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{ "url": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/X6CD4SPXSN4D3B5JM335PVVAJ5W4QXVU/?format=api", "mailinglist": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/?format=api", "message_id": "[email protected]", "message_id_hash": "X6CD4SPXSN4D3B5JM335PVVAJ5W4QXVU", "thread": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/thread/X6CD4SPXSN4D3B5JM335PVVAJ5W4QXVU/?format=api", "sender": { "address": "n1al (a) cds1.net", "mailman_id": "f80746532eeb4ee9a90361b9ab507fdc", "emails": "https://mailman.amsat.org/hyperkitty/api/sender/f80746532eeb4ee9a90361b9ab507fdc/emails/?format=api" }, "sender_name": "Alan Bloom", "subject": "[eagle] SMT heat dissipation in the space environment", "date": "2006-10-10T04:16:11Z", "parent": null, "children": [ "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/UY6FZBY2SJ2VA6GHSVSBUREL5RRFFIYN/?format=api" ], "votes": { "likes": 0, "dislikes": 0, "status": "neutral" }, "content": "This is a note I wrote some years ago during the AO-40 project. If\npeople think it's useful, perhaps I could post it to Wikipedia.\n\nAlan N1AL\n\n-------------------------------------------------------------------\n\nOur local PC board technical expert here at HP has done an extensive\nstudy on power dissipation of surface-mount parts. While his report is\nbased on non-space applications, there are a number of conclusions that\nare very interesting.\n\nI won't include the entire report, partly because it is much too long,\nand partly because it is HP proprietary. But here are a few highlights.\n\nHe found surprisingly little discussion of these issues in the\nliterature. So he performed a series of experiments using an\nInframetrics 760 infared camera system to measure component\ntemperatures. All measurements were taken on similar-sized PC boards\nmounted horizontally with no significant airflow.\n\nOne main conclusion: \"Part ratings by the vendors have little meaning\nby themselves since trace width and part density have a huge impact on\nheat dissipation.\"\n\nFor example, what is the allowable power dissipation of an 0603\nresistor? The answer is somewhere between 12 mw and 526 mw, depending\non layout! \n\nThis is based on a 60 degC temperature rise. (Most resistors have a 125\ndegC max case temperature spec.) Temperature rise is directly\nproportional to power dissipation.\n\nThe 12 mw number was measured on 112 parts packed into a 0.75 square\ninch area and all dissipating the same power. The 526 mw measurement\nwas on a single part soldered between two half-board-sized ground planes\nand tied through many vias to another ground plane on the back side of\nthe board.\n\nManufacturers rate 1206-case resistors at 125 mw, versus 62.5 mw for\n0603-case resistors, a 2:1 ratio. Actually, for most reasonable trace\nwidths, an isolated 0603 resistor can dissipate roughly 80% as much as a\n1206. When parts are packed densely together, power dissipation is\nlimited by the maximum watts per square inch. Note that I used a 30\ndegC temperature rise for the following table:\n\n 0603 1206\nIsolated Resistor:\n Large ground plane: 263 mw 403 mw\n 0.060 inch traces: 170 mw 199 mw\n 0.040 inch traces: 148 mw 177 mw\n 0.012 inch traces: 106 mw 128 mw\n 0.005 inch traces: 77 mw 100 mw\n 0.0025 bond wires: 55 mw 79 mw\nHigh-density part layout, 0.75 square inch (4.84 cm^2) area:\n Number of parts 112 32\n Power per part 6 mw 20 mw\n Total power 672 mw 640 mw\n Number of parts 56 16\n Power per part 11.5 mw 43 mw\n Total power 644 mw 688 mw\n Number of parts 20 8\n Power per part 31 mw 80 mw\n Total power 620 mw 640 mw\n\nSince the power dissipation depends so strongly on trace width, then\nclearly most of the heat must be conducted, not radiated, on SMT\nresistors (and other parts). Even in the thin-trace case, much of the\nheat is conducted to, and radiated from, the PC board. You can see that\non the infared photos: the PC board surrounding the part is quite hot.\n\nThermal resistance of isolated SOT-23 transistors was very similar to\n0603 resistors up to 0.040\" line widths. SOIC-8 voltage regulators had\nabout 1/2 the thermal resistance of SOT-23.\n\nThe above numbers are probably conservative, since they are based on\nstill air, even though most earth-based applications have forced-air\ncooling or at least natural convection (vertical PC board). The rule of\nthumb I generally use is 1 watt per square inch (155 mw/cm^2), which\ngives an average temperature rise of around 35 degC in still air.\n\nI have been told that for space applications, 15 mw/cm^2 is a more\nappropriate limit. Assuming radiation cooling is only 1/10 as efficient\nas convection cooling, then that seems a reasonable spec.\n\nConclusion:\n\nI have heard several rules of thumb on what percentage you should derate\ncomponent power specifications for space applications. For through-hole\ndevices, such a rule of thumb probably makes sense. THD devices\ndissipate most of their heat from the component body, and relatively\nlittle heat is conducted out the leads. On earth, most of that heat is\nconducted to the air; only a little is radiated. In space, radiation is\nthe only heat-dissipating mechanism, so the power must be derated by a\nlarge factor.\n\nBut SMT devices are cooled mainly by conduction out the leads to the\ncopper traces on the PC board, so they are not directly affected by the\nlack of air. It seems to me that as long as I keep within the 15\nmw/cm^2 limit on the PC board, that allowable power dissipation of\nindividual components should be governed by trace width, per the above\ntable. If I have a hot component, I'll connect it to lots of copper and\nmake sure no other hot components are nearby. \n\n<END>\n\n", "attachments": [] }