Proposal of abstract for LEB99, Snowmass, Colorado, 20-24 September 1999The ALICE Inner Tracker (ITS) silicon strip layers will use kapton/aluminium microcables (12/14 um thickness) exclusively for all interconnections to and from the front-end chips and hybrids, completely eliminating traditional wirebonding. Benefits are increased robustness and an extra degree of dimensional freedom. Utilising a low-power, low temperature and low-force (10-15 grams) single-point TAB bonding process, aluminium traces are directly bonded through bonding windows in the kapton foil to bond pads on the chips and the hybrid. The same technique is also used to interconnect these microcables to create multi-layer bus structures with "bonded via's". A double-sided strip detector using prototype cables has been installed in the NA57 experiment in 1998

Borshchov, V N

De Haas, A P

Kiprich, S K

Kuijer, P G

Oskamp, C J

Ruzhitsky, V M

Van den Brink, A

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VERY LOW MASS MICROCABLES FORTHE ALICE SILICON STRIP DETECTORA.P. de Haas, A. van den Brink, P. Kuijer, C.J. OskampNIKHEF,  UtrechtV.N. Borshchov, S.K. Kiprich, V.M. RuzhitskySRTIIM, KharkovFor the ALICE collaborationAbstractThe ALICE Inner Tracker silicon strip detector layers willuse kapton/aluminium microcables (12/14 Pm thickness)exclusively for all interconnections to and from the front-end chips and hybrids, completely eliminating traditionalwirebonding. Benefits are increased robustness and anextra degree of dimensional freedom. Utilising a low-power, low temperature and low-force (10-15 grams)single-point TAB bonding process, aluminium traces aredirectly bonded through bonding windows in the kaptonfoil to bond pads on the chip, detector and hybrid. Thesame technique is also used to interconnect thesemicrocables to create multi-layer bus structures with“bonded via’s”.Working in close collaboration, the SRTIIM Kharkov andNIKHEF UTRECHT groups are involved in the design,production and testing of these cables, as well as researchof positioning and assembly methods. A double-sideddetector using prototype cables has been installed in theNA57 experiment in 1998.1. INTRODUCTIONThe ALICE Inner Tracker (ITS) [1] consists of sixconcentric layers of detectors. Layer 1 and 2 are equippedwith pixel detectors, layers 3 and 4 with silicon driftdetectors and layers 5 and 6 with silicon strip detectors.Layer 5 has a radius of 390 mm. and consists of 34ladders with 23 modules each, layer 6 (radius 435 mm.)has 38 ladders with 26 modules. The Silicon Strip Detector (SSD) system consists of 1770modules, 782 in layer 5 and 988 in layer 6.  A front-endmodule consists of a double-sided (stereo) strip detectorwith 1536 strips and a double-sided hybrid on which the12 (2x6) ALICE128C readout chips [1] ,[2] are mounted.The total number of channels is approximately 2.5million.Layer 5 ladders are 990 mm long, layer 6 ladders 1100mm long, the ladders are carbon-fibre space-frameconstructions and are connected to carbon-fibre end conesto form cylindrical shapes.A freon cooling system consisting of two stainless steeltubes running along each ladder cools the hybrids. Thesetubes have diameter of 2 mm and the wall thickness is 40Pm.On the ladders detectors are mounted in “high” and “low”overlapping positions to eliminate the dead areas of thedetectors in the Z direction (parallel to the beam).Alternate ladders are also mounted in “high” and “low”overlap positions on the circumference to eliminate thesearea’s in the RI  direction.Heavy-ion physics with particle multiplicities up to 8000dictate a very low-mass design.2. MICROCABLE TECHNOLOGYThe microcable technology described here was originallydeveloped by the SRTIIM as a low-mass interconnectsystem for use in space electronics, with microcables withminimum pitch of 150-200 Pm and bus assemblies withup to 10 layers.For use in detector systems, the technology has beenscaled down to a pitch of 88 Pm with typical trace widthsof 35-40 Pm.A microcable is made up of a kapton (polyimide) foil withaluminium traces. The cables are available in 12 Pmkapton with 14 Pm aluminium or 20 Pm kapton with 30Pm aluminium.Bonding windows are etched in the kapton foil, so as tobe able to press the traces through these windows andbond them directly to the bonding pads beneath.The technique is similar to Tape Automated Bonding(TAB) , but with several important differences.In TAB bonding, traditionally gold bumps are used on thepads and all connections