The Effect Of Substrate Surface Conditions On Mechanical Performance Of Electrically Conductive Adhesives

Wan Abdul Rahman, Wan Ahmad (2018) The Effect Of Substrate Surface Conditions On Mechanical Performance Of Electrically Conductive Adhesives. Project Report. UTeM, Melaka, Malaysia. (Submitted)

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Abstract

Nowadays, with increasing awareness to protect the environment, the use of lead-based solder for electronic components interconnection in printed circuit board (PCB) are gradually replaced by the lead-free electrically conducive adhesive (ECA) in microelectronic industry. As interconnect materials, the mechanical strength of ECA is another aspect to be improvised in addition to the electrical conductive performance. This research project investigates the effect of substrate surface treatment in terms of the surface roughness and surface wettability which contributed to shear strength of multi-walled carbon nanotube (MWCNT) filled ECA bonded to aluminium-aluminium substrate, and the effect of varying MWCNT filler loading on ECA mechanical and electrical properties. Surface roughness is measured using a stylus profilometer and the contact angle test is conducted to measure aluminium substrate surface wettability. The result of four-point probe test reveals that the sheet resistance of the ECA decreased with an increase in the MWCNT filler loading from 5 wt.% to 7 wt.%, due to enhanced formation of percolated linkages between MWCNT particles. Surface treated, and untreated aluminium substrate were used as substrate for single-lap shear adhesively bonded experiment. The surface treatments consist of grinding with silicone carbide (SiC) abrasive paper grit 180 and alkaline/acidic etching. The mechanical properties of ECA bonded to as-received and chemically etched aluminium substrates show an increase in shear strength with an increase MWCNT filler loading from 5 wt.% to 6 wt.% and decrease in shear strength with an increase of MWCNT filler loading from 6 wt.% to 7 wt.%. Higher shear strength is obtained when the ECA experience an adhesive-cohesive failure as compared to the adhesively failed ECA. The surface morphology study on fractured surface of ECA following lap shear test reveals high density of hollow structures/voids on the entire surface of ECA with 7 wt.% MWCNT filler loading, possibly due to agglomeration of MWCNT in the composites, which results in poorer mechanical properties. Overall, the alkaline/acidic etched aluminium substrate exhibit the highest surface roughness and the highest wettability as compared to other surface conditions which results in largest effective bond area between ECA/substrate interface, hence, highest shear strength of the ECA is obtained. Meanwhile, the grinded aluminium substrate with SiC abrasive paper grit 180 has the lowest wettability and slightly higher surface roughness than as-received aluminium substrate which yield to the lowest shear strength of the ECA. This is due to low degree of wettability of grinded aluminium substrate which yield in the low effective bond area, results in an insufficient anchoring of ECA towards the substrate surface.

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