Development Of Particle On Bump Method For Ultra-fine Pitch Chip On Glass Technology

Flat panel display including LCD, PDP, OLED etc. is a majorInteractive media in modern society. With the development of multi-pointtouch technology, the smart phone and pad had become very important indaily life. Chip on glass (COG) technology is widely used in liquidcrystal display (LCD) modules for connecting driver ICs to the displaysespecially for middle&small size panels. The most common COGtechnology currently used in display applications is based on anisotropicconductive films (ACF). As the increasing demand in higher resolutionand cost reduction, the bump pitch of the driver ICs becomes finer andfiner. With the reduction of bump pitch, the current ACF based COGtechnology is confronted with two issues: One is the contact resistance.As the cross-sectional area of the bump decreases with the pitch reduction,the number of the conductive particles trapped between the bumps andthe ITO electrodes would decrease. The possibility of having too fewparticles to fulfill the contact resistance requirement or even formingelectrically open joints increases. The other is the electric short. As thegap between the bumps becomes smaller, the chance of forming shortcircuits due to the electrical-bridging by agglomeration of conductive particles between the adjacent bumps increases. A lot of research wascarried out to solve the problems, including the double-layer ACF and theconductive particles with an insulation layer. The researchers havereduced the bump pitch to level below40μm, whereas risks still remain infurther pitch reduction, and there seems likely a bottleneck in presentACF-based COG technique.A new approach for ultra-fine pitch chip on glass bonding, which isnamed “Particle on Bump (POB)” technology, is described in this thesis.The main idea of this technique is to have conductive particles only at thelocations where we need them. To achieve this, the conductive particlesare first fixed on the bumps of a driver IC by Au-Sn inter-metallicconnection. And then, the driver IC is assembled on the glass substrate ofan LCD panel with an insulated adhesive by thermal press. The newmethod yields superior properties of interconnection including fine pitch,low contact resistance, and no short. The work of the thesis mainlyincludes:The Au-Sn solder reflow process for particle planting was developedfor the feasibility test of the new method. The particle planting test wasdesignd in gold bump-tin-particle structure. During the reflow process,the annealing temperature was above eutectic point of the Au80Sn20alloy of278℃, and we introduced the mixture of formic acid (HCOOH)and nitrogen to inhibit the tin layer from being oxidized. The conductive particles formed a nice wetting shape and were well planted on the bumpsurface. Au-Sn alloy formed a homogeneous composition.We introduced the electrospray method for the conductive particledistribution, and found that the stable cone-jet mode was helpful to obtainbetter conductive particle distribution than other modes. With the help ofparameter adjustment which including voltage, flow rate and so on, theconductive particle distribution with large density and good uniformitywas obtained. The evaluation for the particle density and uniformity ofthe distribution through assessment method indicated that the electrospraysatisfied with the requirement of the conductive particle distribution.The mechanical property of the planted particle was investigatedthrough vibration test followed by shear test, which simulated the acturalloading sequence on the planted particle. The shear strength wasmeasured in about0.18g. The result of the investigation showed that theplanted particle was strong enough to fulfill the loadings during themanufacture, transportation and assembly of the POB process.The electrical property of interconnection formed by POB methodwas investigated assisted with the evaluation of contact resistance andinsulated resistance. The POB connection performed better in terms ofelectrical conductivity and insulation than ACF, for the intermetallicconnections of the conductive particle and the forbidden of the particleresidual between bumps. The reliability of the POB method in COG bonding was investigated.Base on the thermal cycling test and hygrothermal test, the reliabilityresults showed that stable interconnections can be established throughPOB interconnection compared with traditional ACF-COG technique.Therefore, it can be concluded that the POB method overcomes theshortcomings of current ACF technique and has the potential to provide aviable ultra-fine pitch flip chip on glass solutions for display applications.