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At IPC APEX EXPO recently, Pete Starkey and Ian Mayoh, Ventec International’s technical support manager, had a chance to sit down and discuss the I-Connect007 micro E-book: The Printed Circuit Designer's Guide to Thermal Management with Insulated Metal Substrates, of which Ian is a co-author.
Pete Starkey: Ian, thanks for joining me. I'm very impressed with this book that you and Didier Mauve have written. Could I start by asking you why you think there was a need for it?
Ian Mayoh: The use of insulated metal substrates and thermal materials has been growing exponentially over the last few years, but it was becoming increasingly clear speaking to designers, some OEMs, and PCB producers, that the need to consider thermal management in the early design stages was seen as a very low priority. And we felt that this profile needed to be raised a little, especially as there are so many of these materials around now.
Starkey: So who is this book primarily aimed at?
Mayoh: It’s primarily aimed at people who are moving into design with thermal management materials, people who are designing high power LED systems, or high-power electronic devices such as inverters and chargers. There's particular growth in the automotive sector because of the big push globally on electric and hybrid vehicles. And these require high power. They need high power substrates, high thermal dissipation substrates, to keep the components comfortable in their working environment.
Starkey: From the point of view of designers’ understanding of some of the implications of thermal management, do you think there is fundamentally a knowledge gap, or just a lot of misunderstanding?
Mayoh: I think it’s a combination of the two; some of it is a knowledge gap, some of it is misunderstanding. For example, putting my chemistry hat on, we know that increasing the temperature of a reaction by 10 degrees will halve the reaction time. Much is the same with electronic devices. If you increase the temperature by 10 degrees above its accepted working temperature, you are potentially reducing the life of that component by 50%.
Starkey: And in many of these applications, failure is not an option. It’s essential that components exhibit high-reliability in the long term.
Mayoh: Absolutely, particularly if we look at the growth of these materials in the automotive sector. A lot of people don't realise this, but the automotive sector, in terms of long-term reliability and long-term reliability testing, is actually much more critical than even the aircraft industry. Because vehicles are on the ground, they tend not to have back-up systems. So if there's a failure, it's a catastrophic failure, even if it's just stopping on the motorway in the fast lane.
Starkey: I agree! Ian, I've read this book and I'm really impressed by its content. It doesn't blind anyone with science, but it provides a sensible explanation for every step of the way. If we just look briefly at the table of contents, after the first introduction, we see a chapter entitled “Management of Junction Temperature and the Concept of Thermal Resistance.” I think that this potentially clarifies an area of considerable misunderstanding, if you know what I mean.
Mayoh: I certainly do, Pete. You will recall that I've been banging the drum on this subject for a number of years now, that there is a basic misunderstanding of thermal materials, and how to specify them. We still receive orders where some people don't even quote the thermal dielectric requirements on these materials, and that's critical. We have actually got people quoting thermal conductivity now, but that's just a coefficient; we also need the thickness of the dielectric. The ideal situation is where people know the thermal impedance they require, along with the dielectric strength. Once we have the figure for the thermal impedance and the dielectric breakdown required, we can instantly advise on what dielectric is required in terms of thermal conductivity and thickness.
Starkey: I can imagine that once people start to understand the concept of thermal impedance, there is a temptation to say, "All I've got to do is make the dielectric layer thinner and thinner and thinner and it will have less and less and less thermal impedance." But then you risk losing some of your margin of safety in dielectric breakdown.
Mayoh: Absolutely. So if you look at thermal conductivity as a constant, once you have the dielectric thickness, you will have the thermal impedance. But it's not a linear curve, and you're also affecting the dielectric breakdown voltage of the material. It all has to be a balance. With the range of materials available now, with thermal conductivities from two watts up to seven or even nine watts per metre Kelvin, there's a place for everything.
Starkey: I see there's a whole chapter on developments in insulated metal substrates, which clearly indicates what is available. And in recent times, there have been more and more developments in the highly thermally conductive materials, which are all well-summarised.
Mayoh: Yes, they are. And things are even moving on from there, Pete. If we look at the latest developments, we know that designers are considering hybrid technologies, building thermal layers within a standard high-Tg or mid-Tg multilayer construction.
Starkey: So you've got materials effectively compatible with FR-4 materials that you can use in combination builds?
Mayoh: Yes, we've recently launched a product designed specifically for that application, called VT-5A2. It's got a 2.2 watt thermal conductivity, but it's also got a very high Tg of around 190. It's fully compatible with high Tg materials, so you could produce cores on this VT-5A2 material, or on normal high-Tg material and bond them with VT-5A2 pre-preg. It gives you the ability to target where you want to dissipate the heat from within the construction.
Starkey: Ian, as technical support manager, I reckon an awful lot of your work with insulated metal substrate materials is really as an applications engineering advisor, where the customer, if he's the right sort of customer, will say, "This is my requirement," and before he starts writing down his specification, he'll say "This is what I want to do. What's the best way to achieve it using available materials? What is technically the best way, what's cost-effectively the best way, what in terms of reliability is the best way?" So I'm sure that is quite a large chunk of the work that you do.
Mayoh: Yes, certainly. I had a case recently where a company was asking one of our producers to produce a design that really was not suited to standard FR-4 materials. But over a short meeting of about 30 minutes, we came up with a hybrid construction using thermal cores and thermal pre-pregs, VT-5A2 in fact, so they can now build their multilayer, and then turn it into an insulated metal substrate by bonding that multilayer onto an aluminium backing substrate using thermal pre-preg.
Starkey: If we go back to the book, there's a beautifully illustrated chapter on applications examples. A lot of it is in automotive because there's so much growth in that sector, but also in power management applications, and other LED lighting applications.
Mayoh: Yes, a big focus at the moment is on automotive LED lighting and power management systems. But in terms of volume, standard utility lighting is going to be a huge market over the coming years, because there will be a global changeover from incandescent bulbs and sodium discharge lamps, and everything is going to be LED, probably within the next 10 to 15 years.
Starkey: Yes, in the UK I live in a little village out in the sticks, and our local authority has already replaced our traditional street lights with LED.
Mayoh: And when they're spending that sort of money, they're looking for long term reliability—people are basically looking for a 15 to 25-year life. And the investment is considerable—it’s more expensive than just changing a sodium discharge lamp; they want the longevity out of it. And this is where the thermal materials come into their own, because they are protecting the LED that is providing the light. Getting the heat away extends the life. What more can I say?