MAVEN CDR May 23-25, 2011 Thermal Considerations for Board Electrical and Mechanical Design Christopher Smith Thermal Engineer Mars Atmosphere and Volatile EvolutioN (MAVEN) Mission

12-2 MAVEN Board Thermal, June 16, 2011 Why Do We Care All RBSP electronics boards were designed without any input from the thermal engineer –Same as we have done forever –Its been working so far, or has it? PWM chip on RBSP LVPS was running 60 ºC above the box temp and was running at 90 ºC before thermal vac test was aborted RBSP LVPS issue was driven by excessive board temp NOT excessive component temp –Power dissipated on the board could not get off the board

12-3 MAVEN Board Thermal, June 16, 2011 RBSP LVPS Board Issues Investigation revealed several issues –Board mounting standoffs were G10 instead of Aluminum –EMI shield was Alodined Aluminum shutting off radiation –More importantly ground / thermal planes did not connect to the mounting areas

12-4 MAVEN Board Thermal, June 16, 2011 RBSP Component Issues After examining the board following the TVAC failure several features were found that were potential component thermal issues. –Two resistors on board were dissipating.5 W each –A diode was dissipating.5 W These were unknown to the thermal engineer even for his lame spreadsheet analysis used for part temperatures

12-5 MAVEN Board Thermal, June 16, 2011 RBSP LVPS Ambient, Top

12-6 MAVEN Board Thermal, June 16, 2011 RBSP LVPS Ambient, Bottom

12-7 MAVEN Board Thermal, June 16, 2011 Peak Power Use Peak Power –It may be possible to ignore short duration peaks but it would need to be looked at on a case by case basis –Component peaks for part stress analysis –Board peaks for boards, etc … –Thermal engineer will need realistic possible combinations of board peaks to produce overall box peaks in analysis –Boards need to be tested to peak power Need GSE to do so If not possible extended test temperatures could substitute with some hand waving

12-8 MAVEN Board Thermal, June 16, 2011 Board Temperature Assumption SWIA, STATIC, SWEA, and DPU currently qual to 50 ºC operational RBSP DCB board was running at Box + 8 ºC –1 Watt total dissipation –Radiatively coupled to the LVPS board –Connected to box by wedge locks LVPS board near hot components was at Box +20 ºC Use 65 ºC for an assumed board temperature to calculate component temperatures in part stress analysis? –Not very conservative –Margin is carried in part stress analysis assumptions –Margin is carried in 10 ºC Qual to predict  –We need to work to keep the board below this temperature –For components that dissipate <= 100 mW, use 70 ºC ?

12-9 MAVEN Board Thermal, June 16, 2011 θ JB, θ JA, θ JC Thermal resistance given, or not, in component data sheets –“Junction” refers to the max temperature inside the component –Units of ºC/W or similar –θ JB : Thermal resistance from components junction to the board –No Convection –ψ JB yet another resistance, different test method, close to but < θ JB –θ JA : Thermal resistance from components junction to “Ambient” –Component and board convection only, board not heat sunk –θ JC : Thermal resistance from components junction to outer case –θ CA : You may occasionally get a θ CA provided where: θ JA = θ CA, + θ JC –You might not get any of these. You can estimate θ JB using: θ JB = θ JC + L/kΣA i –K=thermal conductivity (copper=390 W/mC) –ΣA i = sum of the cross sectional are of your leads to the board –L = length of lead from component to board

12-10 MAVEN Board Thermal, June 16, 2011 θ JA, θ JB, θ JC, Useful with Caveats? θ JB is the only one that is correct for our purposes and even it is wrong? JEDEC standard 51-3 specifically states "It should be emphasized that values measured with these test boards cannot be used to directly predict any particular system application performance but are for the purposes of comparison between packages.” Your package connection to board, ground planes, ground plane connection to thermal ground, and nearby components effect this. How good is it for our purposes? –By definition good to the 1 st order only –Don’t really know until we accumulate more data –For now lets figure it gets us to within 5 ºC in most cases

