Charged Molten Metal Droplet Deposition as a Direct Write Technology

Abstract

The formation of highly uniform charged molten metal droplets from capillary stream breakup has recently attracted significant industrial and academic interest for applications requiring high-speed and high-precision deposition of molten metal droplets such as direct write technologies. Exploitation of the high droplet production rates intrinsic to the phenomenon of capillary stream break-up and the unparalleled uniformity of droplet sizes and speeds attained with proper applied forcing to the capillary stream make many new applications related to the manufacture of electronic packages, circuit board printing and rapid prototyping of structural components feasible. Recent research results have increased the stream stability with novel acoustic excitation methods and enable ultra-precise charged droplet deflection. Unlike other modes of droplet generation such as Drop-on-Demand, droplets can be generated at rates typically on the order of 10,000 to 20,000 droplets per second (depending on droplet diameter and stream speed) and can be electrostatically charged and deflected onto a substrate with a measured accuracy of ±12.5 µm. Droplets are charged on a drop-to-drop basis, enabling the direct writing of fine details at high speed. New results are presented in which fine detailed patterns are “printed” with individual molten metal solder balls, and issues relevant to the attainment of high quality printed artifacts are investigated.

Rapid prototyping based on uniform droplet spraying

“…As each drop breaks from the jet, it keeps the charge it holds at the moment before it breaks free. By virtue of the charge, droplets are prevented from merging in flight; thus, they can remain uniformly sized until they solidify or are collected on a substrate [6].…”

“…Droplets can be generated at rates in the order of 10 000-20 000 droplets/s and can be electrostatically charged and deflected onto a substrate with a measured accuracy of ±12.5 m. Under carefully controlled conditions, droplet formation from capillary stream break-up provides droplet streams with speed dispersions as low as 1 × 10 −6 times the average speed, and angular deviations as small as one micro-radian [6].…”

Enhanced Liquid Metal Micro Droplet Generation by Pneumatic Actuation Based on the StarJet Method

“…The other category of metal dispensers is the group of continuous-mode devices, which generate metal droplets by a capillary break-up of molten metal jets. To this end, disturbances are induced into a liquid metal jet, using for example piezoelectric stack actuators [6,13]. Due to the continuous jetting, these devices eject droplets at much higher frequencies than the DoD-devices.…”

“…However, the control of individual droplet deposition is more complex and requires solutions to deflect unwanted droplets into waste or gutter following continuous inkjet-devices. Orme et al [6,13,14], for example, used a set of capacitors for charging and deflecting single droplets emerging from a continuous jet. Consequently, both kinds of droplet generators have specific advantages and disadvantages, depending of the field of application.…”

Research on lateral instability of the uniform-charged droplet stream during droplet-based freeform fabrication

“…But, the droplet deposition accuracy is difficult to control as the droplet velocity is too high and droplets merge with each other during the spray process. So the deflecting charged droplet method is employed to deposit droplets accurately and controllably [4][5][6][7]. But, once the droplet stream is charged, the electrostatic repelling forces between the charged droplets will disperse the stream.…”