thermal fea update (mainly end of stave) september 2013

Graham Beck (QMUL) LBNL Sept.2013 1

Berlin, March 2013: FEA of Side-Mounted Card + Straight Cooling Pipe (no meander through SMC region):

• For adequate cooling of EoS chips (GBT etc) need thermally conductive core material (obvious choice: graphite foam).

• EoS heat (~3W) causes only a small degradation in:- Runaway headroom (23 => 21.5C) - Sensor temperature uniformity (≤ 5C)

(assuming SMC on CO2 OUTlet side; haven’t forgotten issue raised by Nigel). - Even smaller effect once Foam added. • But many of the FEA ingredients uncertain!

Thermal FEA Update (mainly END of STAVE) September 2013

LV

GBT

VTR

foam

30 mm

50 mmfoam

End of Stave card


(Some) Factors that affect GBT die temperature:

• GBT Power (estimate:1.8W, more recently: 2.2W “flat out”)• GBT Package resistance - unknown at Berlin meeting (assumed 10 C/W).• Thermal resistance of PCB (especially vias)• Foam (+ CFRP) conductivity and distance between GBT and pipe.

FEA prediction (Berlin): +11C (operation, CO2 at -30C) / +56C (commissioning, above dew-point)…

GBT Package: - High terminal count => 20x20x0.8mm pitch Plastic Ball Grid Array package. - Procurement chain: CERN – IMEC – ASE (Adv. Semi. Engineering, Taiwan) <= communications slow!- IMEC advised die thermal resistance to ambient probably high (~ 30 C/W) => package with Copper heat

spreader and remove heat from TOP. As of Berlin mtg:

0.5mm Copper spreader (top) 0.7/0.3mm x 5 x 5mm2 chip3mm Plastic Substrate20x20 BGA ~0.4mm.

~ 8 layer PCB: 1.6mm FR4

17x17 mm2

PCB: Assume 2x17mm Cu planes (! heat spreading near chip). Thermal Vias: Not in FEA (needs design): could reduce DT by ×3?

May 2013: Packaged die at CERN. Power→Ground shorted (most functions test-able after some surgery). Paulo Moreira stressed package NOT optimised for heat removal via BGA (I guess: vias, trace thickness..)One sample obtained by QMUL to investigate thermal resistance to BOTTOM of package.


17

0.54 0.34

1.14

0.5 dia / 0.8 pitch

~ 5.4

Dims (mm) measured at QMUL

Thermal Resistance die-to-bottom measured in “TIMTower”:

Note thin (0.25mm) die. Capacitors, but no clock or Copper heat slug.


Two vertical copper bars sandwiching GBT (+ RS grease); chilled plate stabilises lower T; Heat injected into upper bar: 2-3W (inferred from dT/dx). Typical contact force (load cell) ~ 10-20N.

Bottom photo: larger Cu block to sink heat from full BGA area.

IR profile => ~ 10C discontinuity between upper and lower bars. ~15% (total) correction for grease at die and BGA (biggest uncertainty) gives package DT.

Þ 6.5 (±1.5) C/W through-package resistance.

Suggests heat removal through BOTTOM of package alone is adequate.

FEA comparison between this set-up and pcb-mounted package would be instructive.

GBT mounted on a prototype pcb sample (DESY?) would be very useful.


GBTPCB

Not to Scale !

Appreciable thermal Resistances through GBT Package, PCB and Foam.

Use of graphite foam in EoS region (as thermal and mechanical core) seems sensible, but if GBT is located far from pipe may need foam K > 30W/mK.(Need progress on PCB layout)

Would like to understand the limitations on available K and dimensions (thickness, area) !

Cooling geometry may also be restricted by other structures at the stave end (next slides).

FOAM


Next:

Slides from Peter Sutcliffe, UK engineering meeting (Oxford, 5th Sept.) (+ my annotations)

- CAD drawing of End of Stave

- Electrical Break in Pipes (Richard French investigating these): We are looking at the possibility to embed them in Stave core to avoid stress …

EoS Area• SMC card incorporated the general

idea of flex sandwiched between 2 FR4 pcb’s.

• SMC Size 98 x 30• Interlink isn't right....• Opto height and chips correct to

latest models.