12-11 MAVEN Board Thermal, June 16, 2011 Using θ JB, θ JA, θ JC θ JB is what we want (GREAT) L/kΣA i use if there is no θ JB for non IC bits (GREAT) ψ JB is close to what we want (GOOD) –Includes convection effects –Increase by 15% to estimate θ JB θ JC can be useful (OK?) –θ JB = θ JC + L/kΣA i θ JA isn’t horrible (Who Knows) –θ JB was roughly 50% of θ JA for a few components where they were both listed but have no idea if this is typical –Depends a lot on board size, Small test boards used for θ JA

12-12 MAVEN Board Thermal, June 16, 2011 Other Sources of Info Lots about this online. Here are a couple places to start:

12-13 MAVEN Board Thermal, June 16, 2011 Real Component Temperatures The only way to actually know your component temp is to test it on a board “thermally” equivalent to the flight board –ETU boards should be built as close to flight as possible –Thermal Images (if done correctly) and direct measurements will help determine the temp of your key components on your ETU –Boards should be flight mounted to something equivalent to the flight box mounting –If you use different components on ETU boards than flight, you won’t get this information until the flight board is built Be conservative in this case How conservative depends on weather you have θ JB, θ JA, or θ JC in your pocket Simple TVAC tests at the ETU level can be very useful –STATIC and SWIA stacked board arrangement –High dissipation boards –High dissipation components

12-14 MAVEN Board Thermal, June 16, 2011 Cooling Down Hot Components Stop doing whatever it is your doing to make it a high dissipater Stake as much area as possible with thermal potting compound –Arathane 5753 with 50%, by mass, Boron Nitride k =1 W/mC –Consider putting a pad underneath the component connected to the thermal plane and stake to that –If filling a large gap, fill with a Beryllium Oxcide shim plus thermally conductive potting k =265 W/mC

12-15 MAVEN Board Thermal, June 16, 2011 Cooling Down Hot Components Dead bug components and stake, more surface area Place the component as close to thermal ground as possible –Near wedge locks, screw posts or mounting lip –Always do this with highest dissipaters Consider changes to ground plane under and near component –Get the thermal ground plane underneath it –If you can’t do that, use a separate ground plane as a spreader and “connect” it to thermal ground Heat sink the component –Last resort? –Mount component to the box –Use a flexible thermal strap from the component to box

12-16 MAVEN Board Thermal, June 16, 2011 Thermal / Ground Planes If possible, the board should have 4 ounces of thermal ground plane that covers the entire board –4 oz is a best guess at this point so 2*2 oz, or 4*1oz, etc –2 might be good enough, don’t know yet One thermal ground plane needs to connect to the box well –Direct connection works best so chassis ground = thermal ground If capacitive coupling is a noise issue –Avoid sensitive planes / areas –Get as close as possible –Overlap the perimeter by as much as possible Noise and Thermal are both issues that we may not have a good handle on until the flight board is built –Don’t be too conservative and make it all a thermal problem

12-17 MAVEN Board Thermal, June 16, 2011 Example Board 8” x 8”, 2.0 W board –0.6 W dissipated on “island” plane –1.4 W dissipated on remainder of board Isolated plane separated from thermal ground plane by 0.1” moat Perimeter of board held at 0 ºC –So temps displayed are equal to  from box 2 oz copper for each 2 layers of FR4 between the copper layers

12-18 MAVEN Board Thermal, June 16, 2011 Isolated Thermal Plane, No Radiation

12-19 MAVEN Board Thermal, June 16, 2011 Isolated Thermal Plane, Radiation Enable radiation both side to 10 ºC sink Similar to what it would see with a board on each side

12-20 MAVEN Board Thermal, June 16, 2011 Less Isolated Thermal Plane Reduce the “Moat” to 0, no overlap but no gap either

12-21 MAVEN Board Thermal, June 16, 2011 Un-Isolated Thermal Plane Overlap Ground planes by 0.1” Should do this anywhere noise is not an issue

12-22 MAVEN Board Thermal, June 16, 2011 Back to Less Isolated Thermal Plane “Moat” back to 0 Turn Radiation Off Board Power doubled = 4 W

