The silicon sensors of the Silicon Tracking System of the Compressed Baryonic Matter experiment at FAIR, GSI are connected to the read-out electronics by low mass flexible microcables due to tight material budget restrictions. The cable length of up to 50 cm and its flexible nature make detector module assembly one of the most critical parts in STS. A novel low mass, low capacitance multilayer copper microcable has been designed and produced to facilitate detector assembly. Furthermore, a novel detector production method based on high-density gold stud bump bonding of silicon die on microcable has been developed. We present the Cu microcable design, capacitance simulations and measurements together with the individual steps performed in the STS detector assembly

Blank, T.

Caselle, M.

CBM collaboration

Pfistner, P.

Weber, M.

English

CBM Collaboration

KITopen

PoS(TWEPP2018)144Novel production method for large double-sidedmicrostrip detectors of the CBM Silicon TrackingSystem at FAIRP. Pfistner∗, M. Caselle, T. Blank, M. Weber, for the CBM collaborationKarlsruhe Institute of Technology, GermanyE-mail: patrick.pfistner@kit.eduThe silicon sensors of the Silicon Tracking System of the Compressed Baryonic Matter exper-iment at FAIR, GSI are connected to the read-out electronics by low mass flexible microcablesdue to tight material budget restrictions. The cable length of up to 50 cm and its flexible naturemake detector module assembly one of the most critical parts in STS. A novel low mass, lowcapacitance multilayer copper microcable has been designed and produced to facilitate detec-tor assembly. Furthermore, a novel detector production method based on high-density gold studbump bonding of silicon die on microcable has been developed. We present the Cu microcabledesign, capacitance simulations and measurements together with the individual steps performedin the STS detector assembly.Topical Workshop on Electronics for Particle Physics (TWEPP2018)17-21 September 2018Antwerp, Belgium∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/PoS(TWEPP2018)144CBM STS Cu microcable technology P. Pfistner1. IntroductionBecause of the very low material budget requirements for the Silicon Tracking System (STS)of the Compressed Baryonic Matter (CBM) experiment, the front-end electronics are located farfrom the sensitive sensor region and therefore are connected by low mass flex microcables witha length up to 50 cm. The primary assembly technology for STS is aluminum-aluminum TABbonding based on aluminum polyimide microcables manufactured in Kharkov [2]. The Al-Al TABbonding technology is well established and several dummy modules with varying cable lengths andsensor sizes have been produced [3]. However, there are two main disadvantages. First, the TABbonding process is a highly manual process, requiring lots of man hours and skilled personnel.Second, there is only a single supplier for these cables located in the Ukraine.Therefore a new double-layered copper polyimide microcable has been designed and produced.The microcable design, capacitance simulations and measurements are described in section 2. Withthis Cu microcable we developed a completely different assembly process based on solder pasteprinting on the cable and gold stud bump bonding on the die, a novel interconnection techniquewhich might be of interest also for other future detectors systems. It combines low cost and highautomation capability with good mechanical and electrical reliability. The individual productionsteps can be found in section 3.2. Cu microcable design and productionThe microcable plays an integral part in the detector module. Due to advanced Cu flex tech-nologies and high yields a double-layered cable design is possible leading to a reduction of requiredcables by a factor of two. Figure 1 shows the developed cable schematically.Figure 1: Schematic showing the developed two-layer Cu cable connected to a die. The top Culayer is routed to the bottom side with vias. A meshed PI interposer with a fill factor of 30 %reduces the capacitance and adding only very little material budget.The total thickness of the cable calculates to ≈ 115 µm. The copper line thickness is below10 µm to minimize material budget. Still, due to the low radiation length of Cu the total materialbudget of X/X0 = 0.05% is a bit higher compared to the aluminum cable (X/X0 = 0.03%), but stillconsiderably below the sensor itself (X/X0 = 0.32%).As the total noise in a detector channel depends strongly on the total capacitance connected to theinput of the charge sensitive amplifier, the capacitance of the microcable has been measured ona needle prober. One Cu trace was set to high voltage while both neighboring lines and the four1PoS(TWEPP2018)144CBM STS Cu microcable technology P. Pfistnerlines directly beneath on the opposite side have been connected to GND, as shown schematicallyin figure 2. A capacitance of 8.8 pF was obtained for a 20 cm long cable, which calculates to0.44 pF/cm.FE simulations have been performed to further minimize the capacitance. The simulation resultscan be seen in figure 3. The cable capacitance is shown as a function of meshed interposer thicknessfor Cu trace widths of 18, 24, 30 and 36 µm. Based on this results, the trace width will be reducedfrom 36 µm to 24 µm and the thickness of the interposer will be increased from 50 µm to 75 µmfor the next run of microcables.Figure 2: The setup for the capacitance mea-surements and simulations. One cable is put tohigh voltage while both neighboring lines andthe four lines directly opposite are put to GND.Figure 3: Results of the FE simulations show-ing cable capacitance over interposer thicknessfor different Cu trace widths.3. Detector module production based on Gold stud - solder paste bump bonding3.1 Gold stud bumpingGold stud bump bonding has been described and investigated previously at KIT [4, 5]. It is lowcost, flexible and reliable. It is also fast, up to 20 bumps/s can be placed on the die. For STS, a goldwire diameter of 25 µm produces a Au stud of 60 µm diameter to make