Material Considerations when Selecting a Thermal Laminate
Dave Hunton, Technical Manager at DK Thermal Solutions, simplifies the problems facing Engineers and Buyers of Thermal PCBs
The 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, Technical Manager Dave Hunton, who has been active in these products since they became commercially available, explains why the recent rapid increase in their use may cause some concern.
History & Development
Initially developed in Japan and the United States, 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,
Automotive and Power Conversion industries. It allows higher heat dissipation from a given area hence now making smaller more compact products possible. The practice of soldering devices straight to the surface gives lower profile designs and these materials soon became common place in full and half
brick power conversion modules. The materials are referred to by many names: IMS (insulated metal substrates) is a term created by The Bergquist Company that has become the most common generic phrase in North America.
In Asia the common term is 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’ are also common.
In Europe we don’t seem to have settled on a term, different countries using different variants on the theme – well I suppose that’s what makes us Europe!
The laminate suppliers also have their own trade names: T-Clad or thermal clad is the Bergquist brand name, T-Lam from the Laird company and Hitt Plate from Denka - to name but a few.
The Compromise struggle
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 compromise struggle as these two design elements fight each other. When higher voltages or high current carrying is required the dielectric thickness is increased in this way the dielectric will maintain its strength in operation. As a general rule one would use the thinnest dielectric suitable for the voltage withstand with a further safety margin.
The skills and chemical knowledge to produce the laminated materials has until recently been the domain of specialised companies like The Bergquist Company, Thermagon (now Laird) in the USA and in Japan Denka and NRK.
In recent years we have seen the start of the LED revolution dramatically increasing the demands for volume and lower cost metal boards the industry is changing but is it changing in a controlled manner?
So let’s look more in depth at the considerations 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 does have its downside as it is a more expensive choice and its weight can limit its suitability in certain applications.
Aluminium accounts for around 94% of all usage, copper base a further 5% and other special application metals (eg: 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.
The filling in the sandwich is the dielectric. Here we have the constant battle....heat removal or electrical insulation - which do you want?
Well… it’s a balance - 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 keeping the layer thin.
The reputation and integrity of thermal management material companies is built on pushing this thermal envelope but maintaining the other key properties that offer the long term reliability
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 now demanded higher volumes, higher capacity and lower prices. As the power demands are not the same 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 the time of writing this article DK Thermal are aware of over 40 brands now available. As a niche market this doesn’t compare well to the traditional FR4 markets.
Every few months seems to bring a new supplier name or a new laminate grade but how qualified are these?
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. At DK we understood that not all materials were the same and that some unique selling points were 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, as if it is a magic number, when in reality the thermal resistance and dielectric integrity are far more important.
Ask yourselves this question: the material specialists have taken 40+ years to develop materials up to 3W/mK with high levels of reliability so is it realistic to expect 10W materials in a few years from China sources…..the answer of course is no.
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.
The Trade off
There is an attraction to mass produce the materials by using traditional sheet lamination techniques with thermally impregnated resins on glass cloth carriers (FR4). These materials are low cost, easy for FR4 manufacturers to adapt their current production processes and aimed at the bottom end of the thermal scale.
With laminated materials the surface area compared to the dielectric mass on a resin glass matrix will be very high so moisture can be easily be absorbed if the material is not baked and all moisture removed before solder level, otherwise any other intense heat shock the vapour pressure of moisture release will lead to delamination.
In our experience the cheaper materials may require additional in process baking during assembly and this adds time, leading to a false economy.
LEDs are sold on a 50,000 hour life span and only a handful of metal substrate materials have been around long enough to have real life data and proven quality.
The reliability of the ‘me too’ laminates flooding the markets is a real concern.
It is therefore important that we understand which material, from the now wide range available, is the best suited to you and your application.
The LED lighting market offers the opportunity to enter the arena and sell new material on price alone, unlike Power Electronics where understanding the materials and function is an absolute, the Lighting manufacturers as a rule don’t understand or care… if it’s seen to work then it must be ok! Circuit shops also share this ignorance as the materials are new to most of them.
This explains why the industry is spawning partnerships where PCB fabricators align themselves with one of the laminate companies. This provides these PCB fabricators with the technical support and sales leads they lack but at a cost of their ability to be independent, as it ties them to a single source offering. Another dubious practice is laminate rebranding: there is no doubt we have the same Chinese dielectric being laminated by different factories branded under different names…..
Although the growth continues in both LED and backlight TV markets where these materials are most frequently found, the ability of the industry to support the influx of new materials is questionable, dissecting the material costs, taking metal prices and the cost of production into account its difficult to see it going any lower. We will see many more entering this market in the coming years, which will offset those that are exposed having had to leave.
The Future
We will doubtless observe some very interesting developments on specific purpose laminates as time progresses. For example DK Thermal Solutions has been working with a material specialist to bring to the market a material that can be formed to a shape, post assembly.
Street lighting is a prime example of a product where lights set at angles provide the beam coverage required - traditionally individual modules need to be wired together however, with a formable dielectric, a circuit can be produced as a one piece solution removing interconnection and failures associated with this.
The answer is in the dielectric chemistry - the ability to bend without loosing the properties of electrical isolation. Once again though, it has been a year since the product was launched and already other suppliers are entering the market with the claimed ability to bend as a selling feature, but this is rarely backed up by the test data.
Metal based PCBs are the principal weapon in the fight to remove heat from electronic components as quickly as possible. As an established leader in its field DK Thermal Solutions is the independent authority that is so desperately needed on metal based circuits.
At DK Thermal Solutions we have made thermally managed PCBs both our specialisation and our passion. We offer advice and manufacturing solutions to the highest international standards from our dedicated facilities in the UK and in Asia. As we are a truly independent supplier we constantly have a wide range of materials in stock from all the major suppliers, hence we can not only select the best solution for your application but we can also supply it in the shortest possible time.
The future will no doubt present many new challenges to match the developments and growth in LED technology and, until this aspect of the lighting market matures, we will see both good and bad solutions being promoted as the best…..
The responsibility for the continued integrity of the market lies with us…the established suppliers
The decision as to whom to trust lies with you…
the customer
For more information or a consultation with a Thermal
Materials Specialist please call +44(0) 1992 514200
or visit www.dkthermal.co.uk
Andrew Grisbrooke, MD at DK Thermal, simplifies the problems facing Engineers and Buyers of Thermal PCBs
The 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, Managing Director Andrew Grisbrooke, who has been active in these products since they became commercially available, explains why the recent rapid increase in their use may cause some concern.
History & Development
Initially developed in Japan and the United States, 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,
Automotive and Power Conversion industries. It allows higher heat dissipation from a given area hence now making smaller more compact products possible. The practice of soldering devices straight to the surface gives lower profile designs and these materials soon became common place in full and half brick power conversion modules. The materials are referred to by many names: IMS (insulated metal substrates) is a term created by The Bergquist Company that has become the most common generic phrase in North America.
In Asia the common term is 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’ are also common. In Europe we don’t seem to have settled on a term, different countries use different variants on the theme – well I suppose that’s what makes us Europe!
The laminate suppliers also have their own trade names: T-Clad or thermal clad is the Bergquist brand name, T-Lam from the Laird company and Hitt Plate from Denka - to name but a few.
