On-demand droplet printing based on piezoelectric micro-jet device (PMJD) is considered a flexible and high-precision method to generate metal droplets directly for flip-chip bonding in industrial electronics. However, the quality of flip-chip bonding is closely related to the height deviation of the solidified droplets (the metal bumps), which is influenced by the complicated hydrodynamics of impacting and oscillation of the droplet with oxide film. Here, the numerical and experimental investigations are first conducted to study the effect of the liquid bridge and deposition parameters on the height deviation of the solidified droplets. The rapid oxidation of the liquid bridge and under-oscillation during the deposition process are the main reasons for height deviation. In addition, the undamped oscillation with high-speed impact and instantaneous solidification also deteriorates the height deviation. To this end, an oscillation control strategy based on ultrasonic-assisted metal droplet deposition (UAMDD) is proposed and verified to be a reliable regulation strategy to suppress the height deviation of the printed bumps. The effective regulating range of height deviation is studied experimentally by changing the ultrasonic vibration amplitude. Finally, a 10 × 10 array composed of 100 solidified metal droplets is printed with the UAMDD, which has the height deviation of 554 ± 6 μm. And the dimensionless height deviation (Δ/) of solidified bumps is stayed below 2.1 %.

中文翻译：

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通过超声波振动控制抑制金属凸块制造的高度偏差

基于压电微喷射装置（PMJD）的按需液滴打印被认为是一种灵活且高精度的方法，可直接生成用于工业电子倒装芯片键合的金属液滴。然而，倒装芯片键合的质量与固化液滴（金属凸块）的高度偏差密切相关，其高度偏差受到带有氧化膜的液滴冲击和振荡的复杂流体动力学的影响。在这里，首先进行数值和实验研究，以研究液桥和沉积参数对凝固液滴高度偏差的影响。沉积过程中液桥的快速氧化和欠振荡是高度偏差的主要原因。此外，高速冲击和瞬时凝固的无阻尼振荡也使高度偏差恶化。为此，提出了基于超声波辅助金属液滴沉积（UAMDD）的振荡控制策略，并验证为抑制印刷凸块高度偏差的可靠调节策略。通过改变超声波振动幅度，实验研究了高度偏差的有效调节范围。最后，用UAMDD打印出由100个固化金属液滴组成的10×10阵列，其高度偏差为554±6μm。固化凸块的无量纲高度偏差（Δ/）保持在2.1%以下。