Mechanical assessment of an anisotropic conductive adhesive joint of a direct access sensor on a flexible substrate for a swallowable capsule application

Abstract

Sensor interconnection achieved with Flip Chip (FC) technology and most particularly with Anisotropic Conductive Adhesive (ACA) is a very attractive technique, in achieving a Direct Access Sensor (DAS), in a swallowable diagnostic sensing capsule. This paper describes the work carried out to mechanically characterise the ACA joints when they are inserted in the capsule to determine the smallest capsule diameter that could be used without imparting excessive stress on the interconnect in the DAS integration process for a specific substrate and chip design. Three point inward bending was used to study the effect of the mechanical loading on the joints during the insertion process. The results showed that the insertion force linearly increased and leveled off at a, low friction, constant value. The spring constant of the linear region in a 23mm was 0.2729N/mm and increased to 1.5N/mm for the 15mm diameter hole. It showed that the spring constant decreased linearly as the diameter increased, signifying that for the same distance, less force will be required in a larger diameter hole than in the small diameter holes. During insertion the stress in the assembly, was higher towards the centre of the chip and the window than at the edge of the chip and the ACA fillet. This was characterised by it exhibiting lower voltage values measured in the partial daisy chains that were close to the window rather than the ones that were situated away from the window. The insertion test suggested that the 23mm diameter hole would be the smallest suitable hole for insertion of this assembly. Failure analysis revealed that the depression and the wide crack close to the window imply that the stress was high in this region. The cross sectional analysis showed that failure occurred within silicon/silicon chip pad and that the ACA contacts form a strong joint and was able to withstand the insertion force required to secure the ACA sensor in place before encapsulation.