(GAB) Note the 100 x 50 mm2 SMC area: conservatively large? - implies a long cooling path for GBT, VTR.

Top – bottom flex.

View of Electrical break in the stave

0.2mm Radial Gap toInner CF SkinWrap with Kapton Tape to insulate

New Closeout

ELECTRICAL BREAKS: Ti-Alumina-Ti. Idea to embed in end of stave, to support / avoid stressing them:

This arrangement reduces pipe length that cools module and SMC by about 20mm – will explore with FEA.


Cooling Path & Headroom away from ends of Stave

-33C

-30C

No Sensor heatSP case1C bands

• Most important contribution to thermal R in pipe region is the fluid film.• Foam – pipe glue joint assembly needs care, to avoid poor joint / thermal bottleneck.• 30W/mK Foam is fine here! (+ good coupling geometry)• (Path across stave surface is as important: sensor // facing, through bus tape & glue…)

FEA (2011): Runaway Headroom for -30C coolant is 21.5o (23o if allow for P drop).

MUST update FEA: power, conductivities, CFRP density, hybrid layout, on-sensor DCDC…(in progress: don’t anticipate more than 2C change).

Very safe headroom - largely due to locating cooling pipes in core: distributes cooling across module*AND* naturally uses full pipe length for cooling. (If omit foam around the U-bend, lose ~30% runaway headroom). (v. backup slide for comparison with CMS.)

Thickness L(mm) K(W/mK) L/K (~R) Foam (at small R) ~1 30

0.03Joint:Hysol+BN 0.1 1.6 0.06Ti pipe 0.14 16.4 0.01Fluid Film equiv. 0.1 0.8 0.13 (@ 8000W/m2K)

Relative Thermal Path resistance in cylindrical region, close to 2mm dia pipe:


ACTIONS

• GBT Package: Do we alert CERN (Paulo Moreira) that we want to cool GBT through its base: omit Copper slug and maybe enhance package conductance? (Maybe wait for pcb trial?)

• Understand VTR cooling requirement: maybe not critical (some leads from Todd Huffman).

• Push DESY on PCB dimensions and vias

• Thermal FEA Update

• => Establish conductivity of Foam required to cool GBT etc.

+ … Acceptable chip temperatures depend on our understanding reliability issues


BACKUP

Thicker stave to Accommodate New, slightly smaller, Electrical Break

Old Break

New break:Larger exit tubeSlightly smaller OD of 4.8

ELECTRICAL BREAKS: Dimensions here are indicative and driven by need to try out the concept!


COMMENT re Module Cooling geometry.

CMS Upgrade Strip modules (Andreas Mussgiller, Oxford Tracker Forum, June 2013)

CO2: -34C Þ Module cooling contacts -27C (Sensor Tmax ~ -20C)

(At 3000 fb-1): Runaway when module cooling contacts reach -25CÞ runaway headroom is 2 degrees (trying to improve on this).


Fluid htc (=>4000)

Foam Glue (BN/Hysol)

CFRP Kx

Cable Bus Ky

CFRP Ky

Foam Kxyz

Baseline

Sensor Glue =>IRS2125, 0.44W/mK

ALL Ks halved

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Coolant Temperature Headroom wrt -30C [C]

4 8 126 1021Approximate Power Headroom Factor wrt 0.65mW/mm2(0C)

Plots from Steve McMahon of Current SCT temperature:



NEAR End of Stave (but away from SMC): Temperature variation across Sensor.

• No sensor heat (3000 fb-1,-25C expect ~ 0.4W)

• Hybrid heat 5.6W/module face (~1mW/chan). Sensor T Variation due to: - Hybrid/HCC/Pipe: 3.5C - Asymmetric fluid T: 2C gives for: - SP: ~ 5.5 degrees over sensor area.

- DCDC (edge-mounted): ~ 4 deg. (fluid as shown) Preferred? ~ 7 deg. (flow reversed) Uncomfortable?

Note: The above FEA plot neglects heat flow between modules due to asymmetric hybrid placement.Hybrid placement affects the range of T variation across the sensor: IF symmetrically placed hybrids have become the baseline it would be an advantage to change the model to reflect that!

-33C

-30C

Fluid

No Sensor heatSP case1C bands

dcdc

This perhaps sets the scale for acceptable level of T variation due to presence of SMC.


