Printed Circuit Design & Fab - May 2008 - (Page 30)

LAMINATES Convection Radiation Radiation TABLE 1. The key properties of two standard and two thermally conductive materials compared. Key Properties Dk (10 GHz) Df (10 GHz) Tc (W/m-K) Z Tc (W/m-K) X-Y CTE (X-Y) ppm/oC CTE (Z) Copper Peel (lb/in) Moisture Absorption (%) TC600 6.15 0.0020 1.1 1.4 8 17 8 0.01 AD600 6.15 0.003 0.46 NA 10-11 45 10 0.04 Competitor A 6.15 0.003 0.43 NA 11-13 75 8 0.02 Competitor B 6.15 0.0027 0.63 NA 13-14 47 7 <0.1 Conduction FIGURE 2. Heat removed by the direct conduction method is most effective. 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 30 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 perMAY 2008 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - May 2008 Contents Our Line Market Watch Around the World Happenings ROI EMC For the Real World PCB East Conference Brochure Positive Plating Don't Let your Signals Stub Their Toes Improve PCB Layout With Skill Utility Programs The Next Generation Design Tool Challenge Thermally Conductive Microwave Materials PCB Dielectric Degradation in Lead-Free Assembly Applications A Tale of Two Trade Shows Eliminating Board Defects Off the Shelf Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - May 2008

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