are gang bonded in one go with athermode using heat and force.A more recent development by Bull and Dassault,supported by the European Community ESPRIT programis bumpless single-point TAB bonding.The kapton foils used are much thicker, typically 70 Pmthick and traces are 17.5 Pm copper covered with 1-1.5Pm gold.Bond force is typically 20 grams for TAB bonding,compared to 10-15 grams for the microcables.TAB bonding is an industrial high-volume process whichutilizes a kapton foil with sprocket holes for fast andaccurate positioning and handling, it is uncertain if it willbe possible to use such a system for microcables whichhave much thinner foils and traces and so are much morefragile.Also, automatic bonding machines often do not reactkindly to trying to use bond forces of around 10 grams!The  TAB direct bonding technique is being investigatedby the Strasbourg and Nantes groups and is used in theSTAR silicon strip detector [3]. Also the possibilities tomodify this industrial process using aluminium instead ofcopper is under investigation.In this paper we describe solely the Utrecht/Kharkovaluminium microcables designs.3. INPUT CABLESA front-end module, see Fig.1, consists of a double-sidedstrip detector and a double-sided hybrid on which the 12(2x6) 128 channel readout chips are mounted. Thedetector and the hybrid are decoupled mechanically, thedetector is mounted on the ladder with high accuracywhile the hybrid is connected to the cooling pipes runningalong the ladder. These pipes are loosely coupled to theladder structure except for one fixed point in the middle toaccommodate differences in thermal expansion.The hybrids are mounted in a different plane than thedetector, so the connection between the chips and thedetector must be made with flexible microcables, at thesame time eliminating the need for fan-outs, as this can bedone in the cables.The input cables, shown in Fig. 2, are made of 12 Pmthick kapton foil with 14 Pm thick aluminium traces,width 40 Pm and 88 Pm pitch on the chip side and 95 Pmon the detector side. The traces are directly bonded to thebonding pads through bonding windows in the kapton foilwith specially designed bonding wedges. The kapton foilis glued to the silicon surface to increase the mechanicalstrength. As the chip input pitch of 44 Pm is too small forthe cable design, half the traces run straight from the firstrow of bondpads and the other half folds around toconnect to the fourth row. The cable also includes theoutput connections of the chip to the hybrid, this side ofthe pattern has a fan-out which fits in a 22 pin ZIFconnector. In this way it is possible to test bonded chipsbefore mounting them on the hybrid.After testing of the chip, the test fan-out is removed,leaving only the pattern to be bonded to the hybrid.4. HYBRID BUS CABLE AND LADDERCABLEThe double-sided hybrid is 0.3x10.5x68 mm, just bigenough to fit 12 chips and associated SMT componentsand the two cooling clips connecting the hybrid to thecooling pipes. On one side of the hybrid the front-endchips of the P-side are placed, on the other side the chipsof the N-side.To save material the read-out and power bus connectingthe chips and components consists of a multi-layermicrocable. This part does not have to be supported bythe hybrid, allowing reduction of the hybrid to theminimum size needed to support and cool the chips. Fig. 3shows a prototype of the hybrid and bus cable assemblyincluding chips with input cables.The via’s in this cable are made by bonding traces indifferent layers directly to each other through bondingwindows in both cable layers.The layers are glued together after the bonding process.Here the kapton foil is 20 Pm thick and the aluminium 30Pm and trace widths range from 70 Pm to 2 mm.Only the part of the cable containing the bondpads forconnecting the chips and the solder/glue pads for the SMTcomponents is glued to the hybrid, the rest of the bus isfloating free in the air. The bus has a short “umbilicalstring” on one side which is bonded to the ladder cable.This is a similar cable of variable length (depending onmodule position) which runs along the ladder to theendcap module at the end of the ladder. This cable end isbonded to a very small PC-board, which has a tracepattern that fits into a ZIF connector, so again acompleted front-end assembly can be tested beforemounting it on the ladder. To make assembly easier, theZIF connector system is also used to connect the laddercable to the end-cap electronics.Figure 