Zhoulong Xu

Helix Electrohydrodynamic Printing of Highly Aligned Serpentine Micro/Nanofibers

Micro/nano serpentine structures have widespread applications in flexible/stretchable electronics; however, challenges still exist for low-cost, high-efficiency and controllable manufacturing. Helix electrohydrodynamic printing (HE-printing) has been proposed here to realize controllable direct-writing of large area, highly aligned serpentine micro/nanofibers by introducing the rope coiling effect into printing process. By manipulating the flying trajectory and solidification degree of the micro/nano jet, the solidified micro/nanofiber flying in a stabilized helical manner and versatile serpentine structures deposited on a moving collector have been achieved. Systematic experiments and theoretical analysis were conducted to study the transformation behavior and the size changing rules for various deposited microstructures, and highly aligned serpentine microfibers were directly written by controlling the applied voltage, nozzle-to-collector distance and collector velocity. Furthermore, a hyper-stretchable piezoelectric device that can detect stretching, bending and pressure has been successfully fabricated using the printed serpentine micro/nanofibers, demonstrating the potential of HE-printing in stretchable electronics manufacturing.

Vacuum-based picking-up of thin chip from adhesive tape

The reliable picking-up of thin chip using the vacuum sorption determines the success rate of flip chip assemblies from donor tape to receptor substrate. An analytical solution to model the chip–adhesive–tape structure with vacuum loads is introduced to understand the fracture mechanism of chip picking-up. The critical process parameters (the length of bonded region, vacuum strength, and pick-up displacement, etc.) are investigated. Theoretical predictions are used in combination with virtual crack-closure-based finite-element technique to reveal the detaching behavior between the chip and the adhesive tape. The results show that the length of the bonded region should be controlled less than 40% of chip length to eliminate the effects of chip thickness, and the higher vacuum strength acting on the adhesive tape is able to accelerate the detachment of the chip from the adhesive tape. In particular, a process window is proposed to enhance the reliability and efficiency of picking-up for a thin chip.

Ultra-Stretchable Piezoelectric Nanogenerators via Large-Scale Aligned Fractal Inspired Micro/Nanofibers

Stretchable nanogenerators that directly generate electricity are promising for a wide range of applications in wearable electronics. However, the stretchability of the devices has been a long-standing challenge. Here we present a newly-designed ultra-stretchable nanogenerator based on fractal-inspired piezoelectric nanofibers and liquid metal electrodes that can withstand strain as large as 200%. The large-scale fractal poly(vinylidene fluoride) (PVDF) micro/nanofibers are fabricated by combination of helix electrohydrodynamic printing (HE-Printing) and buckling-driven self-assembly. HE-Printing exploits “whipping/buckling” instability of electrospinning to deposit serpentine fibers with diverse geometries in a programmable, accurately positioned, and individually-controlled manner. Self-organized buckling utilizes the driven force from the prestrained elastomer to assemble serpentine fibers into ultra-stretchable fractal inspired architecture. The nanogenerator with embedded fractal PVDF fibers and liquid-metal microelectrodes demonstrates high stretchability (>200%) and electricity (currents >200 nA), it can harvest energy from all directions by arbitrary mechanical motion, and the rectified output has been applied to charge the commercial capacitor and drive LEDs, which enables wearable electronics applications in sensing and energy harvesting.

Analytical Evaluation of Interfacial Crack Propagation in Vacuum-Based Picking-up Process

Chip picking-up with vacuum sorption is a critical technology in flip-chip packaging for separating chip intactly from donor adhesive tape. With increasing length/thickness ratio of chips, the incomplete separation affects the success rate of this process, even leads to the failure of chips. Here, we present a theoretical model with a bisection algorithm to calculate the effective size of tape that contributes to the crack growth, and introduce an energy release rate and pick-up force to evaluate the ability of thin chip picking-up. The finite-element method with a virtual crack-closure technique is constructed to validate the established process model. The effects of the type of pick-up head, length of chip, thickness, and material of tape on chip picking-up process are also revealed to improve the ability to propagate a crack in the adhesive layer. Furthermore, the crack propagation in the adhesive layer with increasing pick-up displacement is predicted to determine the critical value of the thin chip complete separation from compliant tape. These results suggest that the thinner and more compliant tape should be adopted in the chip picking-up process.

Competing Fracture of Thin-Chip Transferring From/Onto Prestrained Compliant Substrate

The transferring of thin chip from donor to receptor plays a critical role in advanced electronic package, and the productivity is determined by the interfacial behavior between chip and substrate during chip transferring. The paper investigates analytical competing fracture model of chip–adhesive–substrate structure in thin-chip transferring (peeling-off and placing-on), to discover the critical process condition for distinguishing the interfacial delamination and chip crack. The structure is continuously subjected to ejecting needle, vacuum pick-up head, and wafer fixture, which leads to concentrated and distributed loads and dynamic boundary conditions. Additionally, two criterions based on competing fracture model are presented to determine the extreme chip dimension for peeling-off and the elimination of residual stress for placing-on. The theoretical results are validated by the finite-element simulation with virtual crack-closure technique (VCCT). This paper provides an insight for process optimization, to improve the success ratio and productivity of chip transferring.

Reliable thin chip peeling from adhesive tape with inverted needle ejecting and spring buffering

Abstract Nondestructively and efficiently peeling thin chip from the adhesive tape is still one of the crucial techniques in IC packaging technology. In order to improve the process efficiency, a novel approach with the inverted ejecting mechanism and spring buffer head is proposed, where, however, the ejecting and spring forces applied at the chip for generating the interfacial crack are also more likely to resulting in the damage of chip. In view of this, an effective theoretical and finite element models are established considering the tape–adhesive–chip structure with ejecting and spring forces, and in the frame of fracture mechanics, the energy release rate is calculated and introduced to reveal effects of the spring buffer on the interfacial crack growth and chip breaking stresses. It is showed that the spring buffer is able to reduce the stress concentration on chip. Furthermore, according to the competing fracture behavior between the interfacial delamination and the chip cracking, the process window of thin chip peeling is suggested for selecting an appropriate spring coefficient and ejecting needle force.