Issue link: https://iconnect007.uberflip.com/i/1285883
72 DESIGN007 MAGAZINE I SEPTEMBER 2020 LED? However, the thermal conductivity num- ber is not necessarily the most important num- ber because if you're taking a single side of an application, where you have your copper, dielectric layer, and heat sink, which could either be copper or aluminum, you want to move the heat from that active copper layer. If you find the material that has five mils of dielectric but has 10 watts per meter kelvin, on paper, sometimes designers get really excited about the super-high thermal conductivity number. But the number that is important to focus on is either thermal resistance or thermal imped- ance, and where we try to focus our technology is to make that dielectric layer as thin as pos- sible to be able to move the heat as quickly as possible. Sometimes, the thermal conductivity is not as high as 10, but with a solid thermal conductivity number and an ultra-thin dielec- tric layer, the opportunity to move the heat is quite significant. Shaughnessy: You mentioned that designers often over-design boards with thermal require- ments, which makes the board more expen- sive and difficult to manufacture. Brandman: As a laminate manufacturer, we get a lot of PCB shops reaching out to us. They say, "We have this drawing for this PCB. Which of your materials best fits this drawing?" A lot of times, I'll see a specification in a single-sided material where the aluminum back or the cop- per back is a weird thickness. For example, a very common thickness for that copper or alu- minum back is 1.5 millimeters, but then I'll see people stack 2 millimeters or 78 thou, or 2.5 millimeters or 100 thou. What the design- ers may not realize is that although they may be getting a small percentage of other proof thermals for doing something like that, the cost of the PCB could double or triple. Maybe you're going to see 1–2% in the junc- tion point temperature on the LED, but is it really worth all that extra money? Sometimes, designers design ultra-conservatively because that's their nature, and that's good. But at the same time, more communication with the PCB shops and the laminate manufacturers could be really positive to value technology vs. cost. Shaughnessy: It seems like you would have to communicate with the fabricator and the lam- inate maker when you're dealing with these boards where you just see hotspots all over. Brandman: We get a lot of designers from OEMs who reach out to us about their design, ask- ing, "Which would be the best material to suit what we're designing here?" In those cases, I make recommendations based on cost vs. technology. I try to remind them, "If you put 3-ounce copper here, that's going to be a lot more expensive than you think. Is there a way that you can do this with two-ounce copper?" Most of the time, the designers are quite recep- tive to these recommendations. Shaughnessy: If they've never dealt with this sort of material before, does it change the way the designer does the layout in any way? Brandman: For a PCB designer who hasn't had experience working in a heat application, there would be a learning curve, but many PCB designers who have experience with heat, Jeff Brandman