SMT heat dissipation in the space environment
This is a note I wrote some years ago during the AO-40 project. If people think it's useful, perhaps I could post it to Wikipedia.
Alan N1AL
-------------------------------------------------------------------
Our local PC board technical expert here at HP has done an extensive study on power dissipation of surface-mount parts. While his report is based on non-space applications, there are a number of conclusions that are very interesting.
I won't include the entire report, partly because it is much too long, and partly because it is HP proprietary. But here are a few highlights.
He found surprisingly little discussion of these issues in the literature. So he performed a series of experiments using an Inframetrics 760 infared camera system to measure component temperatures. All measurements were taken on similar-sized PC boards mounted horizontally with no significant airflow.
One main conclusion: "Part ratings by the vendors have little meaning by themselves since trace width and part density have a huge impact on heat dissipation."
For example, what is the allowable power dissipation of an 0603 resistor? The answer is somewhere between 12 mw and 526 mw, depending on layout!
This is based on a 60 degC temperature rise. (Most resistors have a 125 degC max case temperature spec.) Temperature rise is directly proportional to power dissipation.
The 12 mw number was measured on 112 parts packed into a 0.75 square inch area and all dissipating the same power. The 526 mw measurement was on a single part soldered between two half-board-sized ground planes and tied through many vias to another ground plane on the back side of the board.
Manufacturers rate 1206-case resistors at 125 mw, versus 62.5 mw for 0603-case resistors, a 2:1 ratio. Actually, for most reasonable trace widths, an isolated 0603 resistor can dissipate roughly 80% as much as a 1206. When parts are packed densely together, power dissipation is limited by the maximum watts per square inch. Note that I used a 30 degC temperature rise for the following table:
0603 1206 Isolated Resistor: Large ground plane: 263 mw 403 mw 0.060 inch traces: 170 mw 199 mw 0.040 inch traces: 148 mw 177 mw 0.012 inch traces: 106 mw 128 mw 0.005 inch traces: 77 mw 100 mw 0.0025 bond wires: 55 mw 79 mw High-density part layout, 0.75 square inch (4.84 cm^2) area: Number of parts 112 32 Power per part 6 mw 20 mw Total power 672 mw 640 mw Number of parts 56 16 Power per part 11.5 mw 43 mw Total power 644 mw 688 mw Number of parts 20 8 Power per part 31 mw 80 mw Total power 620 mw 640 mw
Since the power dissipation depends so strongly on trace width, then clearly most of the heat must be conducted, not radiated, on SMT resistors (and other parts). Even in the thin-trace case, much of the heat is conducted to, and radiated from, the PC board. You can see that on the infared photos: the PC board surrounding the part is quite hot.
Thermal resistance of isolated SOT-23 transistors was very similar to 0603 resistors up to 0.040" line widths. SOIC-8 voltage regulators had about 1/2 the thermal resistance of SOT-23.
The above numbers are probably conservative, since they are based on still air, even though most earth-based applications have forced-air cooling or at least natural convection (vertical PC board). The rule of thumb I generally use is 1 watt per square inch (155 mw/cm^2), which gives an average temperature rise of around 35 degC in still air.
I have been told that for space applications, 15 mw/cm^2 is a more appropriate limit. Assuming radiation cooling is only 1/10 as efficient as convection cooling, then that seems a reasonable spec.
Conclusion:
I have heard several rules of thumb on what percentage you should derate component power specifications for space applications. For through-hole devices, such a rule of thumb probably makes sense. THD devices dissipate most of their heat from the component body, and relatively little heat is conducted out the leads. On earth, most of that heat is conducted to the air; only a little is radiated. In space, radiation is the only heat-dissipating mechanism, so the power must be derated by a large factor.
But SMT devices are cooled mainly by conduction out the leads to the copper traces on the PC board, so they are not directly affected by the lack of air. It seems to me that as long as I keep within the 15 mw/cm^2 limit on the PC board, that allowable power dissipation of individual components should be governed by trace width, per the above table. If I have a hot component, I'll connect it to lots of copper and make sure no other hot components are nearby.
