce the thermal differential required to transfer the requisite energy, and provide an opportunity to electrically isolate the thermal attachment. It is not uncommon for low resistance connections to DC-ground to occur from either the thermal attachment to the board or to passivated die paddles in FCBGA (Flip Chip Ball Grid Array) and will conduct to a thermal solution if chipped, cracked or damaged.

Simplified Thermal Design

Figure 2. Dual-phase CPU core supply, high current multi-layer stitched pathway. Note the gold padsfor attachment of an “over-wire” to further reduce resistance, if necessary.
Thermal design often revolves around figures with units in W.°K-1.m-1, which represents the thermal conductivity of a material. To be useful, the number of degrees of temperature differential and the ratio of area to thickness must be known. Conversely, knowledge of the thermal flux and geometry can be used to compute the resulting thermal gradient. However, for thermal transmission using a given material, the efficacy of a given transmission solution can be determined based entirely on the geometric area of the input and output.

Consider the Intel Dothan 1.4GHz CPU in an exposed flip-chip package. The area of the passivated silicon die (0.490" × 0.275") is approximately 0.13475in2 and represents the maximal window through which heat will flow into a receiving material. The diagonal dimension of the oblong die is just under 0.562" requiring a circular thermal pick-up of at least this diameter in order to make full contact with the exposed die paddle. The choice of material in contact with the die will influence the performance characteristics of the thermal solution. For example, 11000 Copper (388 W·°K-1·m-1) will conduct >230% more heat per unit area than 6061-T6 Aluminum (167 W·°K-1·m-1).

If a copper interface were utilized to pick up thermal energy from a Dothan CPU, and this copper were interference-fit into an aluminum spreader, what thickness of aluminum must be fit to the copper slug to produce 100% thermal transfer from the copper pickup to the aluminum carrier?

Interference-fit copper and aluminum creates very high contact pressures and a nearly ideal transfer situation occurs. Geometry suggests that when the area of the right-hand contact cylinder approaches 230% of the input area, that full thermal transfer will occur, and that beyond this there is no substantial gain in the performance of the solution. To examine this hypothesis, test jigs were constructed using 0.735" diameter 110 alloy copper and 6061 aluminum, and compared to a fully-plugged and co-milled construction of the same material (Figure 4).

Testing was performed by using the same integrated fan-sink unit on all three solutions, connected to a control unit based on an Intel Pentium-M 738 CPU, and driven by Intel’s Dothan MaxPower tool. A phase change thermal interface compound was used between the jig and the integrated fan-sink, and a micronized silver interface compound was used between the die paddle and jig.

In situations where a heat-spreader is in contact with an external case, the required case thickness is a function of the input material and area. Using the 0.13475in2 Dothan example, a 6061 aluminum spreader with output dimensions of approximately 3.50" × 1.88", the 10.76" perimeter achieves 100% of input contact at 0.0125" material thickness when the contact outline is propagated through a 6061 aluminum case wall. Standard 1⁄16" material exceeds this critical thickness by >450%. Large diameter fasteners in such a brick improve contact pressure and provide an opportunity for electrical isolation, if necessary, using thermal compound treated insulating sheets and insulating washers similar to that which is used for TO-3 style power devices.

Enhancements and Benefits

Figure 3. An integrated heat spreader with thermalcontact busses. Such milling can be directlyincorporated into a custom chassis shouldthe need arise, and the contact bosses can beenhanced, as necessary.
In low power applications, boards may be designed to offer thermal attachment at the edges as part of the design standard, making through-the-board cooling an effective option. Designers who opt for such cooling need to be acutely aware of the need to isolate digital and chassis grounds as well. Structures such as interleaved cooling pick-up layers parallel to thermally enhanced ground and power planes can reduce the thermal resistance of FR-4 material to manageable levels. If the component area requirements are below 133% of the square area, such designs are mechanically feasible, without special DC-ground to chassis isolation materials.

While it may be possible to design thermal interfaces and solutions for the average power case, absence of a thermal reservoir to store the excess energy until it can be effectively dissipated will limit the operational performance of the system. Designing the thermal pathways and solutions to accommodate full power operation will ultimately allow the end user to fully consume all available CPU capacity. This brings home the need to consider all power conservation methods, which begin by adopting the lowest power, widest temperature range components available.

A final electrical enhancement is to minimize the strength of pull-up resistors in the design, reducing quiescent and active currents to the minimum necessary while maintaining required signal integrity across the operational temperature range. Such an optimization may seem inconsequential individually, but when applied across buses and active-low, open drain driven signals, the total reduction power consumption, and associated thermal output, differentiates seemingly identical implementations from one another.

Summary

Figure 4. Family of interference fit, copper enhanced empirical test jigs constructed for direct thermalcharacterization.
Consistent application of these techniques enables the production of systems that are capable of thriving within the constraints of a thermo-mechanical envelope. When components specified to handle the desired operational envelope are not readily available, empirical testing is required. Through the added value process of thermal screening each board produced, only units that survive the thermal envelope proceed through manufacture, which may include bonding of high-mass components and underfill of BGAs, as required to survive shock and vibration constraints.

Modern high-density packages often result in limits on the service model, reducing the options for component-level service and repair. Customer requirements and adhesive enhancement result in similar constraints, such that the service model becomes one of board-level replacement. Adhesive enhancement improves the component-to- board thermal pathway and when coupled with precision thermal interface, aids in the control of thermal stress. The final result is greater delivered reliability with a lower total cost of ownership.

This article was written by Martin Mayer, Department Head, Research and Development, ADL Embedded Solutions (San Diego, CA). For more information, Click Here " target="_blank" rel="noopener noreferrer">http://info.hotims.com/34459-500.