# A Novel Compliant 2-DOF Ejector Pin Mechanism for the Mass Transfer of Robotic Mini-LED Chips
## 1 Introduction  
In recent years, due to the display technology of high-pixel, high-brightness, and high color gamut displays, high requirements of high-end display applications are presented, including flexible displays, augmented reality displays, and virtual reality displays [1, 2, 3, 4]. Due to the advantages of high resolution, low power consumption, high brightness, flexibility, fast response and high reliability, micro-LED and mini-LED display technologies play an important role [5, 6, 7]. For example, micro-LED technology (LED microminiaturization and matrix technology) refers to the integration of microminiaturized LEDs into circuit substrates with a very high density of solder joints, which makes each pixel in the LED display a fixed address and emit light separately. The number of micro-LED chips needed to transfer to make a single display is usually millions or even tens of millions. Therefore, the pixel density of display can be achieved from a millimeter range to micron range [8, 9]. Due to the small size and large number of LED chips in mini-LED displays, the main challenge in the mass production of mini-LED display technology is the massive transfer technology with micron precision, high efficiency, high yield and low cost [10, 11]. At present, the mass transfer technology of micro-LED can be mainly summarized as following: (1) the electrostatic transfer method for chip operation by using electrostatic adsorption force [12]; (2) elastic stamp transfer using van der Waals force to absorb and release chips [13, 14, 15]; (3) laser-assisted transfer technology using a laser-point melting chip adhesion layer [16, 17]; and (4) self-assembly technology using a fluids journal [18]. However, there are some problems with these technologies, such as the need for specialmaterials on the chip adhesion layer and the high cost of spare parts. The mechanical chip-eject transfer method uses tungsten needles to directly pierce and eject the dyes on the blue film onto the electrodes to complete the transfer [19]. This mechanical transfer mode has no special requirements for blue film materials and does not require expensive laser equipment. In a word, it is a low-cost and high-efficiency transfer approach. However, this method transfers the chip intermittently. During the transfer process, the motion platform is required to move the chip above the plate. It is necessary to wait until the platform is stable, since the needle is driven to eject the dye. The repetitive process of Mini-LED transfer is needed, whose effectiveness is restricted by the intermittent method. The continuous repetitive operations of the platform greatly reduce the chip transfer efficiency, resulting in low efficiency of the transfer method, which is less than 100 k units per hour (UPH). In order to deal with the limitations of intermittent mechanical transfer, a novel compliant 2-DOF ejector pin mechanism for mini-LED chip mass transfer is proposed in this paper. The ejector pin is always relatively stationary in the horizontal direction in the process of crystal pricking to avoid the frequent starting and stopping of the stage, so as to realize high-speed chip transfer. Moreover, to meet the requirements of mini-LED mass transfer, the chip size is 75-120 \(\upmu\)m and the thickness is about 80 \(\upmu\)m [20]. Additionally, the stroke, working frequency range, and the tracking accuracy of the proposed compliant mechanism are \(\geq\)100 \(\upmu\)m, \(\geq\)30 Hz and \(\pm\)2 \(\upmu\)m, respectively [20; 21]. The compliant mechanism has the advantages of high precision, low winding, and no friction, and can achieve ultra-high-precision and high-frequency motion within a small range [22; 23; 24]. Moreover, it has been demonstrated that it is effective for implementing mini-LED precision positioning by utilizing a 2-DOF-compliant mechanism [25; 26; 27]. Therefore, based on the principle of the compliant mechanism, a novel mini-LED direct transfer device is designed to realize the massive transfer of mini-LEDs with high accuracy and efficiency.