Robert Art on the Importance of Thermal Management
Robert Art, global account manager for IMS materials at Ventec International Group, discusses future market requirements for thermal management materials, the need for a better understanding of the concept of thermal impedance, and an initiative to propose a consistent industry-standard method for measuring thermal conductivity while at electronica 2018.
Pete Starkey: Robert, it's great to meet you. Thank you for taking the time to talk with me.
Robert Art: Thank you.
Starkey: You're an expert in materials for thermal management. We know that over the last several years, Ventec has established a leading position in IMS materials, and you have a good forward vision of the market. Can you tell us what the market is expecting of you now, and in which direction you are going to meet future market needs and expectations?
Art: The market is looking for what it has always looked for—better thermal performance. In every sales meeting and discussion with customers and at trade show events, the engineering community is always looking for better thermal performance. Our VT-4B7 material is a star performer amongst our market-leading VT-4B family of high-performance IMS materials. We continue to push the envelope when it comes to developing stronger, more reliable robust dielectrics that can be thinner to reduce thermal impedance and resistance values.
Starkey: Can you give me a couple of points of reference? What sort of thermal conductivities are achievable with these materials, and when you talk about "thin," what's a point of reference for thinness?
Art: With respect to thermal conductivities, the current Ventec product line-up ranges anywhere from two watts per meter-Kelvin all the way up to roughly 10 watts per meter-Kelvin. From a thickness standpoint, we have one new product out on the market called VT-4B5SP that is available in a thickness of 40 microns. That's an incredibly thin dielectric that still has a very good voltage withstand and excellent thermal conductivity.
Starkey: Is that a reinforced dielectric or a filled-resin dielectric?
Art: It's a filled-resin dielectric with no fiberglass or other carrier reinforcement.
Starkey: And you can get that down as thin as 40 microns?
Starkey: We continue to try very hard to explain to people that the most significant consideration is the concept of thermal impedance rather than just thermal conductivity. Even if you have high thermal conductivity, if the heat has a long path to get through, there’s going to be resistance to the flow. The shorter you can get that path, the better, and it's really important to explain this.
Art: Exactly. We need to continue to educate and help our customer base. Take LEDs, for example; even though IMS substrates have become very commonly used in LED applications, we still come across engineers who focus solely on thermal conductivity. Thermal conductivity is a great characteristic to use for benchmarking materials, but it is not the only attribute that designers should pay attention to. They need to consider dielectric thickness and the overall thermal resistance. That is truly what matters—the resistance to heat flow. As you mentioned, you can have a product at 10 watts per meter-Kelvin, but if it's in a 150- or 200-micron dielectric, then the thermal resistance will be high, and the overall path of heat may not be as good as it could be through a thinner dielectric with a lower thermal conductivity.
Starkey: To try to achieve a dielectric thickness reliably at 40 microns must present a lot of challenges in the manufacturing process. How do you overcome those challenges?
Art: That's a really good question. Ventec has without a doubt the best R&D staff community in the IMS world in my opinion. A lot of time and care is taken to perfect a manufacturing process and speeds of conveyors and lamination cycles to be able to consistently reproduce a dielectric at 40 microns with very high yield and efficiency.
Starkey: The other consideration is that on the one hand, you want to conduct heat, but on the other, you want to maintain electrical insulation.
Art: Yes. Although there are products on the market that are slightly thinner than 40 microns, the challenge is holding to a tighter tolerance. So, when you commit to a thin dielectric, the customer base wants that to be the same dielectric thickness with each and every shipment. A 40-micron dielectric with a ±40-micron thickness tolerance is really of no value. You're absolutely right; it's a hard line between thermal performance, electrical isolation, and peel strength. Trying to juggle all three of those attributes and hold to something that's less than 50 microns is quite a challenge.
Starkey: Where is the main area of application for these high-performance materials?
Art: The primary application today for these extremely thin dielectrics is in LED lighting, which is a high performer.
Starkey: Is this LED lighting for automotive, municipal, industrial, or domestic?
Art: The answer really is all of the above. The hottest market today is automotive lighting for these types of dielectrics, but we also see them in parking lot lights, parking ramp lights, and street lights. Most of those designs today use a number of boards—it's not just one dedicated board; it might be four, six, or eight different boards in a lighting unit. The desire by the engineering community is to get all of those LEDs and connectors onto one board, so it's much more robust and reliable. Each time you add an additional board into a module, you create more risk as far as efficiency and failure. These thin dielectrics allow customers to go to a one-board system, and that represents a great savings in total system cost.
Starkey: You've progressively developed to a very high level of specification, and have a vision of what the market is going to call for in the future. Is there a realistic limit to what you can achieve? Is there any benefit in pushing it any further than that, or do you look for alternative technologies if you have more heat than this sort of material can handle?
Art: Good question! Do we push the envelope? We have to! To stay in the mindset of the customer as a world leader or a supplier to look for the next best-generation product, we have to push the envelope. We consider the characteristics of IMS that are not only focused on the dielectric. There are attributes with the total material stackup that can be altered, changed, or adjusted that produce even better results. There's a whole concern in the industry with IMS because generally speaking, the base plate in an IMS stackup is aluminum. That has wildly different CTE values than the copper foil that becomes the circuit trace. Solder joint reliability is a big concern today for many customers, and not exclusive to automotive; we see it in other markets too.
If you look at our roadmaps, some of the product offerings we're pursuing not only improve the thermal performance or peel strength but also improve the modulus of the dielectric to make it more conducive to providing a really solid solder joint. Solder joint fatigue is a big issue in our marketplace today, and that's an area of focus for our future development.
Starkey: As you've said, when the dielectric layer is very thin, if there's an expansion mismatch between the conductor and heat sink level, then a thin dielectric—even if it has very good elastic modulus—is not going to help much. There needs to be a good match between the two metallic layers.
