VG VG 11W.O. Miller

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Bridge AnalysisBridge Analysis

• ObjectiveObjective– Develop model suited to Develop model suited to

examining effect of low velocity examining effect of low velocity air flow through the isolating air air flow through the isolating air gapgap

• Stagnate air pocket Stagnate air pocket produces maximum effectproduces maximum effect

– Suspect that a CFD solution will Suspect that a CFD solution will show colder chip temperatures, show colder chip temperatures, but possibly higher sensor but possibly higher sensor temperaturestemperatures

• Solution approachSolution approach– Quarter model, similar to ANSYS Quarter model, similar to ANSYS

model reported by othersmodel reported by others– Half model, with air boxHalf model, with air box

Air gap

1/4

1/2

Air box for 1/2

VG VG 22W.O. Miller

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Bridge Model-Foam Support Contacts Bridge Model-Foam Support Contacts FacingFacing

• Two FEA ModelsTwo FEA Models– Quarter modelQuarter model

• Air gap (1mm) confined to region directly beneath the bridge, Air gap (1mm) confined to region directly beneath the bridge, presumably simulated arrangement used by others with ANSYSpresumably simulated arrangement used by others with ANSYS

• Wire bonds simulated with wide thin sheet , 2mils thickWire bonds simulated with wide thin sheet , 2mils thick– Initial mesh problem solved by breaking thin sheet into two Initial mesh problem solved by breaking thin sheet into two

solidssolids

• Heat load, 0.25W/chip, total 10chipsHeat load, 0.25W/chip, total 10chips

• Cooling tube wall reference temperature, -28Cooling tube wall reference temperature, -28ºCºC– Half ModelHalf Model

• Air space encloses bridge, allowing three-dimensional affectsAir space encloses bridge, allowing three-dimensional affects– Space beneath bridge remains 1mmSpace beneath bridge remains 1mm

• Wire bonds, simulated with narrow band, improved aspect ratio to Wire bonds, simulated with narrow band, improved aspect ratio to fix mesh issues observed in the quarter modelfix mesh issues observed in the quarter model

– Treated as one solidTreated as one solid

• Heat load, 0.25W/chip, 40chipsHeat load, 0.25W/chip, 40chips

• Cooling tube reference temperature, -28Cooling tube reference temperature, -28ºCºC

VG VG 33W.O. Miller

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Model: Solid Thermal PropertiesModel: Solid Thermal Properties

Item Thickness W/mK

Tube OD: 2.8mm, ID: 2.1mm 200

Composite Facing Thickness 0.21mm 219/1/110

Cable 0.125 0.2

Detector 0.32mm 148

CC bridge 0.5mm 25/25/700 (X,Y,Z)

Dielectric Hybrid 0.23mm 1

Chips 0.38mm 148

Carbon Foam for Sandwich 4.9mm 3

Carbon Foam for Bridge 4.9mm 45/45/45

Carbon Foam for Tube n/a 45/45/45/45

VG VG 44W.O. Miller

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Model: Adhesives and Cable Model: Adhesives and Cable DefinitionDefinition

Cable definition combines two layers of adhesives with cable

Shell Element Parameters

Item Thickness W/mK

Chip to Hybrid 0.002in 1.55

Hybrid to CC Bridge 0.002in 1.0

CC Bridge to Foam Support 0.002in 1.0

Foam Support to Tube Foam Support 0.002in 1.0

Foam Tube Support to Facing 0.002 1.0

Tube Support Foam to Tube 0.002in 1.0

Sandwich Core to Facing 0.002in 1.0

Cable 0.227mm 0.31

VG VG 55W.O. Miller

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Bridge Model-1/4 SizeBridge Model-1/4 Size

• SolutionSolution– With air and with wire With air and with wire

bondsbonds– Peak chip, -5.84Peak chip, -5.84ºCºC– Range in sensor Range in sensor

temperaturetemperature

• -20.0ºC to -24.4ºC -20.0ºC to -24.4ºC

• Warmest on point Warmest on point sensor is directly sensor is directly beneath bridge where beneath bridge where bridge foam contacts bridge foam contacts the facingthe facing

Sensor -20ºC

VG VG 66W.O. Miller

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Quarter ModelQuarter Model

• End viewEnd view– Illustrates temperature Illustrates temperature

gradient through 1mm thick gradient through 1mm thick air gapair gap

– Temperature gradient in gap Temperature gradient in gap at center plane is 16.75at center plane is 16.75ºCºC

