Thin Si Sensors on Flexible Printed Circuits – Study of Two Bond Methods

Daresbury Laboratory (DL) and Rutherford Appleton Laboratory (RAL) have developed and built radiation detectors for experiments in particle physics since decades. This includes tracker detectors which record the pathway of high energy particles in order to locate the vertexes of decay processes. These tracker detectors need to have low material budget in order to minimize scattering. The most advanced systems comprise thin CMOS sensors (approx. $100 \mu \mathrm{m}$ thick) which require interconnection to a thin (approx. $150 \mu \mathrm{m})$ Flexible Printed Circuit (FPC) board which is in turn electrically connected to the data acquisition system. The thickness of detector components and flexibility of the FPC board create challenges for the assembly of these detectors. For detectors described here, an array of 2x2 CMOS sensor chips (each 30mm x 15mm) is mounted onto an FPC creating a module. The chips are positioned in close proximity to each other (approx. $150 \mu \mathrm{m}$ gap). This paper investigates flip-chip bonding to assemble and interconnect these modules as an alternative to the standard wire bonding technique used in particle physics. For this purpose, two different methods are compared.Method 1: This is considered as the standard method where CMOS sensors are initially bonded to the FPC board with adhesive and subsequently their contact pads are wire bonded to the FPC through large via.Method 2: This is an alternative technique explored in this work where an electrically conductive adhesive is initially printed onto contact pads of the FPC and CMOS sensors are subsequently flip-chip bonded with high precision alignment to the FPC. Prior to this, contact pads on the CMOS sensor are fitted with gold studs.Reported here are considerations on the advantages and disadvantages of the proposed method 2 with respect to the standard method 1. This includes bond yield, mechanical stability of the detector module (adhesion of sensors to FPC), and complexity of the process.