Stencil Printing and Flip-Chip Bonding for Assembly of Pixelated X-ray Detectors using PCB-type Interposer and Flexible Printed Circuit Boards

In recent years multi-layered Flexible Printed Circuit (FPC) boards have advanced considerably in complexity, allowing versatile designs for electronic packaging applications which require interconnect through-via [1]. This makes these boards also attractive for interposers (pitch-convertors) in X-ray detector applications where pixelated sensors are connected to readout Application-Specific Integrated Circuits (ASIC) in order to match different pixel-pitches between sensor and ASIC. Thin boards with total thickness of approx. 140μm are also available and desirable for low radiation absorption of detector packaging in high-energy particle experiments (e.g. CMOS imaging sensor for particle tracker [2,3]). These X-ray detector sensors and ASIC are based on a wide range of materials (Si, Cd(Zn)Te, GaAs). The mismatch in thermal expansion of these different materials is a major challenge. Bonding of sensors and ASIC to flexible boards increases the risk of distortion of the assembly, with potential delamination of components from the board. The paper describes the bonding processes. Stencil printing is used to form an array of dots from electrically conductive adhesive on the sensor, interposer or FPC. ASIC and interposer board are studded with a similar array of gold studs using a ball bonding technique. Subsequently flip-chip bonding is used to bond a studded component to an array of adhesive dots on the printed counterpart. Optimal bonding parameters are evaluated using shear testing and analysis of contact area. The flatness of boards is analysed. Bonded devices are tested for their electronic functionality as X-ray detectors.