• Separation between solids is Separation between solids is space represented by shell space represented by shell elements elements – Account for thermal Account for thermal

resistances of adhesives and resistances of adhesives and cablecable

Air gap

Shell element region, typical

VG VG 77W.O. Miller

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Half Model: Without Air BoxHalf Model: Without Air Box

• Half-Model Bridge Half-Model Bridge Support: Support: – Wire bonds are present. Wire bonds are present. – Peak chip is -1.85ºCPeak chip is -1.85ºC

• Quarter SolutionQuarter Solution– Same conditions, peak chip Same conditions, peak chip

temperature is -2.12ºCtemperature is -2.12ºC

• Good agreementGood agreement– In spite of different mesh In spite of different mesh

size and meshing issuessize and meshing issues

VG VG 88W.O. Miller

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Half Model: With Air BoxHalf Model: With Air Box

• Thermal SolutionThermal Solution– Air box surrounds the half Air box surrounds the half

modelmodel– Result is that the bridge Result is that the bridge

runs a bit coolerruns a bit cooler

• Peak chip is -6.85Peak chip is -6.85ºC ºC versus -5.84ºC from the versus -5.84ºC from the quarter modelquarter model

• Next stepNext step– Investigate low velocity thru Investigate low velocity thru

air boxair box

VG VG 99W.O. Miller

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Half Model: With Air BoxHalf Model: With Air Box

• Temperature Temperature profile with air profile with air box removedbox removed

• Profile with bridge Profile with bridge removedremoved

VG VG 1010W.O. Miller

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Multiple Sensors –Convective Multiple Sensors –Convective CoolingCooling

• ConvectionConvection– -25C air flows over -25C air flows over

and under the bridge and under the bridge – Initial velocity Initial velocity

entering air entering air box=.01m/sbox=.01m/s

Peak chip -6.19C

VG VG 1111W.O. Miller

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Multiple Sensors –Convective CoolingMultiple Sensors –Convective Cooling

• Air boxAir box

VG VG 1212W.O. Miller

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Multiple Sensors –Convective CoolingMultiple Sensors –Convective Cooling

• Isotherm Isotherm taken thru taken thru boxbox

Next step is to solve with -15C

Peak chip=-6.19ºC

VG VG 1313W.O. Miller

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Model ResultsModel Results

Description Chip Peak Temperature

(ºC)

Bridge Gradient

(ºC)

Sensor Max/Min

( ºC)

Quarter Model

a. No air, no wire bonds 4.53 10.2 -17.7/-26.5

b. No air, with wire bonds -2.12 7.81 -19.2/-25.7

c. No wire bonds, with air -1.52 8.2 -15.6/-25.4

d. With air, with wire bonds -5.84 6.58 -20.0/-24.4

Half Model

a. No air, no wire bonds 4.61 10.52

b. No air, with wire bonds -1.85 8.20 -19.28/-25.47 c. With air, with wire bonds -6.29 6.75 -20.15/-24.44 Multiple sensors

d. With -25ºC convection, complete model -6.19 -20/-26.2

VG VG 1414W.O. Miller

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Multiple Sensors –Convective CoolingMultiple Sensors –Convective Cooling

• Solution with -15Solution with -15ºCºC– Use second and third chip to avoid entrance effects and exit effectsUse second and third chip to avoid entrance effects and exit effects– Sensor temperature ranges from -18.7Sensor temperature ranges from -18.7ºC to -24.7ºC

Section cut of isotherms

VG VG 1515W.O. Miller

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Multiple Sensors –Convective CoolingMultiple Sensors –Convective Cooling

• Section cut of velocity flow fieldSection cut of velocity flow field– Air flows over chip, essentially no flow under bridgeAir flows over chip, essentially no flow under bridge– 1515ºC air does not have appreciable effect on sensor, more effect on ºC air does not have appreciable effect on sensor, more effect on

peak chip temperaturepeak chip temperature

Velocity flow over bridge

Essentially zero

~.01m/s

VG VG 1616W.O. Miller

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Multiple Sensors –Convective CoolingMultiple Sensors –Convective Cooling

• Section cuts for temperature and velocity fieldsSection cuts for temperature and velocity fields