# Presentation: 362649

## Bridge Analysis

**Objective**
- Develop model suited to examining effect of low velocity air flow through the isolating air gap
- Stagnate air pocket produces maximum effect
- Suspect that a CFD solution will show colder chip temperatures, but possibly higher sensor temperatures
**Solution approach**
- Quarter model, similar to ANSYS model reported by others
- Half model, with air box

- Air gap

- 1/4

- 1/2

- Air box for 1/2

## Bridge Model-Foam Support Contacts Facing

**Two FEA Models**
- Quarter model
- Air gap (1mm) confined to region directly beneath the bridge, presumably simulated arrangement used by others with ANSYS
- Wire bonds simulated with wide thin sheet , 2mils thick
- Initial mesh problem solved by breaking thin sheet into two solids
- Heat load, 0.25W/chip, total 10chips
- Cooling tube wall reference temperature, -28oC
- Half Model
- Air space encloses bridge, allowing three-dimensional affects
- Space beneath bridge remains 1mm
- Wire bonds, simulated with narrow band, improved aspect ratio to fix mesh issues observed in the quarter model
- Treated as one solid
- Heat load, 0.25W/chip, 40chips
- Cooling tube reference temperature, -28oC

## Model: Solid Thermal Properties

## Model: Adhesives and Cable Definition

- Cable definition combines two layers of adhesives with cable

## Bridge Model-1/4 Size

**Solution**
- With air and with wire bonds
- Peak chip, -5.84oC
- Range in sensor temperature
- -20.0oC to -24.4oC
- Warmest on point sensor is directly beneath bridge where bridge foam contacts the facing

- Sensor -20oC

## Quarter Model

**End view**
- Illustrates temperature gradient through 1mm thick air gap
- Temperature gradient in gap at center plane is 16.75oC
**Separation between solids is space represented by shell elements **
- Account for thermal resistances of adhesives and cable

- Air gap

- Shell element region, typical

## Half Model: Without Air Box

**Half-Model Bridge Support: **
- Wire bonds are present.
- Peak chip is -1.85oC
**Quarter Solution**
- Same conditions, peak chip temperature is -2.12oC
**Good agreement**
- In spite of different mesh size and meshing issues

## Half Model: With Air Box

**Thermal Solution**
- Air box surrounds the half model
- Result is that the bridge runs a bit cooler
- Peak chip is -6.85oC versus -5.84oC from the quarter model
**Next step**
- Investigate low velocity thru air box

## Half Model: With Air Box

**Temperature profile with air box removed**
**Profile with bridge removed**

## Multiple Sensors –Convective Cooling

**Convection**
- -25C air flows over and under the bridge
- Initial velocity entering air box=.01m/s

- Peak chip -6.19C

## Multiple Sensors –Convective Cooling

**Air box**

## Multiple Sensors –Convective Cooling

**Isotherm taken thru box**

- Next step is to solve with -15C

- Peak chip=-6.19oC

## Model Results

## Multiple Sensors –Convective Cooling

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

- Section cut of isotherms

## Multiple Sensors –Convective Cooling

**Section cut of velocity flow field**
- Air flows over chip, essentially no flow under bridge
- 15oC air does not have appreciable effect on sensor, more effect on peak chip temperature

- Velocity flow over bridge

- Essentially zero

- ~.01m/s

## Multiple Sensors –Convective Cooling

**Section cuts for temperature and velocity fields**