Issue link: https://iconnect007.uberflip.com/i/1285883
SEPTEMBER 2020 I DESIGN007 MAGAZINE 55 to three watts per meter Kelvin. Today, we can achieve toward eight or 10 on an IMS substrate. Again, thermal conductivity is important, but there's also thickness. The thinner the substrates, the more heat is transmitted. We ask OEMs what their design characteristics are. First, what's the temperature they have to deal with? And how big is the space? How big is the substrate they have to deal with? The area is always a massively important part of the calculation. Lastly, what voltage withstand is required in the design? This has a signifi- cant impact on what dielectric thickness will be used. Then, we can provide some solutions for emissivity. Given that, we can then design what's required and come up with a material that meets the application requirements. And that approach has proven very successful for a number of automotive companies in particu- lar. They've shared with us their requirements, and based on that, we've come up with an appropriate material. Shaughnessy: Automotive is one big science project area for PCB technologists. Morgan: Automotive brings lots of things with it. People sometimes complain about the dif- ficulty of dealing with some automotive sup- pliers, but we think they're great because they really challenge us. They challenge us on many levels. They challenge us on the technol- ogy and price point. They also challenge us in the supply chain. We have to bear in mind that when you find a solution for a particular appli- cation in a particular market, within the space of a few weeks or months, it has often required attention across the whole supply chain in the world. That's something that is very important for OEMs. They may do some development work in Germany and then move volume very quickly to China, for example. Shaughnessy: Do these laminates change the way the designer has to do his design? Does he have to do anything differently? Morgan: Not so much; although, they are basi- cally single-sided circuits. You have to bear in mind that you only have one side for your circuitry, in general, but that doesn't usually matter. The backside of it is protected during manufacturing. There is a coating provided on that. During the PCB process, it can be dealt with. From the designer's point of view, it's a fairly simple layer design. They just have to design a single-sided board, and the back will be a piece of aluminum. They have to decide how thick the laminate should be because that's important, as well as how thick the dielectric should be and what properties the dielectric requires. Having done that, we can run with it. As our IMS products evolve, we see several requests from the engineering com- munity for multiple layer IMS stackups. This requires multiple stacked layers of circuitry on top of the aluminum base plate that have internal connections through copper vias. In terms of reliability, the other aspect to take into consideration is the thermal man- agement of the expansion of all the different materials. A silicon package has an expansion coefficient of around 10 ppm/°C. For alumi- num, it's about 25 ppm/°C, so more than dou- ble the amount of silicon. Copper is mid-teens, so maybe 17–18 ppm/°C. That could cause an issue for headlamps, for example, which are switched on and off quite frequently. Those are pretty high thermal cycling requirements. Again, the automotive companies have pretty robust testing for this. Failures can occur because of this constant expansion and con- traction of the silicon device on the one hand, and then the aluminum can cause a failure on the other hand. In terms of reliability, the other aspect to take into consideration is the thermal management of the expansion of all the different materials.