Plaquette Assembly

The CMS Forward Pixel (FPix) detector consists of 2 Disks mounted inside Service Cylinders, installed at each end of the Pixel Barrel detector. Twelve Blades are mounted between inner and outer rings, forming 1/2-Disks. The Blades are rotated by 20 degrees around radial axes of the Disks. Plumbing connections between Blade cooling channels are made with short union tubes, providing for active cooling of the pixel detector modules, named Plaquettes. Plaquettes are installed on high density interconnect circuits on Panels, which are mounted on both sides of each Blade cooling channel.

Assembly of the FPix detector was shared between Purdue and Fermilab. Purdue was the assembly and testing center for all of the production Plaquettes. We developed the tools, assembly line, procedures, quality controls, database and staff needed to assemble, test and deliver ~1000 production Plaquettes. Fermilab assembled Blades and Disks using similar tools and techniques developed and evaluated by Purdue for plaquette assembly.

A cross-section view and photo of a Plaquette are shown here. A Plaquette is comprised of a Pixel Sensor bump-bonded to several ROCs (Read Out Chips), joined with adhesive and connected by wire bonds to a VHDI (Very High Density Interconnect) on a supporting silicon plate.

We evaluated the strength, thermal conductivity, and bond line repeatability of a variety of adhesives and selected a film adhesive for joining Sensor-ROC modules to VHDI. The goal was to optimize the thermal and mechanical performance of the joint between ROCs and VHDI. The Sensor-ROC modules were joined to VHDI using parallel plate fixtures aligned on linear rails in a vacuum oven. In close collaboration with Fermilab, we studied the effects of thermal cycling and radiation on the mechanical properties of prototype Plaquettes assembled using the film adhesive and our assembly procedure. The tests demonstrated that the film adhesive and application method resulted in a construction that mitigates warpage due to temperature changes, reliable strength and thermal conductivity, and safe construction for the electrical operation of Plaquettes.

To meet the production goal, simple tooling and procedures were developed to promote safe handling and optimize the assembly rate. We established a four station assembly line for Plaquettes, with a fifth station for special inspection and rework, as follows:

• Station 1: Align and transfer film adhesive to VHDI in vacuum.
• Station 2: Align and join Sensor-ROCs to the VHDI in vacuum oven.
• Station 3: Wirebond ROCs to VHDI and VHDI to fan-out circuit with Plaquette clamped in 'storage box'.
• Station 4: Electrical test of functionality.
• Station 5: Inspection and rework of problems identified during test.

All assembly work took place in our 800 sq. ft. cleanroom. The alignment of components was inspected using an optical probe on our Coordinate Measuring Machine. A flexible plate was used for fine adjustment of the location of components on the parallel plate fixturing at Stations 1 and 2. Alignments between joined components were within +/- 50 microns, meeting the requirement for wirebonding and later alignment of Plaquettes on Panels. The adhesive bond between Plaquette components was made in a vacuum at 50 degrees C to soften the adhesive and prevent air entrapment. An air cylinder was used to apply and finely control the mating pressure, which is limited by the compression that is allowed on the bump-bonds (1 gram-force per bump).

After Plaquette components were mechanically joined, they were clamped on "storage cassettes". Storage cassettes accommodated all processing steps (wirebonding, laser scanning, probing, reworking, shipping to Fermilab, burn-in testing) without removing Plaquettes from the containers. ~400 containers were needed, so we developed a simple design that was inexpensive. Prototype storage cassettes underwent thermal and shock tests to ensure that they reliably clamped and protected the Plaquettes.

Wirebonding of Plaquettes was done using our K&S 1478 automated wedge bonder. Prototype ROC and VHDI bond pad layouts were tested on our bonder for wedge tool accessibility. A microwave plasma etcher was used to clean VHDI bonding surfaces to enhance the pull-strength of wires. Inspection under microscope and gram measurements with a pull tester provided feedback for optimization of bonding parameters, wedge tool and wire specifications.

AUTOMATED BONDER AND ROC-VHDI BONDS

Electrical tests of the basic functionality of Plaquettes were done in parallel to assembly, and rework performed if necessary.

Approximately 1000 Plaquettes in 7 different geometries (varying in sense area and number of ROCs) were required to be assembled. We met the target assembly rate of 6 Plaquettes per day and shipped ~30 Plaquettes per week to Fermilab for assembly onto Blades. A period of 7 months was allocated for Plaquette assembly, leaving a reasonable amount of contingency.