by John Priday, Chief Technical Officer, Teledyne Labtech
As the power generated by microwave semiconductor devices and high-speed processors continues to increase, the printed circuit board (PCB) must play multiple roles: supplying power to the devices, passing the increasingly high-speed signals between devices with as little loss as possible, and aiding in moving heat generated by the devices to heat sinks.
In contemporary consumer electronics such as smartphones, the task of heat dissipation is not left solely to the PCB; areas of high heat concentration are also managed using pre-formed adhesive pads made from synthetic graphite, a good dissipator of heat (Figure 1). These are used to spread the heat energy over a wide area, reducing the temperature-induced stress on individual devices such as RF power amplifiers, maximizing performance and improving life expectancy.
Defense Systems Pose Unique Challenges
Some of the most acute heat management challenges are found in aerospace and defense applications that have stringent requirements for size, weight, and power (SWaP) that make heat management even more challenging. These applications typically sacrifice low cost to achieve the best combination of thermal management, signal fidelity, size, and weight.
Examples include GaN RF power amplifiers, phased array transmit/receive modules, and high-speed digital signal processing cards. The most common heat dissipation technique is the use of copper sheets as ground planes, but we wondered if the benefits of synthetic graphite could also be useful for SWaP-constrained, medium-power applications. If this proved to be true, and as many high-performance PC boards have four to six copper planes or even more, synthetic graphite could be a very effective, lightweight heat-mitigation tool. We decided to find out, and the results are promising.
Why Synthetic Graphite?
Synthetic graphite offers significant weight savings and benefits in terms of thermal management for PCBs that have medium-power RF and microwave devices mounted on them. It can distribute heat efficiently in the X-Y plane out to the perimeter of the board, where it is then extracted via clamps to a cold wall (Figure 2). The result is that semiconductor devices can operate at substantially lower stabilized temperatures, which can increase their mean time between failure (MTBF).
Synthetic graphite has excellent in-plane thermal conductivity of between 1500 to 1600 W/mK, about four times that of copper, and densities between 2.0 to 2.1 g/cm3, about one-quarter of copper. In summary, it’s four times lighter and transfers heat four times better than copper. It is available in thin sheets ranging from 10 μm to 40 μm in thickness and is typically supplied with a self-adhesive coating and on a carrier. It has an excellent ability to spread heat across its width (X, Y). However, it is much worse for transmitting heat in its third dimension (Z), where z-plane conductivity is about 5 W/mK down through the thickness of a PCB.
High-density active devices often mounted in QFN style packages can dissipate a significant amount of heat, so one of many roles of the PCB is to channel it from the underside of the semiconductor devices through to the chosen heatsinking scheme as efficiently as possible. This is generally achieved by mounting devices on thermal/ground pads to the top layer of the PC board,which is electrically and thermally connected to lower layers through plated through-hole (PTH) vias. Sometimes the vias are filled with solder but more often with thermally conductive filler.
To replace copper planes in a PC board sandwich, graphite would have to perform well in several areas to make it worthwhile. It would need to have a significant thermal benefit without adding weight, and the graphite planes would need to minimally impede the passage of microwave signals in their role as ground planes. It would also have to be mechanically robust and survive normal processing stresses during manufacture and use, and finally, it must be easy to process using normal PCB manufacturing techniques.
Among all these requirements, mechanical robustness was the most difficult, and Teledyne tried many different techniques to get the graphite layer to adhere. Thermal conductivity was often very high, but thermal stress tests could induce cracked vias or delamination. Continued development eventually resulted in a successful approach, which is described below.
An evaluation board was designed to allow each aspect of performance to be demonstrated. The board was produced in two versions, one using two graphite planes, the other nearly identical but using copper planes (Figure 3).
The most extreme stress experienced by almost all PCBs is during processing because the thermal shocks experienced by solder processing steps are extreme. Customers usually require PCBs to withstand two excursions from room temperature to solder temperature for double-sided boards and allow for the possibility that three more rework cycles will be required, which is noted as Condition A of the IPC-650-TM-2.6.8 test methods.
Tests were performed with 1 cycle of 10 seconds solder float at 288°C and then five cycles starting from ambient room temperature for each cycle, all of which were successfully passed. The only notable degradation seen was in the via filler that cracked after five thermal stresses, which was the likely result of coefficient of thermal expansion (CTE) mismatch but was not considered significant.
Having shown that graphite can be incorporated into a PCB and survive thermal shocks, the most important question is then whether it actually improves heat dissipation. To explore this, a power resistor was mounted on both PCBs using H20E conductive epoxy to act as a heat source. Thermal measurements were made using a Teledyne FLIR infrared camera. In both cases, the same power (6.7 W) was applied and allowed to stabilize for 5 min. The boards were suspended in the air with no additional heatsinking. The resistor was more than 20° C cooler in the graphite-plane case compared to the equivalent using copper planes. This would significantly improve the Mean Time Between Failure (MTBF) reliability of a semiconductor device in normal use.
For this new heat mitigation material to be useful, it could not substantially degrade signal integrity, so to evaluate microwave performance, the test structures had both stripline and microstrip test paths. To make measurements, 10-mm aluminum plates were bonded to the back of the PCBs to allow for mounting of SMA connector interfaces.
A vector network analyzer was used to conduct measurements from 5 to 20 GHz, and the results are shown in Figure 4 for both stripline and microstrip test structures. The graphite planes caused a maximum degradation in insertion loss of no more than 2 dB. This was for a 100-mm trace length, which is two to three times that of the typical total length of the signal path on most PC boards. For many applications, this performance will be more than acceptable. It should be noted that for results above 18 GHz, the stripline was not optimal because of the mismatch of the connector interface.
In the opinion of Teledyne Labtech, graphite plane PC boards have a great enough thermal advantage to make them an alternative to copper as they can allow components to run at least 20°C cooler. They also weigh less than their copper counterparts, particularly when more ground plane layers are employed. In their role as ground planes, they do not significantly impact the passage of microwave signals for 100mm path lengths in both stripline and microstrip.
The boards are mechanically robust and survive normal processing stresses incurred during manufacture and use and withstand the rigors of IPC-650-TM-2.6.8 thermal shock tests even after five cycles from room temperature to 288°C in 10 seconds with no cracked vias or layer delamination. They operate over the same temperature ranges as regular PC boards and use conventional manufacturing techniques, with only a modest increase in time and cost.
There are certain restrictions resulting from the need to maintain via integrity and we recommend early stage discussions with Teledyne Labtech to ensure all designs for manufacturing considerations are included.
KANEKA Corporation for supplying its synthetic Graphinity graphite sheets.