Electrical conductivity and reliability of nano- and micro-filled conducting adhesives for z-axis interconnections

This paper discusses epoxy-based conducting adhesives for z-axis interconnections. Recent work on adhesives formulated using controlled-sized particles to fill small diameter holes is highlighted, particularly with respect to their integration in laminate chip carrier substrates, and the reliability of the electrically conductive joints formed between the adhesive and metal surfaces. A variety of conductive adhesives with particle sizes ranging from 80 nm to 15 mum were laminated into printed wiring board substrates. SEM and optical microscopy were used to investigate the micro-structures, conducting mechanism and path. Volume resistivity of Cu, Ag and low melting point (LMP) alloy based paste were 5 times 10
^-4
ohm-cm, 5 times 10
^-5
ohm-cm, and 2 times 10
^-5
ohm-cm, respectively. Volume resistivity decreased with increasing curing temperature. The mechanical strength of the various adhesives was characterized by 90 degree peel test and measurement of tensile strength. Adhesives exhibited peel strength with Gould's JTC-treated Cu as high as 2.75 lbs/inch for silver, and as low as 1.00 lb/inch for LMP alloy. Similarly, tensile strength for silver, Cu and LMP alloy was 3370, 2056 and 600 psi, respectively. Reliability of the adhesives was ascertained by IR-reflow, thermal cycling, pressure cooker test (PCT), and solder shock. Change in tensile strength of adhesives was within 10 % after 1000 cycles of deep thermal cycling (DTC) between -55 degC and 125 degC. There was no delamination for silver, copper and LMP alloy samples after 3X IR-reflow, PCT, and solder shock. Among all, silver-based adhesives showed the lowest volume resistivity and highest mechanical strength. It was found that with increasing curing temperature, the volume resistivity of the silver-tilled paste decreased due to sintering of metal particles. Sinterability of silver adhesive was further evaluated using high temperature/pressure lamination, and shows a continuous metallic network when laminated at 365 degC. As a case study, an example of silver-filled conductive adhesives as a z-axis interconnect construction for a flip-chip plastic ball grid array package with a 150 mum die pad pitch is given. This effort is an integrated approach centering on three interrelated fronts: (1) materials development and characterization; (2) fabrication, and (3) integration at the device level