You've said that most of these materials have been generally based on aluminum, but do you have an equivalent range on copper? Is copper a realistic alternative for very high-performance requirements?
Art: Absolutely. We're seeing many more requests for copper-based IMSs from customers. Of course, there’s an adjustment in price and an added raw weight of their component, but to give their components the life and output that they require, copper is becoming a very common choice as base plate material. In addition to copper, we are looking at other alternative base plate options that can simulate the CTE of copper, but may be less weight than copper and certainly cost less.
Starkey: It would be interesting to have a future discussion when you can disclose what some of these materials are.
Art: Yes, at this stage, I will say there are options out there today in base plate technology that offer some huge benefits to customers.
Starkey: We can have fun talking about that on a separate occasion! Robert, going back to thermal impedance considerations, the thermal conductivity of the IMS material doesn't tell the whole story because we are talking about the thermal impedance of the whole heat path and there are a few interfaces in that heat path. How do you make sure that there is good thermal contact between the device you're trying to cool and the heat-sinking substrate?
Art: Well, Pete, that's a very good lead to a discussion about a new product family that we are offering as a key distributor. We've joined forces with a company called EMI Thermal that carries a full line of thermal interface products that marry very well with the IMS materials that we promote today. There aren't many applications out there where a customer using IMS would directly mate them to a heat sink without some form of secondary thermal interface material, whether that's a thermal grease or a Sil-Pad or a void-filler type product. We've decided that there is value in bringing a full product line to our customers—a full family of products that can help them with their heat issues. In addition to our tec-thermal IMS product line, we've engaged with EMI Thermal to offer a secondary thermal interface product to help them completely remove heat not only from component to IMS board but also to housing or heat sink.
Starkey: What sorts of technologies are involved in the range that you offer?
Art: The most popular technology would be soft compressible materials called void fillers. Other industry names for these products are gap pads or gap fillers, which are compressible soft materials that don't require a lot of pressure or force to dissipate heat. The EMI Thermal family of void fillers covers the required gamut of thermal and compression expectations from customers. In addition, we have some product lines that are reinforced with fiberglass and electrically insulating, so we can offer a full line of products that either dissipate heat with very good thermal resistance characteristics and/or some that have thermal resistance and a decent breakdown voltage. We look at these products as a great complement to the tec-thermal IMS product family.
Starkey: Robert, it occurs to me now that one thing we haven't discussed, which might be very important, is test methods. We've talked about all of these thermal properties, but how do we measure them?
Art: We have a lot of discussions with our business partners on that. The truth is there are lots of different test methods. Ventec’s testing is based on ISO methods, but there are also ASTM test methods, an E1461 test method, and a TO-220 test method. The issue with all these test methods is that, if the customer wants to have a thermal conductivity of 5 watts per meter-Kelvin, we need to understand which test method is being used to define the 5 watts per meter-Kelvin. If a customer were to ask for a product of 12 watts per meter-Kelvin, it is indeed possible! Even if the customer wants a product of 80, 90, 100 watts per meter-Kelvin product, within just about one week, it is entirely possible to figure out a test method which fits the 100 watts per meter-Kelvin. It is quite hard to explain that to our customers because there are a lot of different values out there, and the biggest issue is figuring out what is the truth and what fits the customers’ application.
What we are currently working on is for our technicians to meet with our engineers from Suzhou—together with a university and an OEM customer—to agree on an ASTM test method that can be used by everybody to enable the end customer to figure out and compare between different suppliers. The first stage will be a data sheet with a matrix showing our values depending on different test methods. The second stage would be to support IPC and the OEMs to develop a common test method that all of the IMS suppliers could use. Then we can save a lot of time and discussions because, when I go to visit customers, 70% of my time is taken up in discussions about thermal conductivity and tests. Once we can overcome this so that both sides understand the same terms and can relate to the same test method, then we can spend the time discussing their real needs.
Starkey: Yes. To continue on that point, something I've heard many times in the past is, "Don't believe everything you read on people's data sheets."
Art: It's a good point, but it doesn't mean from our side that our competition is quoting wrong values—absolutely not. They most likely have the right values but simply from a different test method.
Starkey: I should re-phrase my comment. Instead of saying, "Don't believe everything that you see on data sheets," I could say, "Make sure you understand the significance of what you see on the data sheet."
Art: Absolutely. We want to produce something that is very simple and consistent so that an engineer can look at two different data sheets and say, "This product has these attributes, and that product has those attributes." Today, when an engineer sits down with two or three data sheets from different suppliers in the IMS industry, it's very difficult to make an assessment based on those data sheets as to which product best fits his requirements.
Starkey: I understand exactly what you're saying and all of the logic of doing what you're proposing.
Art: Just a short example: If I use an ISO test method, a certain material appears to have a thermal conductivity of about 4 watts per meter-Kelvin. But if the same material is tested based on ASTM, you might get a value of about 2.7 watts per meter-Kelvin. That’s a huge difference between 4 watts and 2.7 watts—not just 10% or 15%; we are really talking about two widely different values for the same material.
We want to be as open as possible in supporting this so that people can make realistic comparisons of materials, and this is the approach that we are taking. If we put it all together, I think we can come up with a proposal that we can promote to IPC. Then, it's up to IPC and the OEMs to push in that direction with input from other OEMs and IMS suppliers.
Starkey: Thank you for that explanation, Robert. It's good to talk with people who know their subject and have a passion for it. Thank you very much for being so open with your information; I really appreciate it.
Art: Thanks for your time, Pete. It's been a great pleasure.
To download your copy of Ventec International Group’s micro eBook, The Printed Circuit Designer's Guide to… Thermal Management with Insulated Metal Substrates, click here.