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30 DESIGN007 MAGAZINE I NOVEMBER 2022 is temperature increase is caused by what we call "Joule heating:" Joule heating is the physical effect by which the pass(age) of current through an electrical con- ductor produces thermal energy. is thermal energy is then evidenced through a rise in the conductor material temperature, thus the term "heating." One can see Joule heating as a trans- formation between "electrical energy" and "ther- mal energy," following the energy conservation principle 3 . We are all aware that, in general, for a given trace an increase in current through the trace will correspond to an increase in temperature of the trace. When it comes to vias, the indus- try guidelines have generally been to size the cross-sectional area of a via to equal that of its parent trace. en the via will be the same temperature as the trace. IPC formalizes this guideline in IPC-2152: e cross-sectional area of a via should have at least the same cross-sectional area as the conduc- tor or be larger than the conductor coming into it. If the via has less cross-sectional area than the conductor, then multiple vias can be used to maintain the same cross-sectional area as the conductor 4 . Our research discovered that this guide- line is totally false. In our book 5 we report on experimental results where we put current through a 10-mil diameter, plated (1.0 ounce) via. When we put 4.75 A through the via, we recorded a via temperature of 64.5 o C. When we put 8.6 A through it, the via temperature was only 44.5 o C, some 20 o C cooler. e differ- ence? In the first case the parent trace was 27 mils wide. In the second case the parent trace was 200 mils wide. e much wider trace in the second case provided a significant heat sink for the via. e heat conducts away from the via so rapidly that the via simply cannot get much hotter than the parent trace. e implication of this, of course, is that many fewer vias are needed on our high-current-carrying traces. In electrical engineering we understand that the temperature of a conductor is related to the current. But in the special case of vias, the physics of heat transfer dominates the sit- uation, and the temperature of the via is not directly related to the electrical current. Thermal Vias are Not Very Effective Oen, if we have a hot component on the top layer, we will put a copper pad under it to help spread the heat. Some people recom- mend dropping copper (filled) vias from that pad to "something " on the bottom layer of the board. at "something" might be a cop- per pad the same size as the top pad, or it might be a copper plane of some sor t. ose vias are called "thermal vias," and the internet is filled with articles and advice about them. But almost all the references say that mul- tiple thermal vias (10s of them) are needed because each one is only marginally effective. It turns out there is a very good reason why thermal vias are not very effective. It has to do with physics. A thermal via generally connects between two surfaces. ermal conductivity between two surfaces is a function of, among other things, the parallel overlapping areas. e pad areas are, of course, much larger than the thermal via cross-sectional areas. So, the presence of the underlying pad has already provided a significant path for thermal con- It turns out there is a very good reason why thermal vias are not very effective. It has to do with physics.

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