Energy resolution of hard x-ray detectors can be adversely affected by the input capacitance between the CdZnTe anodes and VLSI readout chip or substrate. This problem is compounded with minimal separation distances between plans that are on the order of 8 - 10 microns using typical Indium Bump Bonding methods. An improved technique is presented and compared that increases the separation between planes by adhesively-bonding single or stacked gold stud bumps, thermo-sonically bonded on an array of pads of a custom VLSI chip, with silver-filled conductive epoxy that is stencil printed on the anode contacts of the CdZnTe detector.
The technique uses low-force and low-temperature processing and can be adapted to avoid direct contact between the conductive epoxy and VLSI surface. The assembly of 24 x 44, 500-micron pitch, pixel array is detailed in this report. This sensor packaging technique is presently being evaluated for the balloon-launched, High Energy Focusing Telescope (HEFT) experiment at the California Institute of Technology.
High pin count flip chip assembly is becoming increasingly necessary for many sensor applications. Electroplated solder bumping technology provides one means for achieving fine-pitched flip chip assembly, but is not an allowable option for many temperature sensitive and microelectromechanical (MEMS) applications. Indium bump bonding, also known as "hybridization", is another common technique, but requires high pressure for cold-welding the bumps together. An alternative polymer adhesive approach is described in this article, which combines Au-wire stud bumped devices with stencil printed conductive epoxy that can be printed at pitches below 200µm and cured at temperatures as low as 80°C. Flip chip test chips with pin counts as high as 8000 I/O have recently been assembled and are discussed and illustrated.
The search for lead solder alternatives is stimulating interest in isotropic conductive adhesive assembly for flip chip interconnection. Originally found principally in low bump count and low-cost applications such as smart cards, adhesive assembly has now moved upscale to applications such as implanted medical devices and large imaging (IR and x-ray/gamma-ray) detector arrays. While indisputably lead-free, the many other advantages of adhesive over solder bump flip chip assembly clearly foretell a wider role for adhesive flip chip assembly in tomorrow's demanding products.
The electrically conductive and non-conductive polymer inks that are used in this process are cured at temperatures as low as 40°C and are therefore important for temperature sensitive component assembly. PAT is believed to be the only company with this area of expertise in the US. High-density interconnects are made using low applied forces onto low-temp curing, silver-filled (non-leaded) conductive epoxies that are applied using a patented stencil printing technique. This technology offers a low-cost alternative to indium (Hybridization) bump bonding.