full use of the pad width. Ina typical die-to-die interconnection, Au bumps can be placed on both dies which are subsequentlybonded in a thermocompression process. However, Au bumping on the flex microcable has beeninvestigated and shown to not lead to a reliable interconnection. The compressibility of the cableis too high for a proper energy transfer to the interconnection site.3.2 Solder paste printingSolder pastes manufacturing has reached very fine granularity, with grain sizes of 2 µm to8 µm for solder paste type 8. This makes solder paste printing also applicable for fine pitch appli-cations in high energy physics such as CBM STS. We performed prints on the microcables withSAC305 solder pastes type 6 (5 µm to 15 µm), type 7 (2 µm to 11 µm) and type 8. The resultsare shown in figures 4a, 4b and 4c. Paste type 6 produces a non-uniform print with a low soldervolume. Type 7 and 8 both lead to homogeneous prints with a sufficient solder volume. Basedon this results, type 7 paste is used for STS, as type 8 has lower shelf life, higher cost and fasteroxidation. The paste is printed on 140 µm×50 µm bond pads on the microcable, as can be seen in2PoS(TWEPP2018)144CBM STS Cu microcable technology P. Pfistnerfigure 4b. For STS, a 40 µm thick stencil has been designed to allow for the concurrent printing ofeight Cu microcables at once, ASIC as well as sensor side. After printing, the cables are directlyreflown in a dedicated reflow oven to prevent oxidation and to make the cables storable. By thisapproach, the cables - similar to the dies - can be prepared in advance in an automated, low cost,fast way.(a) Print result for type 6 sol-der paste showing insufficient sol-der volume and strong variation inshape.(b) Type 7 solder paste print show-ing sufficient solder volume andgood shape homogeneity.(c) Type 8 solder paste print alsoshowing sufficient solder volumeand good shape homogeneity.3.3 Thermocompression bondingBonding of ASIC on flex cable is performed under a Femto fineplacer flip chip machine witha high alignment accuracy of 0.5 µm [6]. Bond head as well as substrate plate are heated to 230 ◦Cto minimize thermal stress induced by thermal mismatch. With a bond force of 40 N the stud ballsare thermocompressed firmly into the solder paste printed on the microcable for about one minuteto establish a strong mechanical connection.The process leads to a solid interconnection for parameters ranging from 30 N to 40 N and 200 ◦Cto 240 ◦C. Moreover, it works under ambient atmosphere, so no additional gas supplies such asnitrogen or formic acid are required. With dedicated jigs it should be possible to automaticallybond at least 8 ASICs in one run in about ten minutes.The sensor - microcable interconnection is more challenging due to the larger sensor size, the factthat several microcables have to be bonded to one solid structure and the double-sided nature ofthe sensor. This time, the cable has to be picked up while the sensor stays on a substrate plate.Therefore a customized bonding machine is developed in-house. It consists of four highly precisestepper motors, a two-camera optical system, force control, automated vacuum control, a heatablebond head and sensor plate and fixed bond head mechanics capable to handle microcables with alength up to 50 cm. The current status of the machine is shown in figure 5. Automation software isunder development in parallel for a fully automated bonding process.3.4 Underfill applicationUnderfill is needed for two main reasons. First, the mechanical strength of the connection hasto be improved further to guarantee that the interconnections withstand the CBM lifetime fluenceof 1×1014 neq/cm2. Second, the low voltage cable has to be protected from the high bias voltagethat is applied to the sensor edge. Bias voltages will reach more than ±250 V by the end of lifetimeto compensate for the radiation damage of the sensor. The small gap between sensor and cable ofroughly 30 µm does not offer sufficient spark protection.3PoS(TWEPP2018)144CBM STS Cu microcable technology P. PfistnerFigure 5: The in-house developed bonder machine currently under construction. It is designedspecifically to handle the STS detector modules and will be used for a fully automated sensor-cableinterconnection process.4. ConclusionAn alternative assembly approach for the CBM STS has been developed. It is based on aspecially designed low mass, low capacitance, double-layered copper microcable manufacturedwith highly advanced PCB technologies. FE simulations have been performed and cross-checkedwith real measurements to optimize the geometry of the microcable towards low capacitance.Based on this cable, a novel die-on-flex production method for the STS detector modules has beendeveloped. It uses Au stud bump bonding on the die side and solder paste printing on the flexmicrocable. A full process workflow has been established from the individual components to thecomplete module. It is geared towards a high degree of automation to enable a low cost, high speedassembly process.It is expected that this interconnection technology proves as a suitable alternative to Al-Al TABbonding for the CBM STS and thus might be of interest also for other future detector systemswhere high-density die on flex interconnections are required.References[1] J. Heuser et al., Technical Design Report for the CBM STS, Darmstadt, 2013[2] V.M. Borshchov et al., CBM Progress Report 2013, p. 45[3] C. Simons et al., CBM Progress Report 2016, p. 48[4] M. Caselle et al., Low-cost bump-bonding processes for high energy physics pixel detectors, JINST 11C01050[5] T. Blank et al., Novel module production methods for the CMS pixel detector, upgrade phase 1, JINST10 C02021[6] FINEPLACER femto, Finetech, Germany4

Novel production method for large double-sided microstrip detectors of the CBM Silicon Tracking System at FAIR

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Novel production method for large double-sided microstrip detectors of the CBM Silicon Tracking System at FAIR

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