Study on non-conducting adhesives onto application of fine pitch flip chip interconnection

Abstract

This study looks at the use of non-conducting adhesives (NCA) in very fine pitch COF application. NCA exists in 2 forms; semi-solid (NCF) and liquid adhesives (NCP). The handling of non-conducting paste (NCP) varies from the film form adhesive, since a dispensing process is needed for NCP application. Dispensing of NCP was difficult in not providing good coverage on the contact pattern of the substrate and this high viscosity NCP would easily entrap voids in these fine pitch interconnections. These voids entrapped in the product could deteriorate electrical contact, and also shorten its reliability performance by providing vulnerable points for moisture attack. For this pair of test chips and substrates with a contact area of about 3x11mm, the NCP was dispensed in a line form onto the substrate. The NCP used was controlled to be 3.5mg, which did not cause contamination of the bond head did provide a good bond line. Control of some parameters and pretreatment of the material used successfully reduced the entrapment of voids in the joints. Polyimide substrate pre-baking at 120oC extracted the moisture from surfaces, and thus increased surface energy and improve wetability of this adhesive on the substrate. A stage heating process reduced the viscosity of the adhesive and so improved spreading and wetting on the substrate. Bonding temperature, descending speed of the bond heating tool, and also bonding pressure induced rheological changes in the adhesive, and thus, with a proper combination of these parameters, a significant reduction of percentage of voids in the interconnections was achieved. NCA bonding depends on direct contact built in the interconnection to achieve good electrical contact. This NCA will react once a temperature is applied. The melting and cross-linking process in the NCA ensured the formation of good contact points in the joint, and also acted as a strong holding force in the interconnections. The results of NCF-bonded chip-on-flex (COFs) at different bonding temperatures showed a high peel strength value at a high bonding temperature. A high bonding temperature accelerated high cure reaction and polymerization process in the adhesive. The degree of cure in the adhesive was influenced by the bonding temperature. This polymerization induced cross-linking between the long chain molecules in the adhesive, which eventually created its high modulus and flexural strength. Also a high bonding temperature gave a build up of cohesive strength in the adhesive and also adhesive strength with the adjacent surfaces. This interlocking force between the adhesive with the chip and substrate was indicated directly by the peel strength. Thus, a high peel strength suggested a better cure in the adhesive as well. This study shows that at the same bonding pressure, an optimum electrical contact was obtained when sufficient melting occurred at the contact points of the joints. A low bonding temperature did not give good melting in the interconnection joint, and increased electrical failure by forming an adhesive barrier insulation layer in the joints. However, an excessively high bonding temperature of 240oC and above, caused instant curing at the contact points in a joint and built up a cured barrier layer in the joint which prohibited a good electrical contact. An aging process by introducing a reflow at 260°C on the COF pieces shows that a degree of cure ~86% and above was able to provide a consistent and reliable electrical contact in these joints. The third part of the study investigated the bonding pressure effect on this NCA application. The bonding pressure effect played an effective role in regulating the electrical contact in the joints especially with the film form NCF. Both the NCF and NCP from different vendors were used for COF bonding at a range of different bonding pressures. A better electrical contact was achieved with NCP bonded COFs compared to NCF bonded COFs. The initial NCP rheological properties which was in liquid paste form was better for the spreading and squeezing procedures which increased the contact area of the joints. The electrical contact formed through NCF bonding faced a few deficiencies as it was initially in semi-solid form which was pre-tacked onto the substrate. The squeezing process of the adhesive from the contact points was not as efficient in this case as in the NCP bonded case. An environmental thermal stress evaluation of these NCA bonded COFs showed that the NCP bonded ones were better. A low CTE value and also a high modulus in the NCP was judged as the contribution factors to achieve this result. A high bonding force NCF bonded COF, at 100N and above, was able to give better squeezing of adhesive from the contact points. Less adhesive remnant trapped in the joint reduced the z-direction expansion. This prevented the interconnection from building up a joint gap at the contact point which broke the electrical connection path in the joints. Thus, a higher bonding pressure in the NCF application was able to improve the reliability of the joints. This study shows that a suitable selection of bonding temperature, pressure and other parameters are able to optimize the electrical and mechanical performance of these NCA bonded COFs. A higher bonding pressure is preferred to overcome the problem with voids, and also create better electrical contact. A low bonding temperature which causes low melting in the adhesive, and also extremely high bonding temperature which induced instant curing of the joint was not a good choice to give a reliable NCA bonded COFs pieces. A low CTE and high modulus adhesive is preferred in NCA bonding applications.

Date of Award	15 Feb 2006
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yan Cheong CHAN (Supervisor)