AHEAD WP 6.5 (Interconnection harness) Status report May 2018

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AHEAD WP 6.5 (Interconnection harness) Status report

May 2018

1 March 2016 – 1 April 2018

Written by Xavier de la Broïse

Checked by

Distribution List

Distribution Date

Ref. Ares(2018)4577105 - 06/09/2018

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1 Introduction.

The aim of this project is to develop high-density superconducting harnesses for thereadout,inspatialcontext,ofverylowimpedancedetectors.Theseharnesseselectricallylinkpieces at different temperatures, minimizing the thermal conduction between them. Theharnesswehavedesignedcontains37tracks,routedonasignallayer(seeFigure1andFigure2).Thisoneisintercalatedbetweentwoshieldinglayersmadeofhatchedshieldingplanes,andthisset isagain intercalatedbetweentwothermalcontact layerswhere largegoldpadsaredesignedatbothextremitiesof theharnessandat itsmiddle, for its thermalizationat threedifferenttemperatures(low,intermediateifneeded,and“high”).Allthegroundsofthefourtoplayersareinterconnectedbymicrovias,inordertoentirelyenclosethelinesbyshielding,andtoimprovethethermalization.

Figure1:Stack-up.

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Figure2:Sectionofthedesignofthetopshieldinglayer(left),thesignallayer(middle),andthebottomshieldinglayer(right).

The width of the signal tracks is 15µm spaced by 15µm. The characteristics andtheoreticalperformancesofeachvariantaregiveninFigure3.

Figure3:Characteristicsandtheoreticalperformancesoftheharness.

Thelengthoftheharnessesis100mm,itsmaximalwidth(atconnectorlevel)is19mm,anditsminimalwidth(betweentwothermalcontacts)is1.89mm(seeFigure4).Twointerconnectionoptionsareavailableatbothextremities:eitherSMDNanoDconnectors,orwire-bondingpadsifverylowcontactresistanceormaximalcompactnessareneeded.

Characteristics andtheoretical performances

Value

Tracks width (µm) 15Tracks spacing (µm) 15

Shielding filling ratio 0,5Tracks quantity 37

Minimal harness width (mm) 1,89Total thermal conduction (W/K) 1,77E-08

Critical current at 4 K (mA) 5,25

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Figure4:Dimensionsandinterconnection.

2 Progress.

2.1 Production history and status.

CEAendedthedesignoftheharnessinAugust2016,andHightec,themanufacturer,hasstartedtheproductioninOctober2016.

Hightecdelivereda firstbatchon1stFebruary2017.Theoverallresultwasgood,butmanylocaldefectshavebeenobserved(onlythreepanelsoverthesixproducedwerepartiallyOK),andcriticaladhesiondefectsweredetectedonalargenumberofpads(seesection2.3.2).

HightecdeliveredaSecondbatch inApril2017.Theyieldwasbetter, severaldefectswerecorrected,buttheadhesiondefectsfullyremain.

Thenithasbeendecidedtostoptheproductionand,fortestingandcomparisonofnewadhesionlayersandactivationmethods,toproduceaspecialbatchcontainingtestsamplesandthreepartialpanels(containingonlythesignallayer).Ithasbeendeliveredon15thJune2017.PullouttestshasbeenperformedbyHightec,andcoldtestsbyCEA,inordertocomparethe different solutions, and finally a new adhesion layer has been validated to solve theproblem.

Thisnewadhesionlayerhasbeenintegratedintheprocessandathirdbatchhasbeenlaunched.IthasbeenstoppedinearlyMarch2018duetoanHightecpreparationerror,butithasbeenimmediatelyrelaunchedfromzeroanditsdeliveryisnowplannedfortheendofJuly2018.

2.2 Realization.

Figure5showsaphotographoftheproducedharness,anddifferentdetailsillustratingthequalityoftherealizations.

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Figure5:Attop:photographoftheharness.Atbottom,left:zoomonthetracksandgridszone;theshieldinggridsareclosedbyvias.Middle:furtherzoomwhere15µm/15µmtracksappearbetweenthetwoshieldinggrids.Right:zoomonatopthermalcontact,withviasconnectingittothetwoshieldingplanes.

2.3 Characterisation.

2.3.1 Measurements.

The critical temperature of the tracks has beenmeasured, by performing a four-wiremeasurement of their electrical resistance when the temperature was varying in bothdirections.Thecriticaltemperaturemeasuredisabout9.2K(seeFigure6),i.e.veryclosetothatofthebulkniobium(9.3K).Thisexcellentresultisduetoagooddepositionquality.

