Investigation Into Mechanism Of Ultrathin Chip Picking-up And Placing-on Process

Chip picking-up and placing-on are both critical technologies in flip-chip packaging for transferring the single die from the donor wafer to the circuit board and assembling. Especially as the silicon die tends to ultrathin and be with higher density, the improvement of the reliability and efficiency of picking-up and placing-on is significant to promise to lower the packaging cost, improve the package performance and reliability of electronic devices. this paper addresses some important problems of the picking-up and placing-on processes including mechanics modeling> technology mechanism analysis and experimental tests as well, and then the theoretical and practical process windows are also established finally. The main research effort and contributions of the presented paper are introduced as follows:Firstly, based on layerwise theory, a mechanical model of sandwich structure with a thick adhesive layer is established, which can be used to obtain a precise prediction of the warpage in chip assembling with anisotropic conductive adhesive (ACA). Besides, in order to predict the interfacial stresses of chip picking-up by theory, we also build an analytical model of the chip-thin adhesive-tape structure with distributed loads.Secondly, considering the actual conditions, we develop a mechanical model under vacuum loads to reveal the mechanism of ultrathin chip picking-up process, where a bisection algorithm is presented to calculate the effective size of tape that contributes to the crack growth. Both energy release rate and pick-up force are introduced to evaluate the ability of picking-up with vacuum sorption. The effects of the key process factors herein are also revealed. Further, the crack propagation in the adhesive layer with increasing pick-up displacement is predicted, which provides an effective route to map out the process conditions for enhancing the reliability of the ultrathin chip picking-up process.Thirdly, based on Hele-Shaw's theory, an analytical model is developed to investigate the process of high density thermal-compression bonding using ACA, where, the total number of particles under compression obtained by the experiment of chip on glass is considered as a supplement. Furthermore, we present the effects of some key factors (bump pitch, viscosity of ACA and pressure) on the assembling process and deformation of particles. On the other hand, the warpage of ultrathin chip induced by thermal stresses in the bonding process is also precisely calculated by the layerwise theory, and then, the effects of structure dimensions and materials of chip-ACA-flexible substrate on the warpage are revealed, which provide some useful suggestions to reduce the warpage of ultrathin chip assembly.Lastly, an experiment platform is established to investigate the process of vacuum-based picking-up. Considering different vacuum pressure below the tape, the changes of pick-up force with the increasing pick-up displacement of chip are obtained by on-line testing. The tests have verified our theoretical results. Further, the technological experiment is also carried out, from which, we also present a new technology and device for picking an ultrathin chip up efficiently and reliably. Besides, through the ultrathin chip assembling experiment, we have a test on the warpage of ultrathin chip under the cooling condition, which results agree nearly with our theoretical predictions.