Influence of process parameters on component assembly and drop test performance using a novel Anisotropic Conductive Adhesive for lead-free surface mount assembly

The novel Anisotropic Conductive Adhesive (ACA) investigated in this research uses a magnetic field to align the conductive particles in the Z-axis direction, during cure, thereby eliminating the need for pressure and the requirement to capture a monolayer of conductive particles. The formation of conductive columns within the adhesive matrix, during cure, provides a very high insulation resistance between adjacent conductors and also eliminates the need for precise printing or dispensing of adhesives onto individual fine pitch pads. The novel ACA can also be mass cured, eliminating the need for sequential component assembly. The formation of columns also alleviates the problems associated with coplanarity errors and varying lead/bump shapes in forming reliable interconnections using traditional AC As. This research incorporated a variety of components, leaded, leadless and bumped, standard, fine and ultrafine pitch, coarse and fine particle filler formulations, different stencil thicknesses, different cure temperatures and times, and different magnetic flux densities. The print process was performed manually using metal stencils of different thicknesses and metal squeegee blades. The findings from this study, including drop tests of 'as assembled' and aged samples (100 hours of thermal and T&H aging) are provided. The filler particle size played a critical role in determining the continuity and the contact resistance of the adhesive joint. Standard pitch devices provided good continuity and contact resistance when assembled with larger diameter filler particles. Fine pitch devices required smaller diameter filler particles. Stencil thickness was found to be a statistically significant factor in determining the contact resistance of the adhesive joint, while magnetic flux and cure temperature were not significant statistically. A magnetic flux of 2000 Gauss and a cure schedule of 150degC for 7 minutes were found to work effectively for different material formulations. A thinner stencil (100 mum - 4 mils) was required to obtain continuity when assembling fine pitch devices whereas a thicker stencil (>200 mum - 8 mils) was required when assembling standard pitch devices. Thicker stencil prints tend to misalign the fine pitch devices considerably after placement and cure. A thick adhesive print, with fine particle material formulation, and high magnetic flux density resulted in the filler particles growing as dendrites past the surface of the adhesive. Vertical orientation for the assemblies during drop test, from a height of 900mm (36 inches), was found to be more reliable when compared to the horizontal orientation, in both the 'as assembled' and thermally aged conditions. The daisy chains for all of the components survived 30 drops in the vertical orientation. MicroLeadframe (MLF) devices survived 30 drops in both the horizontal and vertical orientations. These devices showed the best performance during drop testing. None of the assemblies survived the T&H aging for 100 hours, to carry out the drop tests.