W.O. Miller i T i VG 1 Bridge Analysis Objective Objective –Develop model suited to examining effect of low velocity air flow through the isolating air gap Stagnate air pocket produces maximum effect Stagnate air pocket produces maximum effect –Suspect that a CFD solution will show colder chip temperatures, but possibly higher sensor temperatures Solution approach 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

W.O. Miller i T i VG 2 Bridge Model-Foam Support Contacts Facing Two FEA Models Two FEA Models –Quarter model Air gap (1mm) confined to region directly beneath the bridge, presumably simulated arrangement used by others with ANSYS 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 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 Heat load, 0.25W/chip, total 10chips Cooling tube wall reference temperature, -28ºC Cooling tube wall reference temperature, -28ºC –Half Model Air space encloses bridge, allowing three-dimensional affects 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 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 Heat load, 0.25W/chip, 40chips Cooling tube reference temperature, -28ºC Cooling tube reference temperature, -28ºC

W.O. Miller i T i VG 3 Model: Solid Thermal Properties

W.O. Miller i T i VG 4 Model: Adhesives and Cable Definition Cable definition combines two layers of adhesives with cable

W.O. Miller i T i VG 5 Bridge Model-1/4 Size Solution Solution –With air and with wire bonds –Peak chip, -5.84ºC –Range in sensor temperature -20.0ºC to -24.4ºC -20.0ºC to -24.4ºC Warmest on point sensor is directly beneath bridge where bridge foam contacts the facing Warmest on point sensor is directly beneath bridge where bridge foam contacts the facing Sensor -20ºC

W.O. Miller i T i VG 6 Quarter Model End view 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 Separation between solids is space represented by shell elements –Account for thermal resistances of adhesives and cable Air gap Shell element region, typical

W.O. Miller i T i VG 7 Half Model: Without Air Box Half-Model Bridge Support: Half-Model Bridge Support: –Wire bonds are present. –Peak chip is -1.85ºC Quarter Solution Quarter Solution –Same conditions, peak chip temperature is -2.12ºC Good agreement Good agreement –In spite of different mesh size and meshing issues

W.O. Miller i T i VG 8 Half Model: With Air Box Thermal Solution 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 Peak chip is -6.85ºC versus -5.84ºC from the quarter model Next step Next step –Investigate low velocity thru air box

W.O. Miller i T i VG 9 Half Model: With Air Box Temperature profile with air box removed Temperature profile with air box removed Profile with bridge removed Profile with bridge removed

W.O. Miller i T i VG 10 Multiple Sensors –Convective Cooling Convection Convection –-25C air flows over and under the bridge –Initial velocity entering air box=.01m/s Peak chip -6.19C

W.O. Miller i T i VG 11 Multiple Sensors –Convective Cooling Air box Air box

W.O. Miller i T i VG 12 Multiple Sensors –Convective Cooling Isotherm taken thru box Isotherm taken thru box Next step is to solve with -15C Peak chip=-6.19ºC

W.O. Miller i T i VG 13 Model Results

W.O. Miller i T i VG 14 Multiple Sensors –Convective Cooling Solution with -15ºC Solution with -15ºC –Use second and third chip to avoid entrance effects and exit effects –Sensor temperature ranges from –Sensor temperature ranges from -18.7ºC to -24.7ºC Section cut of isotherms

W.O. Miller i T i VG 15 Multiple Sensors –Convective Cooling Section cut of velocity flow field 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

W.O. Miller i T i VG 16 Multiple Sensors –Convective Cooling Section cuts for temperature and velocity fields Section cuts for temperature and velocity fields