<END>
Thank you Alan for this most useful and significant note. Operating in space, where we depend upon radiation as our primary mode of heat transfer, is truly different from all of our earth-bound experiences. Since the PCB size for most of the Eagle modules will be 125x180mm, an area of 225cm^2, following your 15mW/cm^2 becomes, in my mind and calculation, a bit difficult. That power density, if it were applied to the whole PCB, would amount to a total dissipation of 3.375W. This is clearly an excessive PCB dissipation. I would be much more comfortable if the dissipation were less than 4mW/cm^2 based on PCB average power. This power level is stated for those PCBs that do not employ steps for heat transfer augmentation.
This discussion also amplifies the fact that the thermal design capabilities of the Eagle project must be involved in the design aspects of each of the modules. We must not depend upon one-way space travel for our modules to do the heat transfer experiments for those modules.
Dick Jansson --------------------------- mailto:[email protected] [email protected] --------------------------- -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Alan Bloom Sent: Tuesday, 10 October, 2006 0416 To: [email protected] Subject: [eagle] SMT heat dissipation in the space environment
This is a note I wrote some years ago during the AO-40 project. If people think it's useful, perhaps I could post it to Wikipedia.
Alan N1AL -------------------------------------------------------------------
Our local PC board technical expert here at HP has done an extensive study on power dissipation of surface-mount parts. While his report is based on non-space applications, there are a number of conclusions that are very interesting.
I won't include the entire report, partly because it is much too long, and partly because it is HP proprietary. But here are a few highlights.
He found surprisingly little discussion of these issues in the literature. So he performed a series of experiments using an Inframetrics 760 infared camera system to measure component temperatures. All measurements were taken on similar-sized PC boards mounted horizontally with no significant airflow.
One main conclusion: "Part ratings by the vendors have little meaning by themselves since trace width and part density have a huge impact on heat dissipation."
For example, what is the allowable power dissipation of an 0603 resistor? The answer is somewhere between 12 mw and 526 mw, depending on layout!
This is based on a 60 degC temperature rise. (Most resistors have a 125 degC max case temperature spec.) Temperature rise is directly proportional to power dissipation.
The 12 mw number was measured on 112 parts packed into a 0.75 square inch area and all dissipating the same power. The 526 mw measurement was on a single part soldered between two half-board-sized ground planes and tied through many vias to another ground plane on the back side of the board.
Manufacturers rate 1206-case resistors at 125 mw, versus 62.5 mw for 0603-case resistors, a 2:1 ratio. Actually, for most reasonable trace widths, an isolated 0603 resistor can dissipate roughly 80% as much as a 1206. When parts are packed densely together, power dissipation is limited by the maximum watts per square inch. Note that I used a 30 degC temperature rise for the following table:
0603 1206 Isolated Resistor: Large ground plane: 263 mw 403 mw 0.060 inch traces: 170 mw 199 mw 0.040 inch traces: 148 mw 177 mw 0.012 inch traces: 106 mw 128 mw 0.005 inch traces: 77 mw 100 mw 0.0025 bond wires: 55 mw 79 mw
High-density part layout, 0.75 square inch (4.84 cm^2) area: Number of parts 112 32 Power per part 6 mw 20 mw Total power 672 mw 640 mw
Number of parts 56 16 Power per part 1.5 mw 43 mw Total power 644 mw 688 mw
Number of parts 20 8 Power per part 31 mw 80 mw Total power 620 mw 640 mw
Since the power dissipation depends so strongly on trace width, then clearly most of the heat must be conducted, not radiated, on SMT resistors (and other parts). Even in the thin-trace case, much of the heat is conducted to, and radiated from, the PC board. You can see that on the infared photos: the PC board surrounding the part is quite hot.
Thermal resistance of isolated SOT-23 transistors was very similar to 0603 resistors up to 0.040" line widths. SOIC-8 voltage regulators had about 1/2 the thermal resistance of SOT-23.
The above numbers are probably conservative, since they are based on still air, even though most earth-based applications have forced-air cooling or at least natural convection (vertical PC board). The rule of thumb I generally use is 1 watt per square inch (155 mw/cm^2), which gives an average temperature rise of around 35 degC in still air.
