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Design007-Sept2020

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SEPTEMBER 2020 I DESIGN007 MAGAZINE 51 use the aforementioned techniques to identify, mitigate, evaluate, and optimize the thermal operating conditions of your product. Often, this means implementing a somewhat blind first attempt, using the best practices you can within your constraints, and then checking whether the performance is adequate. Draw- ing upon previous experience is a key part of this, whether that's the capability of a device, heat sink, or enclosure or understanding the environment in which it operates. In general, thermal management is some- thing of an iterative process, and explicit tech- nical requirements aren't available nor know- able. For example, the most common require- ment is related to the electronic components on the board not exceeding their rated junction temperatures. How to do that is a combination of the system implementation operating in the field conditions, which usually requires proto- typing or simulation to baseline. Knowing that a part will dissipate 3W of power is simply not enough. Once you have an idea of where you're at, then you can assess opportunities for improvement. Generally, these optimizations involve: • Lower the power dissipation of the device (component selection, functional operating load) • Improve the heat transfer conditions (thermal pathways through PCB, heat sinks, etc.) • Constrain the operating environment/ use case (software-controlled limits, environmental ratings) Whenever something is a risk, we must miti- gate it, and because the nature of thermal man- agement is heavily influenced by environmen- tal conditions, prototyping is key. Even if it's just a small portion of the system, being able to characterize the limits will enable an under- standing of whether simple thermal pathway improvements can succeed or whether drastic architecture changes are required. The latter is quite important to know as soon as possible. Lastly, use thermal vias and heat sinks where possible. To remove the heat created by power dissi- pation of the components, and especially the power components, one solution is to improve the vertical heat transfer through the support- ing material. Typically, this is achieved by providing thermal through-contacts, or thermal vias, going through the substrate from the top side to the bottom side. It is typical to use thermal vias beneath the rear contact or mounting surfaces of the com- ponents and particularly the power components, and then plate the through-holes through the entire thickness of the material/substrate. A second remedy for the heat dissipation problem relates to external heat removal. For this, the substrate may be mounted on a metallic cooling body or heat sink, such as a copper plate, which conveys the power dissi- pation heat to a cooling system. Such a cool- ing body may be separated from the support- ing substrate by an electrical insulating layer, such as an insulating film or foil. After apply- ing the components to the mounting surface of the substrate, the components are electrically connected with contact surfaces or specifically contact pads and the traces. To achieve this, a solder paste is printed onto the contact surfaces and the top surface of the thermal vias and is then melted in a reflow soldering process to solder-connect the com- ponents. Conclusion I hope this column has helped to explain just a few ways to stay out of the hot seat. And, as always, I appreciate any feedback. I can be reached at markt@msoon.com. DESIGN007 Mark Thompson, CID+, is a senior PCB technologist at Monsoon Solutions Inc. To read past col- umns or contact Thompson, click here. Thompson is also the author of The Printed Circuit Designer's Guide to… Producing the Perfect Data Package. Visit I-007eBooks.com to download this book and other free, educational titles.

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