MAVEN CDR May 23-25, 2011

Thermal Considerations forBoard Electrical and Mechanical Design

Christopher SmithThermal Engineer

Mars Atmosphere and Volatile EvolutioN (MAVEN) Mission

12-2MAVEN 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

12-3MAVEN 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

12-4MAVEN 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

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RBSP LVPS Ambient, Top

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RBSP LVPS Ambient, Bottom

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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

12-8MAVEN 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 D– We need to work to keep the board below this temperature– For components that dissipate <= 100 mW, use 70 ºC ?

12-9MAVEN Board Thermal, June 16, 2011

θJB, θJA, θJC

• Thermal resistance given, or not, in component data sheets– “Junction” refers to the max temperature inside the component– Units of ºC/W or similar– θJB: Thermal resistance from components junction to the board

– No Convection– ψJB yet another resistance, different test method, close to but < θJB

– θJA: Thermal resistance from components junction to “Ambient”– Component and board convection only, board not heat sunk

– θJC: Thermal resistance from components junction to outer case

– θCA: You may occasionally get a θCA provided where:θJA= θCA, + θJC

– You might not get any of these. You can estimate θJB using:θJB= θJC + L/kΣAi

– K=thermal conductivity (copper=390 W/mC)– ΣAi = sum of the cross sectional are of your leads to the board– L = length of lead from component to board

12-10MAVEN Board Thermal, June 16, 2011

θJA, θJB, θJC, Useful with Caveats?

• θJB 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 1st 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

12-11MAVEN Board Thermal, June 16, 2011

Using θJB, θJA, θJC

• θJB is what we want (GREAT)

• L/kΣAi use if there is no θJB for non IC bits (GREAT)

• ψJB is close to what we want (GOOD)– Includes convection effects– Increase by 15% to estimate θJB

• θJC can be useful (OK?)– θJB= θJC + L/kΣAi

• θJA isn’t horrible (Who Knows)– θJB was roughly 50% of θJA 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 θJA

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Other Sources of Info

• Lots about this online. Here are a couple places to start:

http://www.intersil.com/data/tb/tb379.pdfhttp://www.coolingzone.com/library.php?read=519http://www.coolingzone.com/library.php?read=520http://focus.ti.com/lit/an/spra953a/spra953a.pdf

12-13MAVEN 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 θJB, θJA, or θJC 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

12-14MAVEN 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

12-15MAVEN 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

12-16MAVEN 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

12-17MAVEN 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

D from box• 2 oz copper for each• 2 layers of FR4 between the

copper layers

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Isolated Thermal Plane, No Radiation

12-19MAVEN 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

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Less Isolated Thermal Plane

• Reduce the “Moat” to 0, no overlap but no gap either

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Un-Isolated Thermal Plane

• Overlap Ground planes by 0.1”• Should do this anywhere noise is not an

issue

12-22MAVEN Board Thermal, June 16, 2011

Back to Less Isolated Thermal Plane

• “Moat” back to 0• Turn Radiation Off• Board Power doubled 1.2 +2.8 = 4 W

12-23MAVEN Board Thermal, June 16, 2011

4 oz copper

• Copper layers 2 oz → 4 oz

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“Island” Near Edge

• Move Island closer to edge

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Ground Plane Results Summary

Ground Plane Model ExperimentationSlide Number: 18 19 20 21 22 23 N/A 24 N/A

Gap Between Isolated Ground Plane an Thermal Ground (in): 0.1 0.1 0 -0.1 0 0 0 0 0

Power on Isolated Plane (W): 0.6 0.6 0.6 0.6 1.2 1.2 1.2 1.2 1.6

Power on Thermal Ground (W): 1.4 1.4 1.4 1.4 2.8 2.8 2.8 2.8 3.4

Total Board Power (W): 2 2 2 2 4 4 4 4 5

Board Edge Sink Temp (ºC): 0 0 0 0 0 0 0 0 0

Radiation Active?: No YES YES Yes No No No No No

Radiation Sink Temp (ºC): N/A 10 10 10 N/A N/A N/A N/A N/A

Copper Thickness (oz): 2 2 2 2 2 4 4 4 5

FR4 Thickness Beteween Grounds (in): 0.016 0.016 0.016 0.016 0.016 0.016 0.031 0.016 0.016

Island Distance From Board "Sink" Edge (in): 2.5 2.5 2.5 2.5 2.5 2.5 2.5 0.5 0.5

Temperature of Isolated Ground Plane (ºC): 94.5 22.3 12.7 11.6 19.7 11.9 14.4 9.8 12

Modified from Previous Run:  

12-26MAVEN 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

12-27MAVEN 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

12-28MAVEN 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

12-29MAVEN 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

12-30MAVEN 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?

12-31MAVEN Board Thermal, June 16, 2011

Wrapup

• Identify all parts > 50 mW– Hunt down the best θJB, θJA, θJC , L/kΣAi 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