Precision Dispensing Purdue takes advantage of the gantry’s precision by fixing a 150 micron inner diameter dispensing tip to an EFD dispensing pressure multiplier mounted on the gantry head. To interface with the hardware, a LabVIEW program performs the vector algebra to systematically deposit encapsulant with enough degrees of freedom to account for variance of parts and positions. Figure 3: Token bit manager encapsulation The Token Bit Manager (TBM) is a custom integrated circuit on the HDI that is responsible for coordinating 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 encapsulation because of the fine pitch of the wires. We prefer to encapsulate only the feet of the wire bonds which achieves the main objectives 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 properties of the pixels that are in contact with the encapsulant. The motion of the dispensing tip 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 optics of the gantry, a 3D point can be measured with the XY positions based on the image and the Z position from the focus of the Camera. In conjunction with the acquired positions 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 operation. Movements such as retracting, shown below, are used to ensure an even glue deposition. Figure 4: Needle retract movement (not to scale) Introduction The pixel detectors used in the CMS experiment at CERN will be replaced by an upgraded detector system in Modules consisting of a pixel sensor and 16 readout chips are being assembled at Purdue with electrical connections 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 protection and to prevent electrolytic corrosion. Presented here are the techniques developed for depositing this viscous compound with a precision of 100 μm. 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 connections 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 μm wire to make the wire bonds. Sylgard 186, a silicone based elastomer, is used to encapsulate the wire bonds. Sylgard is a very viscous polymer before curing, which after curing has flexibility, high shear strength, and excellent dielectric properties. Equipment To encapsulate at the 100 μm precision, Purdue uses an Aerotech AGS10000 robotic gantry system which is capable of 1 μm positioning precision over large distances. An Edmund Optics machine vision camera with 2560 x 1920 resolution is used to index the wire bonds to the gantry. Figure 2: Indexing the last wire bond of a line. The camera’s precision allows the operator to gather positions of wire bonds in any configuration in all three dimensions. To deposit the encapsulant at high pressure, an EFD Ultimus V pressure control dispenser with pressure multiplier is used. Figure 5: ROC to HDI encapsulation results Benefits of Encapsulation For the CMS Forward Pixel detectors, there are three main reasons for encapsulation: Mechanical protection Prevention of electrolytic corrosion Resonance damping The encapsulant provides mechanical protection 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 impractical. The encapsulant also prevents water and other electrolytic catalysts from accelerating the entropic corrosion process [1]. Figure 6: Wire bond resonance from Lorentz force and wire bond breaking at the heel. Forced harmonic oscillations in the wire bonds can result from currents on some wire bonds in the presence of the 3.8 Tesla magnetic field used in CMS [2]. Periodic currents at a resonant frequency can result in large amplitude mechanical vibrations which could eventually result in bond failures, as shown in Figure 6 [3]. The encapsulant damps these resonances, preventing large amplitude vibrations from developing. Wire Bond Encapsulation for the CMS Forward Pixel Upgrade Sam Higginbotham Prof. Matthew Jones Purdue High Energy Physics Conclusion We have developed a process for the selective encapsulation of wire bonds used in the phase-1 upgrade of the CMS forward pixel detector. Using an Aerotech robotic ganrty system, Purdue can encapsulate a module with 100 μm precision in approximately 20 minutes. Encapsulation provides mechanical protection, prevents electrolytic corrosion, and damps mechanical vibrations. This process will be used throughout 2015 in the production of approximately 500 sensor modules. References 1.D.R. Sparks, Chemically-accelerated corrosion tests for aluminum metallized ICs. Thin Solid Films 235 (1993) S. Chatrchyan, et al. (CMS collaboration.) The CMS experiment at the CERN LHC, JINST 3 (2008) S G. Bolla, et al., Wire-bonds failures Induced by resonant vibrations in the CDF silicon detector. IEEE NSS 3 (2003) HDI Sensor ROCs 5 mm 6.5 mm 0.75mm