I have been told that for space applications, 15 mw/cm^2 is a more appropriate limit. Assuming radiation cooling is only 1/10 as efficient as convection cooling, then that seems a reasonable spec.
Conclusion:
I have heard several rules of thumb on what percentage you should derate component power specifications for space applications. For through-hole devices, such a rule of thumb probably makes sense. THD devices dissipate most of their heat from the component body, and relatively little heat is conducted out the leads. On earth, most of that heat is conducted to the air; only a little is radiated. In space, radiation is the only heat-dissipating mechanism, so the power must be derated by a large factor.
But SMT devices are cooled mainly by conduction out the leads to the copper traces on the PC board, so they are not directly affected by the lack of air. It seems to me that as long as I keep within the 15 mw/cm^2 limit on the PC board, that allowable power dissipation of individual components should be governed by trace width, per the above table. If I have a hot component, I'll connect it to lots of copper and make sure no other hot components are nearby.
<END>
_______________________________________________ Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle http://amsat.org/mailman/listinfo/eagle
Dick, Why don't you take Alan's data and come up with a revised set of guidelines for designers. It would be helpful for designers to get help from the starting point rather than after they have completed their designs.
On Oct 10, 2006, at 6:14 AM, Dick Jansson-rr wrote:
Thank you Alan for this most useful and significant note. Operating in space, where we depend upon radiation as our primary mode of heat transfer, is truly different from all of our earth-bound experiences. Since the PCB size for most of the Eagle modules will be 125x180mm, an area of 225cm^2, following your 15mW/cm^2 becomes, in my mind and calculation, a bit difficult. That power density, if it were applied to the whole PCB, would amount to a total dissipation of 3.375W. This is clearly an excessive PCB dissipation. I would be much more comfortable if the dissipation were less than 4mW/cm^2 based on PCB average power. This power level is stated for those PCBs that do not employ steps for heat transfer augmentation.
This discussion also amplifies the fact that the thermal design capabilities of the Eagle project must be involved in the design aspects of each of the modules. We must not depend upon one-way space travel for our modules to do the heat transfer experiments for those modules.
Dick Jansson
[email protected]
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Alan Bloom Sent: Tuesday, 10 October, 2006 0416 To: [email protected] Subject: [eagle] SMT heat dissipation in the space environment
This is a note I wrote some years ago during the AO-40 project. If people think it's useful, perhaps I could post it to Wikipedia.
Alan N1AL
Our local PC board technical expert here at HP has done an extensive study on power dissipation of surface-mount parts. While his report is based on non-space applications, there are a number of conclusions that are very interesting.
I won't include the entire report, partly because it is much too long, and partly because it is HP proprietary. But here are a few highlights.
He found surprisingly little discussion of these issues in the literature. So he performed a series of experiments using an Inframetrics 760 infared camera system to measure component temperatures. All measurements were taken on similar-sized PC boards mounted horizontally with no significant airflow.
One main conclusion: "Part ratings by the vendors have little meaning by themselves since trace width and part density have a huge impact on heat dissipation."
For example, what is the allowable power dissipation of an 0603 resistor? The answer is somewhere between 12 mw and 526 mw, depending on layout!
This is based on a 60 degC temperature rise. (Most resistors have a 125 degC max case temperature spec.) Temperature rise is directly proportional to power dissipation.
The 12 mw number was measured on 112 parts packed into a 0.75 square inch area and all dissipating the same power. The 526 mw measurement was on a single part soldered between two half-board- sized ground planes and tied through many vias to another ground plane on the back side of the board.