12-23 MAVEN Board Thermal, June 16, oz copper Copper layers 2 oz → 4 oz

12-24 MAVEN Board Thermal, June 16, 2011 “Island” Near Edge Move Island closer to edge

12-25 MAVEN Board Thermal, June 16, 2011 Ground Plane Results Summary Ground Plane Model Experimentation Slide Number: N/A24N/A Gap Between Isolated Ground Plane an Thermal Ground (in): Power on Isolated Plane (W): Power on Thermal Ground (W): Total Board Power (W): Board Edge Sink Temp ( º C): Radiation Active?:NoYES YesNo Radiation Sink Temp ( º C):N/A10 N/A Copper Thickness (oz): FR4 Thickness Beteween Grounds (in): Island Distance From Board "Sink" Edge (in): Temperature of Isolated Ground Plane ( º C): Modified from Previous Run:

12-26 MAVEN Board Thermal, June 16, 2011 Ground Plane Action Plan Roughly 1 oz of thermal plane per Watt –2 oz minimum Thermal plane needs to go right up to noise sensitive ground planes –If not possible need to work closely with thermal engineer to solve heat path issue If other ground planes are not sensitive, thermal plane should completely overlap them Minimize FR4 between thermal plane and important dissipaters, one or two layers of FR4 assumed here Isolated planes that are high dissipaters should move as close as possible to thermal ground

12-27 MAVEN Board Thermal, June 16, 2011 When Good Thermal Planes Go Bad All of the above analysis assumes that the thermal plane has good connection to the sink RBSP had lots of overlapping copper but the heat had no where to go Chassis ground was quite small and carefully avoided “keep out” zones around fasteners Standoffs were G10 by accident Ground plane not connected to all but one of the screw mounting locations

12-28 MAVEN Board Thermal, June 16, 2011 Increase Mounting Area The MAVEN perimeter lip on the box frames offers a lot of area to conduct heat (kA/L) –To take advantage of the area you need lots of screws to the lip Lots of smaller screws better than a few bigger ones –Can also take better advantage of this area by bonding with thermal epoxy Still need to get energy from the thermal plane to the box Thermal Planes Thermal Pad VIAs Thermal Epoxy (Potting) −Create a copper pad around perimeter at mounting surface −Connect pad to thermal planes with lots of VIAs −Connect pad to box with lots of screws, epoxy, or potting compound

12-29 MAVEN Board Thermal, June 16, 2011 Screws and Standoffs Need to increase area as much as practical –More standoffs better –Bigger standoffs.25” to.30” diameter –SWIA and STATIC should try to fit in 0.3” standoffs since there are only 4 to connect all the boards Standoffs need to be thermally conductive, Aluminum No thermal breaks in standoff stacks –PEMs are ok if there is good contact to thermally pad on the other side –Stainless is a thermal insulator, even a washer Stainless = 16 W/mK Aluminum = 160 W/mK G10 =.25 W/mK

12-30 MAVEN Board Thermal, June 16, 2011 Screws, the Details Need to carefully connect standoff and screw pad to thermal plane –Plated through holes –Pad on both sides connected with vias –No wagon wheels Beware of misinterpreted keep out zones Bigger Pads Thermal Planes Thermal Pad Standoff Thermal Pad Vias Keep Out?

12-31 MAVEN Board Thermal, June 16, 2011 Wrapup Identify all parts > 50 mW –Hunt down the best θ JB, θ JA, θ JC, L/kΣA i for these components Might use a value from a very similar part you have good data for –Identify components that need special heat sinking –Verify you are correct using a thermal imager Identify Component, and Board Peak power Plan ground planes and connection to chassis –Combine thermal ground plane with chassis ground plane if possible –1 oz of thermal ground plane per watt, 2 oz minimum –Generate good instructions for standoff pads, lip pads, etc Provide data to thermal engineer, probably not me to produce predicts and iterate toward solution All done before we fabricate flight PWDs –Errr … that means this month