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What technology creates the biggest challenge for cooling and thermal management?

Wed, 05/28/2014 - 9:13am

Andrew Smail, Product & Solutions Marketing Manager, Software & Modular Solutions Division, Agilent Technologies

With the introduction of 4G LTE technology, mobile communications were able to achieve higher data throughput than ever before. However that achievement is accompanied by unprecedented battery requirements for mobile terminals. LTE employs the SC-FDMA modulation format in the uplink and it has a higher Peak-to-Average Ratio (PAR) than W-CDMA. One of the most power-hungry components of a mobile terminal is the power amplifier (PA). The power level of the LTE uplink signal stays relatively low most of the time and goes to peak power only occasionally, but the PA is designed to deliver the highest efficiency only at peak power. Since the high power supplied to the PA won’t be used most of the time, it is mostly dissipated as heat and causes battery drain, impacting the thermal design power (TDP) of the mobile terminals.

Envelope tracking (ET) has come to the fore¬front as a possible solution for this issue in mobile RF front end design. Agilent’s RF power amplifier test  Reference Solution, which includes a graphical user interface (GUI) program for easy solution evaluation, example source code optimized for speed, extremely fast modulation analysis, and excellent accuracy and measurement repeatability, uses envelope tracking techniques, which dynami¬cally adjusts DC supply voltage based on the “envelope” of the PA input signal and delivers higher voltages only when needed, improving battery consumption and heat dissipation in the PA.

Matt Burns, global IP&E technical marketing specialist, Avnet Electronics Marketing

FPGAs and SOCs with integrated programmable logic provide unique challenges for appropriate coiling and thermal management.  There are two reasons.  First, the configurability and programmability of FPGAs and SOCs with integrated programmable logic makes thermal management and cooling dependent on device utilization and operating speeds.  With MCUs, DSPs and other fixed function semiconductors, this is not a large concern.

Secondly, the large variety of FPGA packages (lidless, ceramic, flip-chip, BGA, QFP, etc.) forces engineers to consider several design parameters when choosing the proper thermal management scheme.  For example, consider TIM selection for FPGA.  Standard double-sided tape may work on a BGA or QFP package, but it won’t work on a lidless flip-chip package.  How do I secure my heat sink?  Thermal epoxy? Thermal grease? Do I even need a TIM?  With more and more OEMs not having thermal experts, FPGA designers are often making these choices without the appropriate knowledge base.

Fortunately, the FPGA manufacturers provide needed assistance.  They provide power estimation tools and thermal design guides for selecting the right cooling solution.  Heat sink manufacturers are increasingly providing selection guides pairing their solutions with standard FPGA packages.  Distributors are assisting too with instructional videos, white papers and other technical tools to choose the right cooling solution for the right FPGA.

Doug Edwards, Market Manager

A big challenge for thermal and mechanical engineers in many markets is the ongoing increased integration of functions into smaller and smaller form factors.  Reduced device geometries in ICs and increased integration of various functions within a given module results in ever-increasing power densities. Constant pressure to reduce costs while still ensuring consistent performance and high reliability of the product also adds to the challenge of thermal management.

Advanced process technologies allow smaller devices and lower power, but they also allow chip designers to pack more transistors and functionality into a smaller silicon area, often resulting in the chip dissipating the same power as a previous version, but in a smaller footprint.

Continuing the trend on the module level often results in multiple ICs interfacing to a common heat sink, leaving a variety of air gap thicknesses to fill in order to dissipate the heat,  in ever-thinner stack-ups. Compounding this challenge is pressure to reduce manufacturing/materials costs where varying gap widths can have a tolerance of up to +/- 30%. Thinner circuit boards may be cheaper, but they also cannot withstand much mechanical stress.

Dispensable liquid gap fillers that cure in place can fill a variety of gap thicknesses, can absorb very high percentages of manufacturing tolerances, put little mechanical stress on assemblies, have exceptional reliability, be cost-effective, and be re-workable.

Bruce Bolliger, Head of the Semiconductor Business, Element Six

Many types of devices have significant thermal management challenges, including laser diodes, processor cores, and even some ASIC’s. However, high-power gallium nitride (GaN) RF power amplifiers present the highest power densities, having tiny hot spots with heat fluxes ten times that of the sun’s surface. RF design engineers have struggled to surpass thermal barriers in achieving the intrinsic performance of GaN; typically, when a GaN device delivers very high RF power, lifetime degrades as heat is unable to sufficiently evacuate the device’s channel.

