Precision Dispensing Purdue takes advantage of the gantrys precision by xing a 150 micron inner diameter dispensing =p to an EFD dispensing pressure mul=plier mounted on the gantry head. To interface with the hardware, a LabVIEW program performs the vector algebra to systema=cally deposit encapsulant with enough degrees of freedom to account for variance of parts and posi=ons. Figure 3: Token bit manager encapsula=on The Token Bit Manager (TBM) is a custom integrated circuit on the HDI that is responsible for coordina=ng readout of data from the ROCs. Wire bonds on the TBM are placed at Fermilab and are encapsulated at Purdue. The geometry of the chip poses a challenge for encapsula=on because of the ne pitch of the wires. We prefer to encapsulate only the feet of the wire bonds which achieves the main objec=ves while avoiding encapsulant encroaching on unwanted places. For example, encapsulant seeping into the gap between the sensor and a ROC has been seen to slightly alter the electrical proper=es of the pixels that are in contact with the encapsulant. The mo=on of the dispensing =p is piecewise linear and the LabVIEW program is used to acquire points along its path from the absolute coordinate system of the gantry. With the precise op=cs of the gantry, a 3D point can be measured with the XY posi=ons based on the image and the Z posi=on from the focus of the Camera. In conjunc=on with the acquired posi=ons of the wire bond feet, a CAD model of the part is used to deposit encapsulant in 8 sets of 35 bonds in a single opera=on. Movements such as retrac=ng, shown below, are used to ensure an even glue deposi=on. Figure 4: Needle retract movement (not to scale)

Introduc1on The pixel detectors used in the CMS experiment at CERN will be replaced by an upgraded detector system in 2016. Modules consis=ng of a pixel sensor and 16 readout chips are being assembled at Purdue with electrical connec=ons to the support circuits made using aluminum wire bonds. We have developed a process to encapsulate these wire bonds in a silicone compound to provide mechanical protec=on and to prevent electroly=c corrosion. Presented here are the techniques developed for deposi=ng this viscous compound with a precision of 100 m.

Module Assembly Purdue is responsible for delivering at least 500 modules for the phase-1 upgrade of the CMS detector. These modules consist of an array of silicon pixels (sensor) that are bump-bonded to silicon read out chips (ROCs) and glued to a high density interconnect (HDI) circuit. There are 560 wire bonds that make electrical connec=ons between the ROCs and the HDI on each module. Figure 1: Forward pixel detector module stack up. A Devoltek F&K 6400 ultrasonic wire bonder is used with 38 m wire to place the wire bonds. Sylgard 186, a silicone based elastomer, is used to encapsulate the wire bonds. Sylgard is a very viscous polymer before curing, and aber curing has exibility, high shear strength, and excellent dielectric proper=es.

Equipment To encapsulate at the 100 m precision, Purdue uses an Aerotech AGS10000 robo=c gantry system which is capable of 1 m posi=oning precision over large distances. An Edmund OpAcs machine vision camera with 2560 x 1920 resolu=on is used to index the wire bonds to the gantry. Figure 2: Indexing the last wire bond of a line. The cameras precision allows the operator to gather posi=ons of wire bonds in any congura=on in all three dimensions. To deposit the encapsulant at high pressure, an EFD UlAmus V pressure control dispenser with pressure mul=plier is used.

Figure 5: ROC to HDI encapsula=on results

Benets of Encapsula1on For the CMS Forward Pixel detectors, there are three main reasons for encapsula=on: Mechanical protec=on Preven=on of electroly=c corrosion Resonance damping The encapsulant provides mechanical protec=on for the wire bonds, ensuring the longevity of the part once it has been installed at the center of the CMS detector, where access is imprac=cal. The encapsulant also prevents water and other electroly=c catalysts from accelera=ng the entropic corrosion process [1]. Figure 6: Wire bond resonance from Lorentz force and wire bond breaking at the heel. Forced harmonic oscilla=ons in the wire bonds can result from currents on some wire bonds in the presence of the 3.8 Tesla magne=c eld used in CMS [2]. Periodic currents at a resonant frequency can result in large amplitude mechanical vibra=ons which could eventually result in bond failures, as shown in Figure 6 [3]. The encapsulant damps these resonances, preven=ng large amplitude vibra=ons from developing.

Conclusion We have developed a process for the selec=ve encapsula=on of wire bonds used in the phase-1 upgrade of the CMS forward pixel detector. Using an Aerotech robo=c gantry system, Purdue can encapsulate a module with 100 m precision in approximately 20 minutes. Encapsula=on provides mechanical protec=on, prevents electroly=c corrosion, and damps mechanical vibra=ons. This process will be used throughout 2015 in the produc=on of approximately 500 sensor modules.

References 1. D.R. Sparks, Chemically-accelerated corrosion tests for

aluminum metallized ICs. Thin Solid Films 235 (1993) 108-111. 2. S. Chatrchyan, et al. (CMS collabora=on.) The CMS experiment

at the CERN LHC, JINST 3 (2008) S08004. 3. G. Bolla, et al., Wire-bonds failures Induced by resonant

vibraAons in the CDF silicon detector. IEEE NSS 3 (2003) 1641-1645.

Wire Bond Encapsula1on for the CMS Forward Pixel Upgrade

Sam Higginbotham Prof. MaIhew Jones

Purdue High Energy Physics

HDI

Sensor

ROCs

5 mm

6.5 mm

0.75mm