04/23/2012LGDRAFT

STAR PXL Ladder Assembly

DescriptionWe will be fabricating PXL detector ladder assemblies that consist of 10 pixel sensors bonded to a hybrid flex PCB all sitting on a carbon fiber stiffener plate. These ladders are the basic unit of the PXL detector which consists of 4 ladders per sector with 10 sectors making up the complete detector. The detector geometry is shown Figure 1.

Figure 1 PXL detector geometry showing the sector numbering system.

The full pixel naming conventions may be found here http://www-rnc.lbl.gov/~wieman/NamingConventionHFTpixe_v3.pptx. A ladder is composed of three main components. Sensors that are glued and wire bonded to a hybrid cable and a carbon fiber composite backer (stiffener plate) along the bottom of the assembly (Figure 2, Figure 3).

Figure 2 Exploded view of ladder components.

Figure 3Ladder structure seen from the end view.

Components

Sensors The sensors are MAPS technology with integrated column discriminators, zero suppression, and two 160 MHz LVDS outputs per sensor. The user manual giving a full description may be found here http://rnc.lbl.gov/hft/hardware/docs/ultimate/Ultimate_UserManual.pdf. The sensors are 20.24 mm × 22.73 mm. There are 10 sensors in each ladder. Sensors are thinned to 50 µm thickness. See Figure 4 for a photograph of the sensor

Figure 4 Photograph of PXL sensor. The sensor is 20.24 mm wide × 22.73 mm high.

Hybrid cablesThe hybrid cables are composed of two main elements. The first is a 2 layer kapton with aluminum conductor hybrid that sits in the low mass region of the detector under the sensors. The second is a multi-layer kapton with copper traces that holds the various driver chips that buffer the signals on and off of the low mass section and forms a flex cable connection to a connector on the “D” tube. The electrical connection between the two sections is made with wire bonds. This arrangement is shown schematically in Figure 5. The two types of driver section cable required for inner and outer ladders on a sector are shown in Figure 6.

Figure 5 schematic representation of the two-part hybrid cable used in the PXL detector.

Figure 6 The driver section of the hybrid cable is different for inner and outer ladders. The outer ladders have the interface connector located at the end of the ladder PCB. The inner ladder driver section incorporates a kapton flex region that allows the ladder interface connector to be brought to the top of the sector.

Cable sectionsThe hybrid cables needed for the PXL sensor ladders are composed of 3 PCBs. The primary PCB is the low mass ladder PCB shown in Figure 7.

Figure 7 Low mass ladder PCB.

The ladders located on the outer sections of the sector are equipped with the simple rectangular driver region PCBs shown in Figure 8.

Figure 8 Driver section PCB for outer ladders.

The ladders located in the inner section of the sectors are equipped with a more complex PCB that incorporates a kapton flex region to allow the ladder interface connector to be routed to the top of the sector. This PCB is shown in Figure 9.

Figure 9 Driver section PCB for outer inner ladders.

Cable/Ladder Handling FixtureThe ladder assembly is accomplished using a set of vacuum chucks and transfer tools described later. The primary handling fixture is a FR-4 “C” shaped handle shown below in Figure 10. All of the fixturing and locating actions are keyed from precision alignment holes in the cable.

Figure 10 Cable handling fixture fabricated from FR-4. Cutting the shown tabs after the ladder is mounted to the sector releases the ladder from the handling fixture.

AdhesivesWe are using an acrylic based adhesive 0.002” thick, 3M type 467MP thin film laminating adhesive for adhering the sensors to the hybrid cable, the hybrid cable to the stiffener plate, and the low mass sensor region of the hybrid cable to the driver region of the hybrid cable.

Composite Backer (Stiffener Plate)The Stiffener plate is composed of K13C2U [0,90,0] 45gsm FAW carbon fiber.

FixturingA full set of fixturing including vacuum chucks and transfer tools for the assembly of ladders and the placement of ladders onto sectors have been designed in CAD, fabricated, and used to fabricate the mechanical models used in our prototype testing. This fixturing was used to assemble Phase-2 based sensor ladders but will be modified to accommodate the slightly different sized PXL sensors. A full set of mechanical models for the PXL detector and associated fixturing can be found at http://www-rnc.lbl.gov/~wieman/HFT_PXL_model_inventory_jan_2011.htm.

Assembly sequence and testingWe have generated a detailed set of assembly procedures using the fixturing that has been fabricated. The detailed assembly sequence for ladders may be found at http://www-rnc.lbl.gov/~wieman/LadderAssemblySteps.ppt. The current assembly sequence for gluing ladders to sectors may be found at http://www-rnc.lbl.gov/~wieman/gluing%20ladder%20to%20sector.ppt. The basic assembly sequence is show in the diagram in Figure 11

Figure 11 Basic assembly sequence for the PXL ladders.

As was mentioned previously, the fixturing and assembly instructions have been used previously to construct the prototype ladders and sectors used in our mechanical testing for air cooling, vibration and displacement. We will be modifying the procedures as necessary when we begin production to produce both the most efficient construction and to maximize our yield of good ladders adhered to sectors. This will likely be an iterative process.A preliminary yield model for this process may be found at http://rnc.lbl.gov/hft/hardware/docs/PXL_yield_model_draft.pdf. We will update this as prototypes are manufactured and tested.

Testing and measuringWe will be doing electrical testing and measuring the mass of each ladder at various points in the assembly sequence. Each basic component is tested before assembly into ladders. The sensors are probe tested and characterized before being labeled good for use in ladder assembly. A description of the tests performed during probe testing can be found at http://rnc.lbl.gov/hft/hardware/docs/ultimate/Testing_PXL_sensor_draft.doc. The hybrid cables will be mechanically and electrically tested at the CERN PCB shop prior to shipment to LBNL. They will be inspected and weighed after arrival at LBNL before use. The CF stiffener plates will be fabricated at LBNL and inspected and weighed before use.In the assembly sequence outlined above, we measure the weight at various points throughout the assembly sequence that will allow us to determine the mass of each added component of different radiation length. This will allow us to determine a complete radiation length profile for each ladder and sector.We plan on testing the wire bond strength as we assemble the ladders. There will be one extra wire bond per sensor that will be pull tested to assess the quality of the bonds before we add encapsulant. Each ladder will be tested electrically with readout through the assembled and wire bonded hybrid cable. The testing will test sensor aliveness and check the characteristics of each sensor as measured during initial probe testing.