Figure6:Electricalresistanceofthreetracks(blue,red,green)versustemperature:transitionaround9.2K.

2.3.2 Defects.

Defectsofdifferentnatureshavebeenobservedontheproducedharnesses.Somearecritical,becausetheypreventtouseproperlytheharness.Othersarenotcritical,andaremoreappearancedefects.

The category of critical defects is composed of the delamination of large pads. This

problemoccurredinthefirstbatchandpersisted,eventoalargerextent,inthesecondbatch.

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Thisdelaminationappearsontheconnectorlargepads(seeexamplesinFigure7),wherepadsare completely or partially delaminated. It appears also on the thermal contact pads (seeexampleonFigure8).Concerningtheconnectorpads,themicroscopephotographsshowsalsopadswithabubbleformingbelowthemetallisation,whatcanbeconfirmedwhentheharnessis observed from the bottom side. The bubble formation indicates that the adhesion isdeterioratingovertimeorduringprocessing.Itisobviousthattheharnessbecameunusableandthestabilityofremainingstickingpadswastobequestioned.

Hence,HightecandCEAhaveagreedthatthisadhesionproblemhastobecontrolledorsolvedbeforecontinuingwiththefabricationofthewholebatches.Thisisthereasonwhyaspecial batch has been produced to test solutions (see section 2.1). A solution has beenvalidatedandanewbatchisbeingproduced.

Figure7:Delaminationonconnector’spads

Figure8:Delaminationonthermalcontactpads.

Othernotcriticaldefectshavebeenalsoobserved,butdonotpreventtousetheharnessandarerelativelyinfrequent.Nevertheless,effortswillbeproducedtofurtherdecreasetheirfrequencyinnextbatches.AtypologyofthesedefectsisshowninFigure9.

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Figure9:Typologyofnon-criticaldefects.

2.4 Assembly.

Theharnessesshouldbeequippedtobeusable.Connectorsorwirebondingshouldbeimplementedatbothextremities,forelectricalinterconnectionoftheharness.Toallowthis,stiffenersshouldbegluedatbothextremities.Thesestiffenersarealsousedtothermalizetheharness.2.4.1 Stiffener gluing.

ThestiffenersarecomposedofgoldplattedOFHCcopper.TheyaremechanicallybondedtotheflexiblePCBbydeepcryo-epoxy.Tothisaim,aspecificpressureandheatingcontrolledpresshasbeendeveloped(seeFigure10andFigure11).

Figure10:Heatingpress.

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Figure11:Gluingtest.

2.4.2 Connectors soldering.

Fortheimplementationoftheconnectors,alowtemperaturesolderpaste(96°C)andan«almost»classicalPCBsolderbrazingprocess(solderpastethroughstencil)areused.Tothisend,manydevelopments,toolsandtrialsforsettingthesolderingprocesshavebeenperformed(seeFigure12andFigure13).

Figure12:Fromlefttorightandtoptobottom:brazingmachine,stencil,dummyharnessinFR4PCB,spreadofsolder

onstencil,passingintheoven,oOven.

Figure13:Supportfortheharnessinthebrazingmachine.Atright,inblack,thetoppartofthehandlingtoolislaid.

Once assembled, harnesses remain very fragile, which imposed the development of atransportsupportandhandlingtools(seeinblackthetoppartofthemonFigure13,right).

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2.5 Measurements tools.

Onceequipped,manytestsshouldbeperformedontheharnessestovalidatethem.Forthesetests,toolsshouldbedesignedandmanufactured.

Forexample,wehavedesignedeightdifferentinterconnectionboards(seeFigure14)forthemeasurementofthetransitiontemperature(Tc)andoftheaccessresistanceoftheharness,includingbothconfigurationsofinterconnectiondevices(connectororwire-bonding).Theseboardsallowthe(4-wires)measurementoftheelectricalresistanceofthe37tracksinparallelwhenthetemperatureisvarying(withfinemeasurementsaroundTc),andthechainingofthe37tracksfortheprecisemeasurementoftheaccessresistancebellowTc.

Figure14:Interconnectionoftheharness:connectorconfiguration(left)andbondingconfiguration(right).

To perform these tests, a cryo-generator has been equipped with tools and cablingspeciallydesignedforit(seeFigure15).

Figure15:Cryo-generatorforthecyclingoftheharnesses.Cablingforthe4-wiresmeasurementoftheelectricalresistanceofthe37tracksofanharness.