Chemical-vapor-deposition (CVD) synthetic diamond can address these thermal management challenges because of its extremely high thermal conductivity, up to 5 times higher than copper. Element Six offers two potential methods of using CVD diamond to address the thermal challenges of GaN RF devices: metallized diamond heat spreaders and GaN-on-diamond wafer substrates.

Diamond heat spreaders rapidly pull heat away from GaN die by mounting these die on top of a 300-micron thick diamond piece laterally only slightly bigger than the die. Diamond heat spreaders can reduce gate temperatures by more than 20 percent. The key is to ensure the interface between the heat spreader and die has the lowest thermal resistance possible by:

• Polishing the diamond to enable very thin metal layers between die and diamond;
• Sputtering 100-nm-thick layers of metal onto the diamond, using Ti to form a carbide with to prevent delamination and Au, with a barrier layer, to enable effective soldering; and
• Using solder only up to 10 microns thick that is compliant enough to handle any CTE mismatch without increasing thermal resistance substantially.

To reduce gate temperatures even further, GaN RF devices can be made using GaN-on-diamond wafer substrates. These substrates bring diamond to within less than a micron of the gate junction by growing the diamond on a 30-nm-thick interfacial layer at the GaN buffer. Not only does the GaN-on-diamond substrate lower the gate temperature by as much as 50 percent, but it also enables more than 3 times the power density of GaN/SiC devices. So for example, the GaN RF device can be more than 3 times smaller with the same power output, allowing 3 times more devices on a wafer as it goes through the GaN fab.

Both of these CVD diamond solutions effectively manage the thermal challenges of high-power GaN RF devices. Heat spreaders work well with existing GaN devices while GaN-on-diamond substrates maximize power density for new GaN RF devices.

Wilson Lee, Newark element14

As far as products are concerned, all design engineers are under pressure to create solutions that are smaller, more powerful, and can operate faster. In a way these three elements are the perfect storm relative to all of the thermal management issues engineers need to work through.

Three product ranges come to mind regarding the challenges of cooling and thermal management. They are, in no particular order: microcontrollers, particularly in applications such as high-end servers and data rooms; power transistors and IGBTs, particularly in applications such as solar and wind inverters; and LEDs, in a variety of applications like lighting and instrumentation.

In each of these three product ranges, what are consistent in the design requirements are higher densities of power, smaller board design needs and the characteristic of “always being on.”  Many design considerations for each can be aided by the following trends:

The first is the growing focus on device efficiency rating vs. any other electrical specification. No longer are designers so focused on output power in lumens or watts, but efficiency’s impact to (lower) generation of heat, and (lower) overall design cost. The designs which are winning production orders today are taking device efficiency as first order of design.

Second, there are a number of newer technologies which are more robust in dissipating heat. Newer technologies like gallium nitride and silicon carbide – which are more robust in handling more power density – are newer options for design engineers to manage through traditional thermal issues caused by unknowingly overdriving product. Silicon carbide diodes come to mind in today’s power applications, and we see more and more of our customers coming to us for them.

Lastly, there are thermal cooling techniques. More and more designs use liquid cooling (chill plates) vs. the traditional force air cooling. There is also the transition to newer conductive heat sink materials such as carbon and graphene, which have much better dissipative properties at higher temperatures than traditional aluminum or copper heat sinks.

David Luna, VP of Marketing at Orion Fans

Higher system clock speeds and higher density physical packaging are creating significant challenges for effective and cost-efficient cooling and thermal management.  In fact, this trend has led designers to add fans to systems that already utilize passive cooling technologies, such as heat sinks and heat spreaders.

Fans offer the solutions to increase heat dissipation in smaller system packages, but can also burden the designer with problems such as mechanical fan failures, increased acoustic noise, and increased power consumption by the fan itself. Due to a system’s reliance on the removal of heat by the fan, it is important for the cooling solution to have a fault detection mechanism.

System designers who do not address the management of the fans within the system are subject to increased costs due to increased down time and field replacements. The customer’s perception of the overall system quality and reliability of the equipment is also adversely affected.

AC Fans, DC Fans and blowers are now available with features that address the problems of noise, reliability, and power consumption. Fan functions, such as smart temperature controls, can reduce power consumption by as much 30% while reducing noise. Air flow monitors and locked rotor alarms provide alerts when fans are not at optimum performance to ensure a reliable cooling solution that meets most any application requirement.
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