The Research On The Formation Mechanism Of Micro Aca Joints Resistance In The Flip Chip Bonding

As the increasing demand for high-density and environmental protection, the traditional lead solder process is hard to meet these needs. Due to the advantages of fine pitch capacity, process simplicity and environmental compatibility, the interconnection process based on the anisotropic conductive adhesive (ACA), one of the flip chip bonding process, has been widely used in the packaging of liquid crystal display (LCD), radio frequency identification (RFID) and other consumer electronics. During the bonding, the chip is inverted 180°to directly attach to the substrate, and the chip bumps and substrate pads can be interconnected with the conductive particles to form electronical road. It makes the ACA's joints resistance very large and susceptible to change with the external environment, which will cause the reliablility problem. This issue has become the bottleneck for ACA to further exploit in high density packaging applications. Many scholars have pay their attention to this issue and carried out a lot of research, including a large number of experimental study and some theoretical and numerical models. However, because there are some assumptions and simplifications, and many of the key factors have not been effectively taken into account, the joints resistance calculated by these models is much smaller than the experimental data, so they cannot provide quantitative guidance to real design and manufacture. Therefore, deeply understanding the formation mechanism of ACA jonts resistance and establishing a more accurate jonits resistance calculation model are very essential.This paper adopts the ACA with solid nickel particles (rigid particles) and the ACA with gold/nickel coated polymer particles (elastic particles) as the object and considers almost all factors during the ACA bonding process, finally reveals the formation mechanism of the ACA joints resistance on the microscopic scale and the physical level. An accurate model of ACA joints resistance is established and validated by experiments. Using this moded, the relationships between joints resistance and process parameters, material properties and geometry are found out, then the reasonable ranges of key parameters are obtained. In conculusion, this paper can guide us to find out the method for obtain a small and reliable ACA joints resistance. The main content and results are summarize as follows.1. It is found that the current in the particle will flow along the spherical surface of the particle and bend. An bulk resistance model is established considering the current bending effect, and the results show that this bending effect is very important on bulk resistance and even play a decisive role. Based on the new model, the effects of the deformation degree and geometry on bulk resistance is discuss, then the reasonable ranges of the geometric dimensions of the particle is obtained.2. Based on the contact theory and statistical methods, this paper studies the microstructure and mechanical behavior of the contact surfaces between particles and bump/ pad. A constriction resistance model considering the rough surface is established, and reveals the effects of the random distribution of conductive spots, the interaction of conductive spots and the nano-scale electron scattering on the constriction resistance. The relationships between the surface roughness, the contact pressure and the nominal contact area and constriction resistance are discussed. It is found that the constriction resistance will be significantly underestimated if the rough surface and the scale effect are neglected.3. The tunneling resistance is measured by experiments and the relationships between the contact pressure and the tunneling resistivity are buided. A fitting equation of the tunneling resistivity and the contact pressure is established. It reveals that the tunneling resistance is large enough and cannot be neglected.4. The curing shrinkage rate, the curing degree, the elastic modulus and the critical interface stress of ACA are measured. The relationship of the curing degree with the bonding temperature and the bonding time is established. An finite element model of the ACA bonding process is presented, synthetically considering the interface crack growth, elastic recovery, cureing shrinkage and cooling process. Then the relationships between ACA joints resistance and process parameters are obtained.Taking the rigid particles for example and according to the measured material properties, a set of optimal process parameters can be given:the bonding pressure per particle is 1128μN, the bonding temperature is 156℃, and the bonding time is 8s (the curing degree is 88%).5. This paper presents a structure-electric field coupling method based on the cohesive model to analyze the crack behavior of the elastic particle. It reveals the effect of the elastic modulus of the polymer particle on the ACA joints resistance, and finds that when the elastic modulus of the polymer particle is less than 0.3GPa, the contact surface of the particle and the bump/pad will change to a ring from a circle. In this case, the ACA joints resistance will increase significantly.6. An model to calculate the ACA joints resistance of the multi-particles is presented. This model considers the normal distribution of particles diameter, the particles spatial location with uniform random distribution.It discusses the effects of the bonding pressure on Multi-particles, the particles number and the standard deviation of the particle diameter on the joints resistance.Finally An experimental verification is performed to validate the model presented by this paper. The results show that the joints resistance of multi-particles and the bonding pressure meet the power function. However, because of the randomness of the particle diameter and spatial distribution, the joints resistance may be different even if the bonding force is the same, and the smaller the bonding pressure, the greater the variation range is. As the standard deviation of the particle diameter increases, the mean value and the variation range of the joints resistance will increase.In order to make the joints resistance small and stable, the standard deviation of the particle diameter should be controlled at less than 0.87μm. With the decrease of the particles number, the joints resistance increases too. However, when the particles number is greater than 18, the joints resistance did not change significantly. Therefore, in order to avoid short circuit, the concentration of ACA should be controlled to ensure that the number of particles captured by a bump is less than18.