Increasing the thermal conductivity of the base materials improves thermal management in RF designs.

Looking at the thermal issues which are impacting microwave design in today’s marketplace, we see several drivers that will continue to require better and more effective thermal management in the near future. Some of these factors are relevant to all electronics packaging, such as increased watt density on smaller package footprints, but others are of particular importance to the microwave and RF industries. Among the more important considerations for the microwave engineer are:

• Power density (Watts/square inch of PCB area) continues to increase
• Packaging is getting smaller & hotter (inherently increasing watt density still more)
• Tower mounted & outdoor electronics increase environmental exposure while requiring higher reliability
• Complex waveforms decrease amplifier efficiency, resulting in more energy lost to heat
• Higher temperatures reduce component reliability
• Dielectric constant of many materials varies significantly with temperature
  

When the heat generated by an active device mounted on a PCB cannot be dissipated, then the temperature of the device rises. With all the drivers pointing toward more power and smaller board surface area, the watt density becomes such that temperatures can easily exceed the qualificated temperature for many of the individual components. Since reliability of components follows a first order Arrhenius equation, a 10ËšC increases in temperature doubles failure rate, so even 1ËšC temperature increase matters. Figure 1 shows data presented by Eli Reese of TriQuint, and presented at IWPC in November 2005, relating junction temperature to mean time-to-failure. The key to reducing junction temperature is to increase the rate at which heat can be removed from the device and from the working area of the PCB immediately adjacent to the device.

The cost of device failure can be significant since device failure accounts for as much as 70 to 90% of equipment downtime, and in the case of tower-mounted electronics, the cost can run several thousand dollars for a simple repair that results in a short downtime. In bad weather this can extend to a full day and cost as much as $25,000. Between actual repair costs and potential lost revenue the implications of this trend towards increased operating temperatures of PCBs is substantial. The industry is looking seriously at solutions that can provide a cost-effective way to mitigate the risk of failure.

Heat is generated every time an active device is in operation. Heat itself does not become a problem until there is an increase in temperature above a critical point (typically 105 to 120ËšF). In simple terms, the whole business of managing heat in a PCB assembly is about preventing the junction temperature from getting high enough to “fry” the active devices.

Heat is moved by one of three basic modes: conduction, convection or radiation. In a PCB assembly all three are in effect to one degree or another. The most effective is direct conduction, where a warmer body is in direct contact with a cooler one, and the heat moves from the hot to cold material (Figure 2). The rate at which heat is carried from one to the other depends on the thermal gradient, the coefficient of heat transfer of both materials, the quality of the interface and to a lesser extent, the heat capacity of the cooler body – that is its ability to absorb heat.

Convection is the transfer of heat from a hot body to a cooler gas or liquid, which carries the heat away. Convection may be aided by forcing the cooling gas or liquid to flow past the warm body, removing the heat more quickly. Radiation is the removal of heat from a body by the emission of energy in the form of electromagnetic radiation, which may be in the infrared (heat) or even visible (light) parts of the spectrum, depending on the temperature of the radiating body.

Corollary to consideration of the thermal conductivity (Tc) of laminate substrate materials is also the fact that the heat transfer coefficient in the perpendicular direction (through the PCB) is different from that in the plane of the board. Unfortunately most modeling software assumes isotropy, which may result in over or underestimation of the heat removal.

Traditionally, a number of approaches have been put forward to reduce temperature in PCBs by removal of heat from active devices. The IWPC has been actively involved in assessment and discussion of the various thermal management techniques, which include the following:

• Laminated heavy metal backplanes (i.e. copper, aluminum, brass from 1 to 10 mm in thickness)
• Electrically and thermally conductive adhesion to plate (a good reference here is Ruwel)
• Thermal vias
• Thermal coins (a good example is the Merix E-Coin)
• Heat sinks, heat spreaders, heat risers (good references found at GrafTech)
• Thermally conductive adhesives, gap fillers, grease, etc.
• Active cooling – forced air, conditioned air
• Water cooled, vapor cooled, direct, indirect
  

While all of these methods (note the predominance of the more efficient conductive approach) are effective to varying degrees in the reduction of junction temperatures, many of them have attributes that make them less than ideal for use in microwave or RF circuit boards. Thermal vias (a clusters of plated-through holes located beneath an active device) are effective in removing heat because of the high thermal conductivity of copper (370-400 W/m-K) but the plated through holes near the signal traces can affect signal integrity, and a large number of plated through holes can affect the mechanical strength of the PCB itself.

