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GET /hyperkitty/api/list/[email protected]/email/MWGDETRP5X7GDVYSLXLKEV7ZYKNMVTJE/?format=api
{ "url": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/MWGDETRP5X7GDVYSLXLKEV7ZYKNMVTJE/?format=api", "mailinglist": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/?format=api", "message_id": "[email protected]", "message_id_hash": "MWGDETRP5X7GDVYSLXLKEV7ZYKNMVTJE", "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] Re: SMT heat dissipation in the space environment", "date": "2006-10-11T06:09:36Z", "parent": "https://mailman.amsat.org/hyperkitty/api/list/[email protected]/email/UY6FZBY2SJ2VA6GHSVSBUREL5RRFFIYN/?format=api", "children": [], "votes": { "likes": 0, "dislikes": 0, "status": "neutral" }, "content": "> Since the PCB size for most of the Eagle modules will be 125x180mm,\n> an area of 225cm^2, following your 15mW/cm^2 becomes, in my mind and\n> calculation, a bit difficult. That power density, if it were applied\n> to the whole PCB, would amount to a total dissipation of 3.375W. This\n> is clearly an excessive PCB dissipation.\n\nI think the maximum power per cm^2 and the maximum power for an entire\nboard are separate issues. That is, make sure your total dissipation is\nwithin the module limits _AND_ no portion of the circuitry exceeds 15\nmW/cm^2.\n\nAlan\n\n\nOn Tue, 2006-10-10 at 06:14, Dick Jansson-rr wrote:\n> Thank you Alan for this most useful and significant note. Operating in\n> space, where we depend upon radiation as our primary mode of heat\n> transfer, is truly different from all of our earth-bound experiences.\n> Since the PCB size for most of the Eagle modules will be 125x180mm, an\n> area of 225cm^2, following your 15mW/cm^2 becomes, in my mind and\n> calculation, a bit difficult. That power density, if it were applied\n> to the whole PCB, would amount to a total dissipation of 3.375W. This\n> is clearly an excessive PCB dissipation. I would be much more\n> comfortable if the dissipation were less than 4mW/cm^2 based on PCB\n> average power. This power level is stated for those PCBs that do not\n> employ steps for heat transfer augmentation.\n> \n> This discussion also amplifies the fact that the thermal design\n> capabilities of the Eagle project must be involved in the design\n> aspects of each of the modules. We must not depend upon one-way space\n> travel for our modules to do the heat transfer experiments for those\n> modules.\n> \n> Dick Jansson\n> ---------------------------\n> [email protected]\n> ---------------------------\n> -----Original Message-----\n> From: [email protected] [mailto:[email protected]] On\n> Behalf Of Alan Bloom\n> Sent: Tuesday, 10 October, 2006 0416\n> To: [email protected]\n> Subject: [eagle] SMT heat dissipation in the space environment\n> \n> \n> \n> This is a note I wrote some years ago during the AO-40 project. If\n> people think it's useful, perhaps I could post it to Wikipedia.\n> \n> Alan N1AL\n> -------------------------------------------------------------------\n> \n> Our local PC board technical expert here at HP has done an extensive\n> study on power dissipation of surface-mount parts. While his report\n> is based on non-space applications, there are a number of conclusions\n> that are very interesting.\n> \n> I won't include the entire report, partly because it is much too long,\n> and partly because it is HP proprietary. But here are a few\n> highlights.\n> \n> He found surprisingly little discussion of these issues in the\n> literature. So he performed a series of experiments using an\n> Inframetrics 760 infared camera system to measure component\n> temperatures. All measurements were taken on similar-sized PC boards\n> mounted horizontally with no significant airflow.\n> \n> One main conclusion: \"Part ratings by the vendors have little meaning\n> by themselves since trace width and part density have a huge impact on\n> heat dissipation.\"\n> \n> For example, what is the allowable power dissipation of an 0603\n> resistor? The answer is somewhere between 12 mw and 526 mw, depending\n> on layout! \n> \n> This is based on a 60 degC temperature rise. (Most resistors have a\n> 125 degC max case temperature spec.) Temperature rise is directly\n> proportional to power dissipation.\n> \n> The 12 mw number was measured on 112 parts packed into a 0.75 square\n> inch area and all dissipating the same power. The 526 mw measurement\n> was on a single part soldered between two half-board-sized ground\n> planes and tied through many vias to another ground plane on the back\n> side of the board.\n> \n> Manufacturers rate 1206-case resistors at 125 mw, versus 62.5 mw for\n> 0603-case resistors, a 2:1 ratio. Actually, for most reasonable trace\n> widths, an isolated 0603 resistor can dissipate roughly 80% as much as\n> a 1206. When parts are packed densely together, power dissipation is\n> limited by the maximum watts per square inch. Note that I used a 30\n> degC temperature rise for the following table:\n> \n> 0603 1206\n> Isolated 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\n> \n> High-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> \n> Number of parts 56 16\n> Power per part 1.5 mw 43 mw\n> Total power 644 mw 688 mw\n> \n> Number of parts 20 8\n> Power per part 31 mw 80 mw\n> Total power 620 mw 640 mw\n> \n> Since the power dissipation depends so strongly on trace width, then\n> clearly most of the heat must be conducted, not radiated, on SMT\n> resistors (and other parts). Even in the thin-trace case, much of the\n> heat is conducted to, and radiated from, the PC board. You can see\n> that on the infared photos: the PC board surrounding the part is\n> quite hot.\n> \n> Thermal resistance of isolated SOT-23 transistors was very similar to\n> 0603 resistors up to 0.040\" line widths. SOIC-8 voltage regulators\n> had about 1/2 the thermal resistance of SOT-23.\n> \n> The above numbers are probably conservative, since they are based on\n> still air, even though most earth-based applications have forced-air\n> cooling or at least natural convection (vertical PC board). The rule\n> of thumb I generally use is 1 watt per square inch (155 mw/cm^2),\n> which gives an average temperature rise of around 35 degC in still\n> air.\n> \n> I have been told that for space applications, 15 mw/cm^2 is a more\n> appropriate limit. Assuming radiation cooling is only 1/10 as\n> efficient as convection cooling, then that seems a reasonable spec.\n> \n> Conclusion:\n> \n> I have heard several rules of thumb on what percentage you should\n> derate component power specifications for space applications. For\n> through-hole devices, such a rule of thumb probably makes sense. THD\n> devices dissipate most of their heat from the component body, and\n> relatively little heat is conducted out the leads. On earth, most of\n> that heat is conducted to the air; only a little is radiated. In\n> space, radiation is the only heat-dissipating mechanism, so the power\n> must be derated by a large factor.\n> \n> But SMT devices are cooled mainly by conduction out the leads to the\n> copper traces on the PC board, so they are not directly affected by\n> the lack of air. It seems to me that as long as I keep within the 15\n> mw/cm^2 limit on the PC board, that allowable power dissipation of\n> individual components should be governed by trace width, per the above\n> table. If I have a hot component, I'll connect it to lots of copper\n> and make sure no other hot components are nearby. \n> \n> <END>\n> \n> _______________________________________________\n> Via the Eagle mailing list courtesy of AMSAT-NA\n> [email protected]\n> http://amsat.org/mailman/listinfo/eagle\n> \n> \n\n", "attachments": [] }