Manufacturers rate 1206-case resistors at 125 mw, versus 62.5 mw for 0603-case resistors, a 2:1 ratio. Actually, for most reasonable trace widths, an isolated 0603 resistor can dissipate roughly 80% as much as a 1206. When parts are packed densely together, power dissipation is limited by the maximum watts per square inch. Note that I used a 30 degC temperature rise for the following table:
0603 1206Isolated Resistor: Large ground plane: 263 mw 403 mw 0.060 inch traces: 170 mw 199 mw 0.040 inch traces: 148 mw 177 mw 0.012 inch traces: 106 mw 128 mw 0.005 inch traces: 77 mw 100 mw 0.0025 bond wires: 55 mw 79 mw
High-density part layout, 0.75 square inch (4.84 cm^2) area: Number of parts 112 32 Power per part 6 mw 20 mw Total power 672 mw 640 mw
Number of parts 56 16 Power per part 1.5 mw 43 mw Total power 644 mw 688 mw Number of parts 20 8 Power per part 31 mw 80 mw Total power 620 mw 640 mwSince the power dissipation depends so strongly on trace width, then clearly most of the heat must be conducted, not radiated, on SMT resistors (and other parts). Even in the thin-trace case, much of the heat is conducted to, and radiated from, the PC board. You can see that on the infared photos: the PC board surrounding the part is quite hot.
Thermal resistance of isolated SOT-23 transistors was very similar to 0603 resistors up to 0.040" line widths. SOIC-8 voltage regulators had about 1/2 the thermal resistance of SOT-23.
The above numbers are probably conservative, since they are based on still air, even though most earth-based applications have forced- air cooling or at least natural convection (vertical PC board). The rule of thumb I generally use is 1 watt per square inch (155 mw/ cm^2), which gives an average temperature rise of around 35 degC in still air.
I have been told that for space applications, 15 mw/cm^2 is a more appropriate limit. Assuming radiation cooling is only 1/10 as efficient as convection cooling, then that seems a reasonable spec.
Conclusion:
I have heard several rules of thumb on what percentage you should derate component power specifications for space applications. For through-hole devices, such a rule of thumb probably makes sense. THD devices dissipate most of their heat from the component body, and relatively little heat is conducted out the leads. On earth, most of that heat is conducted to the air; only a little is radiated. In space, radiation is the only heat-dissipating mechanism, so the power must be derated by a large factor.
But SMT devices are cooled mainly by conduction out the leads to the copper traces on the PC board, so they are not directly affected by the lack of air. It seems to me that as long as I keep within the 15 mw/cm^2 limit on the PC board, that allowable power dissipation of individual components should be governed by trace width, per the above table. If I have a hot component, I'll connect it to lots of copper and make sure no other hot components are nearby.
<END>
Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
MessageSo 70% of the heat generated in the 70 cm receiver must be conducted to the case. Heat can be conducted from exposed pads on the bottom of the IC packages to one or two ground planes and then through aluminum standoffs to the case, so this seems possible. Modules like the ACP will be much harder as 90% of the heat must be conducted away and many IC packages will be designed for convection cooling using heat sinks mounted to the top surface.
73
John KD6OZH ----- Original Message ----- From: Dick Jansson-rr To: [email protected] ; [email protected] Sent: Tuesday, October 10, 2006 13:14 UTC Subject: [eagle] Re: SMT heat dissipation in the space environment
Thank you Alan for this most useful and significant note. Operating in space, where we depend upon radiation as our primary mode of heat transfer, is truly different from all of our earth-bound experiences. Since the PCB size for most of the Eagle modules will be 125x180mm, an area of 225cm^2, following your 15mW/cm^2 becomes, in my mind and calculation, a bit difficult. That power density, if it were applied to the whole PCB, would amount to a total dissipation of 3.375W. This is clearly an excessive PCB dissipation. I would be much more comfortable if the dissipation were less than 4mW/cm^2 based on PCB average power. This power level is stated for those PCBs that do not employ steps for heat transfer augmentation.
This discussion also amplifies the fact that the thermal design capabilities of the Eagle project must be involved in the design aspects of each of the modules. We must not depend upon one-way space travel for our modules to do the heat transfer experiments for those modules.
Dick Jansson --------------------------- [email protected] --------------------------- -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Alan Bloom Sent: Tuesday, 10 October, 2006 0416 To: [email protected] Subject: [eagle] SMT heat dissipation in the space environment
This is a note I wrote some years ago during the AO-40 project. If people think it's useful, perhaps I could post it to Wikipedia.
Alan N1AL -------------------------------------------------------------------
Our local PC board technical expert here at HP has done an extensive study on power dissipation of surface-mount parts. While his report is based on non-space applications, there are a number of conclusions that are very interesting.