Heat sinks and heat spreaders are heavy and expensive (a 3mm copper plate can add as much as $25 to a single PCB) and devices such as thermal coins that require cutouts result in increased fabrication complexity and assembly costs. More sophisticated approaches such as forced convection are costly, and may themselves be subject to risk of electromechanical failure.

An attractive alternative to total reliance on the traditional approaches of thermal management is to build the underlying PCB with a dielectric material that is inherently more thermally conductive. Conventional PCB materials having a thermal conductivity of 0.2 to 0.25 W/m-K, and filled products range from 0.4 to 0.6 W/m-K do not provide enough heat removal and heat spreading capacity to do the job alone.

There are new materials entering the market that have a 6.15 Dk and are suitable for use as microwave substrate materials. These materials can provide thermal conductivity (Tc) of 1.1 W/m-K perpendicular to the plane of the board, and 1.4 W/m-K in-plane. This is more than double the best thermal conductivity currently available in a standard microwave PCB substrate materials, and high enough to allow designers to reduce dependency on more costly approaches.

What benefit does increased thermal conductivity of microwave PCB substrates offer to the PCB designer?

• Component and solder joint reliability improvements would drive down warranty costs
  
• At constant heat rise, the improvement in heat transfer can be used to increase power handling by as much as 5-10%
  Thermal stability of dielectric constant reduces dead bandwidth and increases phase stability over the temperature range reducing design limits and complexity
  
• It compliments all other alternative sources of thermal heat extraction at no additional cost
  
• Alternatively, it potentially simplifies or lowers costs of other thermal solutions, such as facilitating cast vs. machined heat sinks, and reduces in copper plate thickness from 3mm to 1mm
  

The design objectives for the development of these thermally conductive materials included:

• Maintaining the current material cost structure while improving dielectric loss (loss tangent) and insertion loss to minimize heat generation in signal transmission
  
• Increasing base laminate thermal conductivity (both perpendicular and in-plane) to reduce heat generation
  
• Maintaining laminate integrity in terms of key properties such as copper peel adhesion and low water absorption.
  

The specific target products were to produced as 6.15 and 3.50 Dk materials to meet the predominant industry demand for RF amplifier designs. Table 1 compares the key properties of two standard and two thermally enhanced conductive materials.

The unique chemistry of the engineered laminate material includes a significantly reduced TCEr (thermal coefficient of dielectric constant) as seen in Figure 3. This translates into a more stable signal within any range of temperature fluctuation.

The improvements of this thermally enhanced material include:

• Temperature insensitive materials help amplifier and antenna designers minimize dead bandwidth, which is lost to dielectric constant drift as operating temperature changes
  
• For antenna designs, a significant shift in resonance frequency and bandwidth roll off at specific frequencies, results in lower gain performance, and
  
• Thermal stability is critical to phase sensitive devices, such as impedance network transformers, utilized for matching networks of power amplifiers
  

To illustrate the impact of thermal conductivity both in the perpendicular direction, as indicated by reduced temperatures measured at the active device, and in-plane, as determined by the degree of heat spreading, a sample circuit based on an RF power field effect transistor was developed and thermograms obtained using thermally conductive laminate and a standard material with a thermal conductivity of 0.46.

The results were significant both in terms of the heat reduction at the top surface and the spreading of the heat in the plane of the board. The resulting thermograms dramatically illustrate the effect of increased thermal conductivity. The maximum temperature reached at constant wattage was 82ËšC for the conventional material, 73ËšC for the thermally enhanced material, a reduction of 10% and more importantly, a reduction of 9ËšC that could be the difference between life and death for sensitive junctions. (Figures 4a and 4b).

Summary

Traditional laminates provide the greatest resistance (insulation) to heat transfer. Increasing thermal conductivity of the base materials is a new approach to thermal management in RF designs, with negligible expected cost implications or impact on signal integrity. These materials can provide improved component reliability as a result of improved solder joint reliability. There is less work hardening, less thermal expansion from lower retained heat and the materials have an inherently lower CTE. Products designed with these materials will see improved power handling, and these new thermally enhanced laminates complement other more traditional thermal management tools. While designed to meet the demands of RF circuits, these materials can also be used as a replacement for traditional high performance FR-4 substrates where thermal management is a critical design constraint. PCD&F

Russ Hornung is technical marketing manager at Arlon; This email address is being protected from spambots. You need JavaScript enabled to view it.. Mike Smith is VP of marketing and R&D, also with Arlon, and can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it.. Chet Guiles is an Arlon retiree and industry consultant, and can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..