Thermal Management
Material considerations
Dave Hunton looks to simplify the problems that regularly confront engineers and buyers when it comes to selecting the correct thermal laminate
T
he Thermal PCB industry has recently experienced an onslaught of alternative laminate options. This has
resulted in inaccuracies and lack of accurate detail with respect to the most suitable choice of materials. Although metal substrate technology has been available to the electronics industry for over 40 years the recent rapid increase in their use is causing some concerns. Thermal laminates sought to provide a solution for the increase in watt densities offered up by advancements in power component development. The concept of the system was simple: it offered a circuit printed on a heat spreading plate material, usually aluminium. These would be assembled, tested and attached to a heat sink or direct to the chassis of the product for final heat dispersal through convection. This was the ideal solution for electronic designers in the Motor Drives, brick power conversion modules. The materials are referred to by many names including IMS (insulated metal substrates), MPCB ( Metal printed circuit board ) or Aluminium Board, IMPCB (insulated metal printed circuit board), MCB (metal clad board), TPCB (Thermal Printed Circuit Board ) and ‘metal backed’, all are commonly used.
Designers in the Motor Drive, Automotive and Power Conversion markets work closely with the material designers to achieve the optimum balance between thermal efficiency and electrical isolation. There has always been a degree of compromise. When higher voltages or high current carrying is required the dielectric thickness is increased, in this way the dielectric will maintain its strength in
12 March 2012
operation. As a general rule one would use the thinnest dielectric suitable for the voltage with an additional margin for safety.
So what are the considerations that need to be taken into account when using the materials and the key functions of the layers? The base material in most cases is Aluminium as this offers a low cost metal with good thermal conductivity, which is light in weight and available in a range of thicknesses. The laminate suppliers offer a choice of alloy grades, harder material for machining and softer grades for punch tool processing.
Copper base material can be used when the design calls for higher thermal demands as copper has a superior conductivity level. If ceramic devices are to be used copper offers a closer CTE match (co-efficient of thermal expansion) so less stress is induced in the device through constant thermal cycling. Copper base materials can also be used to selectively ground parts of the circuit to the base by drill & plate technology. Copper is the more expensive choice and its weight can limit its suitability in some applications. Aluminium accounts for around 94% of all usage, copper base a further 5% and other special application metals (e.g. steel & iron) account for the final 1%. The top layer would be copper foil electro deposited or rolled, depending on the copper weight. Conductors tend to be thicker than those in traditional FR4 boards. Current carrying capacity and thermal spread are key design considerations, 70 & 105 micron foils are the norm and the copper is etched to create the conductor pattern.
Components in Electronics The thicker the dielectric the less
effective it is at passing heat, but thicker material increases the safety margin for electrical isolation.
Fillers added to the dielectric increase the thermal transfer. Add too much filler and the dielectric will weaken and the peel strength of the copper conductors deteriorates. Generally speaking the dielectic layers in thermal materials are not the traditional reinforced glass weave materials as these decrease the thermal performance dramatically. Compared to the base plate and the copper conductor layer where the metals move the heat quickly with conductivity levels of between 200 & 400 W/mK, the dielectric would have a typical range of 1-3 W/mK - so this presents a real heat barrier, compare this to traditional FR4 materials down at 0.25 W/mK and you can see the benefits of the thermal enhancements and in keeping the layer thin.
The LED boom Growing demand for thermal laminates in recent years is being fuelled by the boom in the LED industry. Light emitting diodes are essentially a power semi-conductor and generate heat. Unlike a filament bulb, where most heat is lost through radiation, the source of the heat is buried within the device and must be conducted through the chip into the heat slug and down through the circuit and laminate, for disposal by the heat sink. The materials required for cooling the new LED devices were already in existence - these thermal laminates, being proven with a track record of quality and performance, could be used immediately. Initially the board prices were viewed as
high. With limited capacity and a price expectation from an industry where higher thermal and electrical demands were the norm, the LED market is demanding higher volumes, higher capacity and lower prices. As the power demands are not the same as in the power industry, the LED
industry could accept lower performance materials. With market growth and more competition amongst material suppliers for increased market share, suppliers without the expert knowledge in thermal chemistry have entered the supply chain. A basic competence level has allowed offerings of laminates that are thermally enhanced with a lower ‘fit for function’ specification. The forecasted growth has fuelled an air of excitement, with traditional or new laminate suppliers adding metal backed to their portfolio. At present DK Thermal are aware of over 40 brands that are now available. As a niche market this doesn’t compare well to the traditional FR4 markets.
Track record
The traditional laminate suppliers have spent years in R+D working on dielectric recipes to meet certain criteria that solved engineering problems in removing heat from devices. Not all materials are the same and some unique selling points are available to certain laminates. There is a plethora of new board suppliers, generally from Asia, using Asian laminates through Asian board shops that mostly sell on cost and perceived performance. They have technology roadmaps that claim they are working on laminates with 5 - 10W/mK performance and that these laminates will be available in the next few years. The focus is on thermal conductivity, when in reality the thermal resistance and dielectric integrity are far more important. The performance figures stated as fact on some data sheets should be taken with a pinch of salt and, until there is a globally accepted criteria for the measurement of the values, spurious claims will exist.
Trade off There is an attraction to mass produce the materials by using traditional sheet lamination techniques with thermally
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