I won't include the entire report, partly because it is much too long, and partly because it is HP proprietary. But here are a few highlights.
He found surprisingly little discussion of these issues in the literature. So he performed a series of experiments using an Inframetrics 760 infared camera system to measure component temperatures. All measurements were taken on similar-sized PC boards mounted horizontally with no significant airflow.
One main conclusion: "Part ratings by the vendors have little meaning by themselves since trace width and part density have a huge impact on heat dissipation."
For example, what is the allowable power dissipation of an 0603 resistor? The answer is somewhere between 12 mw and 526 mw, depending on layout!
This is based on a 60 degC temperature rise. (Most resistors have a 125 degC max case temperature spec.) Temperature rise is directly proportional to power dissipation.
The 12 mw number was measured on 112 parts packed into a 0.75 square inch area and all dissipating the same power. The 526 mw measurement was on a single part soldered between two half-board-sized ground planes and tied through many vias to another ground plane on the back side of the board.
Manufacturers rate 1206-case resistors at 125 mw, versus 62.5 mw for 0603-case resistors, a 2:1 ratio. Actually, for most reasonable trace widths, an isolated 0603 resistor can dissipate roughly 80% as much as a 1206. When parts are packed densely together, power dissipation is limited by the maximum watts per square inch. Note that I used a 30 degC temperature rise for the following table:
0603 1206 Isolated Resistor: Large ground plane: 263 mw 403 mw 0.060 inch traces: 170 mw 199 mw 0.040 inch traces: 148 mw 177 mw 0.012 inch traces: 106 mw 128 mw 0.005 inch traces: 77 mw 100 mw 0.0025 bond wires: 55 mw 79 mw
High-density part layout, 0.75 square inch (4.84 cm^2) area: Number of parts 112 32 Power per part 6 mw 20 mw Total power 672 mw 640 mw
Number of parts 56 16 Power per part 1.5 mw 43 mw Total power 644 mw 688 mw
Number of parts 20 8 Power per part 31 mw 80 mw Total power 620 mw 640 mw
Since the power dissipation depends so strongly on trace width, then clearly most of the heat must be conducted, not radiated, on SMT resistors (and other parts). Even in the thin-trace case, much of the heat is conducted to, and radiated from, the PC board. You can see that on the infared photos: the PC board surrounding the part is quite hot.
Thermal resistance of isolated SOT-23 transistors was very similar to 0603 resistors up to 0.040" line widths. SOIC-8 voltage regulators had about 1/2 the thermal resistance of SOT-23.
The above numbers are probably conservative, since they are based on still air, even though most earth-based applications have forced-air cooling or at least natural convection (vertical PC board). The rule of thumb I generally use is 1 watt per square inch (155 mw/cm^2), which gives an average temperature rise of around 35 degC in still air.
I have been told that for space applications, 15 mw/cm^2 is a more appropriate limit. Assuming radiation cooling is only 1/10 as efficient as convection cooling, then that seems a reasonable spec.
Conclusion:
I have heard several rules of thumb on what percentage you should derate component power specifications for space applications. For through-hole devices, such a rule of thumb probably makes sense. THD devices dissipate most of their heat from the component body, and relatively little heat is conducted out the leads. On earth, most of that heat is conducted to the air; only a little is radiated. In space, radiation is the only heat-dissipating mechanism, so the power must be derated by a large factor.
But SMT devices are cooled mainly by conduction out the leads to the copper traces on the PC board, so they are not directly affected by the lack of air. It seems to me that as long as I keep within the 15 mw/cm^2 limit on the PC board, that allowable power dissipation of individual components should be governed by trace width, per the above table. If I have a hot component, I'll connect it to lots of copper and make sure no other hot components are nearby.
<END>
_______________________________________________ Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
------------------------------------------------------------------------------
_______________________________________________ Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
So 70% of the heat generated in the 70 cm receiver must be conducted to the case. Heat can be conducted from exposed pads on the bottom of the IC packages to one or two ground planes and then through aluminum standoffs to the case, so this seems possible. Modules like the ACP will be much harder as 90% of the heat must be conducted away and many IC packages will be designed for convection cooling using heat sinks mounted to the top surface.