2.6 Summary of the assembly and test facilities readiness.

Thereadinessoftheassemblyandtestfacilitiescanbesum-upasbelow:

• Stiffenersassemblyatbothextremities:• Thestiffenersaredesignedandmanufactured.• Thetoolsforhandlingofassembledsetaredesignedandmanufactured.• Theheatingpressismanufacturedandvalidated.• Thegluingprocesshasbeentestedandvalidated.

• Connectorassemblyatbothextremities:

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• The tools forpositioningofharness (mobilesupport) andspreadingofsolder(stencil)havebeendesignedandmanufactured.

• Adummyharness(FR4PCB),forprocesstrialspreservingharnesses,hasbeendesignedandmanufactured.

• Firsttestsofsolderspreadingandthenconnectorsolderinginovenhavebeenperformedondummyharnesses.

• Stilltodo:thesamestepsshouldbenowperformedontrue(andnotdummy)harnesses.Thisisexpectedtobeachievedinthenextfewmonths.

• Resistancemeasurementatroomtemperature:• Itisperformedsystematicallyforalltracksofallharnessesbythemanufacturer,

usingitsmobileprobetester.Thisisagoodwitnessofthemetallizationquality.

• Criticaltemperaturemeasurement:• ItisperformedforeverybatchesforasetoftracksofflexiblePCBsamples(see

theset-upinFigure16).

Figure16:Cryo-coolerforthemeasurementofthecriticaltemperatureofsetoftracks.

• Interconnectionresistancemeasurementatcoldtemperature:

• The aim is the 4-wire measurement of the critical temperature and of theinterconnectionresistanceofallthetracksofanharness.

• The interconnection boards are designed and manufactured, as well as themechanicalpiecesandcables.

• Stilltodo:theassemblyofthetestbench,andthewritingoftheacquisitionandcontrolsoftware.

• Thistestfacilitywillbeabletobefullyvalidatedonlywhenequippedharnesses(withconnectors)willbeavailable.

• Thermalconductionmeasurement:• Thetestbenchhasbeendesignedandmanufactured(seeFigure17, left),and

firstmeasurementshavebeenperformed.• Still to do: the calibration of the test bench for absolutemeasurements. This

achievementisexpectedinthenextmonths.

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• Detailedvisualinspectionoftheharnesses:

• Anadvanceddigitalmicroscopehasbeensupplied(seeFigure17,right).• Eachsampleisinspectedindetailtolistallmajororminordefects.

Figure17:Left:testbenchforthethermalconductionmeasurement.Right:advanceddigitalmicroscopefordetailedvisualinspection.

Figure18:Cryo-coolerforthe4-wiresmeasurementoftheelectricalresistanceofthe37tracksofanharness,accordingtothetemperature.

3 Conclusion.

Theprogressofthemanufacturinghasbeendelayedbytheoccurringofcriticaldefectsofadhesion(delamination)forlargepads(connectorpadsandthermalcontactpads).ButagoodcooperationhasbeenperformedwithHightectoidentifyandsolvetheproblem.Twoextrasamplesandthreespecial(partial)panelshavebeenproducedforthecomparisonandthe(thermal)controloftheenvisagedsolutionstofixtheproblem.Amodifiedprocesshasbeenvalidatedandanewproductionusingitisinprogress;thedeliveryisplanedforJuly2018.Hightecisconfidentthatthiswillsolvetheproblem.

WehavemadeinMarch2018avisitatHightec.Wehavevisitedtheproductionrooms,wehavemadeadetailedreviewoftheprocesses(inspectionofthefull“productmanufacturinginstructionfiles”),wehavediscussedabouttheproductionandqualityassurances,andwehavemade a detailed reviewof all the productiondefects,major orminor.Wehave had a goodoverallimpression.

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Inparallel,wehavedevelopedallthemechanicalpartsneededfortheequipmentoftheharnesses (stiffeners, handling tools, storage tools, heating press), for the soldering of theconnectors(stencil,supportforbrazing,trialboard),andforthecoldtests(thermalresistancetestbench,electricalresistancetestbench,interconnectionboards),etc.

Veryfirsttestsofconnectorsolderinghavebeenperformedondummyharnesses(lowtemperaturesolderlayout,oventests),andtestontruesampleswillbeperformedverysoon.Butthefinaltestsonthefinalsampleswillbecarriedoutwhenthenewproduction,withpaddelaminationproblemsolved,willbeavailable.

AHEAD WP 6.5 (Interconnection harness) Status report May 2018

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