1. Front end modules on ladder structure.Figure 2. Input cable within template.5. TEST RESULTS5.1 IntroductionBond tests and bond pull strength tests have been doneand are still ongoing. Also research has been and is stilldone on several subjects: glue pull strength for the inputcables, tests using different bond wedge foot profiles,bonding and assembly methods and jigs to accomplishthemThis is necessary, as there is little or no industrial know-how concerning this kind of cables. Bonding these cableslooks deceptively easy compared to traditional wirebonding, but one has to realize that to make a reliablebond connection, now not only the workpiece(chip,detector, hybrid) has to be held stable in position,but the microcable template as well. The cable must beheld absolutely flat and tension-free against the surface tobe bonded to get reliable, consistent results.Also visual inspection of the bonds is not easy, it isdifficult to discern any difference between a successfulbonding and a total failure!5.2 ResultsFor a typical input cable the pull strength of thealuminium trace with 40 Pm width has been measured, aconsistent value of 6 grams was found.Also for the input cable, the glue strength of a narrowglue strip (200-300 Pm) was determined, values of 40-60grams were found. The glue pull strength is much largerthan the total weight of a single module, about 7 grams,so that it can effectively protect the aluminium tracesduring handling of the module.In Fig. 4 bond pull strength figures as a function ofbonding energy for four different bonding wedge footprofiles are shown. The results are quite good, there is asufficiently wide energy range in which acceptable bondsare realized. In nearly all cases the trace is pulled off thepad leaving a patch of aluminium behind, see Fig. 5, itdoes not tear off in the “heel” of the bonding. Inwirebonding practice the conclusion would be that thebonding energy should be increased up to the point thatthis effect starts to appear. With microcables anotherparameter must be accounted for; if the bonding energy isincreased too much the trace will smear out too much andwith the small pitch the risk of shorts becomes very high.The hybrid bus cable and the ladder cable are producedusing 20 Pm kapton foil with 30 Pm aluminium. TraceFigure 3. Alice doublesided hybrid with input cables and output bus cables.Bond tests by 15 g wedge force1.02.03.04.05.06.05.0 10.0 15.0 20.0Energy [ - ]Tensil force [ g ]7000 7100 7145 7400Figure 4. Bond pull strength tests for 4 differentbonding wedges.Figure 5. Footprint of good quality micro cable bonds.widths vary from 100 Pm to several mm for power lines,in the case of power lines several bondings are made inparallel, the trace width in the bonding window area isalways 100 Pm wide. Bond pull strength of thesebondings is 18-20 grams.6. CONCLUSIONS The use of microcables gives the designers of front-endmodules and inner tracker structures more possibilities;3D freedom, longer interconnections, some relativemovement allowed, robust and yet low mass.Bonding on active surfaces is inherently safer comparedto wire bonding, no lost wire bonding possible and lowsettings of  bonding parameters.Component qualification can be taken one step further, itis possible to do chip acceptance tests with bonded chips.The SRTIIM institute and production facilities inKharkov have much experience with large-scale use ofthis technology using minimum pitch 150-200 Pm.One should keep in mind that this is a new technology forthis field, that there is –especially for small pitch cables-not yet very much experience. Also there is no industrialproduction know-how in industry here yet, although at themoment two firms (Selmic, Detexis) seem interested.But we are convinced that without these microcables itwould not be possible to assemble the inner tracker layersand get anywhere near the present specs concerning massand coverage of active area.7. REFERENCES[1]  ALICE ITS Technical Design ReportCERN/LHCC 99-12[2]  Proceedings of Third Workshop on Electronics forLHC Experiments CERN/LHCC 97-60: L. Hebrard et.al.:ALICE128C: A CMOS Full Custom ASIC for the readoutof Silicon Strip Detectors in the ALICE Experiment.[3]  Proceedings of Fourth Workshop on Electronics forLHCC Experiments: S. Bouvier:  TAB: A PackagingTechnology Used for Silicon Strip Detector to Front EndElectronics Interconnection.

Very low mass microcables for the ALICE silicon strip detector

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