For Rudak on AO-40 we designed it so the high heat parts were on the top PCB, and then machined a custom module cover so that it was in contact with the IC pakages. Gap filler was then applied and the assembly screwed together.
For the RAM moduls, which mounted at right angels to the main PC board, we ran an aluminum strip heat sink across the module, also in contact with the memory ICs and with gap filler, and then attached to the sides of the module.
Lots of conductive paths!
73,
Lyle KK7P
John:
I am currently working on this model. Have patience and I will be back to you and the project.
Dick Jansson --------------------------- mailto:[email protected] [email protected] --------------------------- -----Original Message----- From: John B. Stephensen [mailto:[email protected]] Sent: Tuesday, 10 October, 2006 2004 To: Dick Jansson-rr; [email protected]; [email protected] Subject: Re: [eagle] Re: SMT heat dissipation in the space environment
So 70% of the heat generated in the 70 cm receiver must be conducted to the case. Heat can be conducted from exposed pads on the bottom of the IC packages to one or two ground planes and then through aluminum standoffs to the case, so this seems possible. Modules like the ACP will be much harder as 90% of the heat must be conducted away and many IC packages will be designed for convection cooling using heat sinks mounted to the top surface.
73
John KD6OZH
----- Original Message ----- From: Dick mailto:[email protected] Jansson-rr To: [email protected] ; [email protected] Sent: Tuesday, October 10, 2006 13:14 UTC Subject: [eagle] Re: SMT heat dissipation in the space environment
Thank you Alan for this most useful and significant note. Operating in space, where we depend upon radiation as our primary mode of heat transfer, is truly different from all of our earth-bound experiences. Since the PCB size for most of the Eagle modules will be 125x180mm, an area of 225cm^2, following your 15mW/cm^2 becomes, in my mind and calculation, a bit difficult. That power density, if it were applied to the whole PCB, would amount to a total dissipation of 3.375W. This is clearly an excessive PCB dissipation. I would be much more comfortable if the dissipation were less than 4mW/cm^2 based on PCB average power. This power level is stated for those PCBs that do not employ steps for heat transfer augmentation.
This discussion also amplifies the fact that the thermal design capabilities of the Eagle project must be involved in the design aspects of each of the modules. We must not depend upon one-way space travel for our modules to do the heat transfer experiments for those modules.
Dick Jansson --------------------------- mailto:[email protected] [email protected] --------------------------- -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Alan Bloom Sent: Tuesday, 10 October, 2006 0416 To: [email protected] Subject: [eagle] SMT heat dissipation in the space environment
This is a note I wrote some years ago during the AO-40 project. If people think it's useful, perhaps I could post it to Wikipedia.
Alan N1AL -------------------------------------------------------------------
Our local PC board technical expert here at HP has done an extensive study on power dissipation of surface-mount parts. While his report is based on non-space applications, there are a number of conclusions that are very interesting.
I won't include the entire report, partly because it is much too long, and partly because it is HP proprietary. But here are a few highlights.
He found surprisingly little discussion of these issues in the literature. So he performed a series of experiments using an Inframetrics 760 infared camera system to measure component temperatures. All measurements were taken on similar-sized PC boards mounted horizontally with no significant airflow.
One main conclusion: "Part ratings by the vendors have little meaning by themselves since trace width and part density have a huge impact on heat dissipation."
For example, what is the allowable power dissipation of an 0603 resistor? The answer is somewhere between 12 mw and 526 mw, depending on layout!
This is based on a 60 degC temperature rise. (Most resistors have a 125 degC max case temperature spec.) Temperature rise is directly proportional to power dissipation.
The 12 mw number was measured on 112 parts packed into a 0.75 square inch area and all dissipating the same power. The 526 mw measurement was on a single part soldered between two half-board-sized ground planes and tied through many vias to another ground plane on the back side of the board.