-30C-33C

-31.5C

-31C

-32C

LV

GBT VTR30 mm

(DCDC)

FEA Model Conduction to adjacent module is ~ 20%: neglected here. LV and VTR are cosmetic only, apart from heat injected into PCB footprint! GBT is modelled, to have some estimate of die temperature.DESY meeting: wider PCB (50mm?)(For convenience have suppressed honeycomb volumes: were modelled as air)

CO2 Temperature (Boundary Conditions) – refinement of previous FEA!Pressure drop => Temperature drop of ~ 3C along pipe (will be measured at CERN).- Mean fluid T is 1.5 degrees below input T. (=> correction to Runaway Headroom: now “23C wrt input fluid”) - Fluid T vs Z: ~ equal at Z=0, increasingly asymmetric towards EoS.

Þ In presence of other sources of asymmetry, direction of flow matters!


Convection +Radiation - appreciable since the SMC and chips are at relatively high T.- included crudely in the model as a surface film effect at the pcb surface (biggest contribution):

htc 10 W/m2K ; ambient T -23.5C (approx. mean stave surface T).

- accounts for about 0.4W (15% ) heat loss from the SMC. - lowers the GBT chip T by typically 5C.

Where does the heat go? At worst, it is a 0.5W load on the module below ( <10% hybrid heat ) - Ignored in the FEA.


Additional Strategy: Divert the Cooling Pipe towards the SMC –

Somehow shape pipe (e.g. zig-zag to reduce distance to SMC? Tricky. . . e.g. Shift pipe 20mm towards SMC => Further cools GBT by 3C.

BUT: Knocks 4C off Runaway Headroom!WHY?- STAVE Cooling philosophy is to distribute the cooling pipes evenly across the sensor. . .Disturbing this increases thermal resistance RT (Sensor Heat => Coolant) (above: by 16%).(A change in R that increases the sensor T (in absence of leakage current) by DT reduces the headroom by ~ 3xDT. Changing R is 3x as dangerous as changing Q, cf straight pipe result).

- Introducing a small meander into the pipe to help cool the SMC (or for other reasons) can seriously harm the runaway headroom: could be done but would involve some TLC and yet more foam ….

Material Cost of Foam:

Foam 56 x 98 x 4mm total, r = 0.25, X0 = 43g/cm2: 0.13 cm2

cf Ti pipe wiggle: 36cm x 2mm id x 0.14mm wall, r = 4.51, X0 = 16.2 g/cm2 0.1 cm2

SMC boards - !!FR4 ONLY 3 x 10 x 0.16 cm3, X0 = 16.8cm : 0.6 cm2

- conclude no need to worry about foam inserts (use more if helps?).

20


VTR: Assume physically similar to SMU design (Dual Tx for Calo): Transmitter: 217mW (LpGBLD 40% lower. Still in 130nm)Receiver: 120mW (+ PIN diode ff. Rad.damage ?)(? Jan Troska, AUW: “VTR doesn’t need active cooling”) FEA: VTR NOT modelled thermally (only cosmetic)…. Inject VTR Heat into connector footprint.

EoS COMPONENTS - THERMAL

Not final package? Thermal R (die => board) not known (aim to measure!).FEA: Assume 10C/W (experience with Motorola MCP7447A ceramic). Model as isotropic, K = 1.4W/m-K.

GBT: 1.8W (Lp version - 65nm Schedule+Reliability issues.) Package: ASE(Taiwan) via IMEC:

0.5mm Copper spreader (top) 0.7/0.3mm x 5 x 5mm2 chip3mm Plastic Substrate20x20 BGA ~0.4mm.

~ 8 layer PCB: 1.6mm FR4

17x17 mm2

PCB: Assume 2x17mm Cu planes (! heat spreading near chip). Thermal Vias: Not in FEA (needs design): could reduce DT by ×3?

Power Conversion: Useful Power 2.14W Assume (aggressive) 75% efficiency => 0.7 W (conversion)

Total SMC power (per face) 2.84W (equiv.to ~ ½ hybrid power)

thermal fea update (mainly end of stave) september 2013

Documents

pcbmounted package

taiwan package

eos heat

thermal resistance of

heat removal

foamgraham beck qmul

copper heat spreader

copper heat slug