Manufacturers rate 1206-case resistors at 125 mw, versus 62.5 mw for 0603-case resistors, a 2:1 ratio. Actually, for most reasonable trace widths, an isolated 0603 resistor can dissipate roughly 80% as much as a 1206. When parts are packed densely together, power dissipation is limited by the maximum watts per square inch. Note that I used a 30 degC temperature rise for the following table:
0603 1206 Isolated Resistor: Large ground plane: 263 mw 403 mw 0.060 inch traces: 170 mw 199 mw 0.040 inch traces: 148 mw 177 mw 0.012 inch traces: 106 mw 128 mw 0.005 inch traces: 77 mw 100 mw 0.0025 bond wires: 55 mw 79 mw
High-density part layout, 0.75 square inch (4.84 cm^2) area: Number of parts 112 32 Power per part 6 mw 20 mw Total power 672 mw 640 mw
Number of parts 56 16 Power per part 1.5 mw 43 mw Total power 644 mw 688 mw
Number of parts 20 8 Power per part 31 mw 80 mw Total power 620 mw 640 mw
Since the power dissipation depends so strongly on trace width, then clearly most of the heat must be conducted, not radiated, on SMT resistors (and other parts). Even in the thin-trace case, much of the heat is conducted to, and radiated from, the PC board. You can see that on the infared photos: the PC board surrounding the part is quite hot.
Thermal resistance of isolated SOT-23 transistors was very similar to 0603 resistors up to 0.040" line widths. SOIC-8 voltage regulators had about 1/2 the thermal resistance of SOT-23.
The above numbers are probably conservative, since they are based on still air, even though most earth-based applications have forced-air cooling or at least natural convection (vertical PC board). The rule of thumb I generally use is 1 watt per square inch (155 mw/cm^2), which gives an average temperature rise of around 35 degC in still air.
I have been told that for space applications, 15 mw/cm^2 is a more appropriate limit. Assuming radiation cooling is only 1/10 as efficient as convection cooling, then that seems a reasonable spec.
Conclusion:
I have heard several rules of thumb on what percentage you should derate component power specifications for space applications. For through-hole devices, such a rule of thumb probably makes sense. THD devices dissipate most of their heat from the component body, and relatively little heat is conducted out the leads. On earth, most of that heat is conducted to the air; only a little is radiated. In space, radiation is the only heat-dissipating mechanism, so the power must be derated by a large factor.
But SMT devices are cooled mainly by conduction out the leads to the copper traces on the PC board, so they are not directly affected by the lack of air. It seems to me that as long as I keep within the 15 mw/cm^2 limit on the PC board, that allowable power dissipation of individual components should be governed by trace width, per the above table. If I have a hot component, I'll connect it to lots of copper and make sure no other hot components are nearby.
<END>
_______________________________________________ Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle http://amsat.org/mailman/listinfo/eagle
_____
_______________________________________________ Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
Since the PCB size for most of the Eagle modules will be 125x180mm, an area of 225cm^2, following your 15mW/cm^2 becomes, in my mind and calculation, a bit difficult. That power density, if it were applied to the whole PCB, would amount to a total dissipation of 3.375W. This is clearly an excessive PCB dissipation.
I think the maximum power per cm^2 and the maximum power for an entire board are separate issues. That is, make sure your total dissipation is within the module limits _AND_ no portion of the circuitry exceeds 15 mW/cm^2.
Alan
On Tue, 2006-10-10 at 06:14, Dick Jansson-rr wrote:
Thank you Alan for this most useful and significant note. Operating in space, where we depend upon radiation as our primary mode of heat transfer, is truly different from all of our earth-bound experiences. Since the PCB size for most of the Eagle modules will be 125x180mm, an area of 225cm^2, following your 15mW/cm^2 becomes, in my mind and calculation, a bit difficult. That power density, if it were applied to the whole PCB, would amount to a total dissipation of 3.375W. This is clearly an excessive PCB dissipation. I would be much more comfortable if the dissipation were less than 4mW/cm^2 based on PCB average power. This power level is stated for those PCBs that do not employ steps for heat transfer augmentation.
This discussion also amplifies the fact that the thermal design capabilities of the Eagle project must be involved in the design aspects of each of the modules. We must not depend upon one-way space travel for our modules to do the heat transfer experiments for those modules.
Dick Jansson
[email protected]
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Alan Bloom Sent: Tuesday, 10 October, 2006 0416 To: [email protected] Subject: [eagle] SMT heat dissipation in the space environment
This is a note I wrote some years ago during the AO-40 project. If people think it's useful, perhaps I could post it to Wikipedia.
Alan N1AL
Our local PC board technical expert here at HP has done an extensive study on power dissipation of surface-mount parts. While his report is based on non-space applications, there are a number of conclusions that are very interesting.
I won't include the entire report, partly because it is much too long, and partly because it is HP proprietary. But here are a few highlights.
He found surprisingly little discussion of these issues in the literature. So he performed a series of experiments using an Inframetrics 760 infared camera system to measure component temperatures. All measurements were taken on similar-sized PC boards mounted horizontally with no significant airflow.
One main conclusion: "Part ratings by the vendors have little meaning by themselves since trace width and part density have a huge impact on heat dissipation."
For example, what is the allowable power dissipation of an 0603 resistor? The answer is somewhere between 12 mw and 526 mw, depending on layout!
This is based on a 60 degC temperature rise. (Most resistors have a 125 degC max case temperature spec.) Temperature rise is directly proportional to power dissipation.
The 12 mw number was measured on 112 parts packed into a 0.75 square inch area and all dissipating the same power. The 526 mw measurement was on a single part soldered between two half-board-sized ground planes and tied through many vias to another ground plane on the back side of the board.
Manufacturers rate 1206-case resistors at 125 mw, versus 62.5 mw for 0603-case resistors, a 2:1 ratio. Actually, for most reasonable trace widths, an isolated 0603 resistor can dissipate roughly 80% as much as a 1206. When parts are packed densely together, power dissipation is limited by the maximum watts per square inch. Note that I used a 30 degC temperature rise for the following table:
0603 1206Isolated Resistor: Large ground plane: 263 mw 403 mw 0.060 inch traces: 170 mw 199 mw 0.040 inch traces: 148 mw 177 mw 0.012 inch traces: 106 mw 128 mw 0.005 inch traces: 77 mw 100 mw 0.0025 bond wires: 55 mw 79 mw
High-density part layout, 0.75 square inch (4.84 cm^2) area: Number of parts 112 32 Power per part 6 mw 20 mw Total power 672 mw 640 mw
Number of parts 56 16 Power per part 1.5 mw 43 mw Total power 644 mw 688 mw Number of parts 20 8 Power per part 31 mw 80 mw Total power 620 mw 640 mwSince the power dissipation depends so strongly on trace width, then clearly most of the heat must be conducted, not radiated, on SMT resistors (and other parts). Even in the thin-trace case, much of the heat is conducted to, and radiated from, the PC board. You can see that on the infared photos: the PC board surrounding the part is quite hot.
Thermal resistance of isolated SOT-23 transistors was very similar to 0603 resistors up to 0.040" line widths. SOIC-8 voltage regulators had about 1/2 the thermal resistance of SOT-23.
The above numbers are probably conservative, since they are based on still air, even though most earth-based applications have forced-air cooling or at least natural convection (vertical PC board). The rule of thumb I generally use is 1 watt per square inch (155 mw/cm^2), which gives an average temperature rise of around 35 degC in still air.
I have been told that for space applications, 15 mw/cm^2 is a more appropriate limit. Assuming radiation cooling is only 1/10 as efficient as convection cooling, then that seems a reasonable spec.
Conclusion:
I have heard several rules of thumb on what percentage you should derate component power specifications for space applications. For through-hole devices, such a rule of thumb probably makes sense. THD devices dissipate most of their heat from the component body, and relatively little heat is conducted out the leads. On earth, most of that heat is conducted to the air; only a little is radiated. In space, radiation is the only heat-dissipating mechanism, so the power must be derated by a large factor.
But SMT devices are cooled mainly by conduction out the leads to the copper traces on the PC board, so they are not directly affected by the lack of air. It seems to me that as long as I keep within the 15 mw/cm^2 limit on the PC board, that allowable power dissipation of individual components should be governed by trace width, per the above table. If I have a hot component, I'll connect it to lots of copper and make sure no other hot components are nearby.
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participants (5)
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Alan Bloom -
Dick Jansson-rr -
John B. Stephensen -
Louis McFadin -
Lyle Johnson