A 2 in. active‐matrix light‐emitting diode (AMLED) display by integration of the micro‐LED onto the oxide thin‐film transistor (TFT) backplane using flip chip bonding is reported. A blue‐emitting micro‐LED (µ‐LED) with a size of 90 × 50 µm2 is fabricated on the GaN epi grown on a sapphire substrate. The amorphous indium‐gallium‐zinc‐oxide (a‐IGZO) TFT on glass exhibiting the mobility of 18.4 cm2 V−1 s−1, turn‐on voltage (V ON) of 0.2 V, and subthreshold swing 0.25 V dec−1, is used for LED backplane. A two TFT and one capacitance pixel structure is utilized for driving 128 × 384 AMLED with 120 Hz frame rate. The laser lift‐off process with flip‐chip bond allows the transfer of the µ‐LED chips with 49 152 pixels onto the TFT backplane, demonstrating a 2 in. AMLED display with a good gray scale image. The current efficiency of µ‐LED is found to be 12.9 Cd A−1 at the luminance of 630 Cd m−2. Therefore, a‐IGZO TFT backplane can be used for µ‐LED displays.
Purpose-To investigate fluxless plasma ball bumping and effect of under bump metallization (UBM) thickness on joint properties using lead-free solders. Design/methodology/approach-A fluxless soldering process was investigated in this study using Ar-10 percent H 2 plasma reflow. Balls made from two lead-free solders (Sn-3.5 weight percent Ag and Sn-3.5 weight percent Ag-0.7 weight percent Cu) were reflowed and, also Sn-37 weight percent Pb as a reference. In particular, the effects of the UBM thickness on the interfacial metallurgical bonding and joint strength were studied. The UBM (Au/Cu/ Ni/Al layers) thicknesses were 20 nm/0.3 mm/0.4 mm/0.4 mm and 20 nm/4 mm/4 mm/0.4 mm, respectively. Findings-The experimental results showed that in the case of a thin UBM the shear strengths of the soldered joints were relatively low (about 19-27 MPa) due to cracks observed along the bond interfaces. The thick UBM improved joint strength to 32-42 MPa as the consumption of the Cu and Ni layers by reaction with the solder was reduced and hence the interfacial cracks were avoided. To provide a benchmark, reflow of the solders in air using flux was also carried out. Originality/value-This paper provides information about the effect of UBM thickness on joint strength for plasma fluxless soldering to researchers and engineers.
An Ar-10 vol%H 2 plasma was applied as a cleaning medium on UBMs (Under Bump Metallizations) to improve the bondability of solder balls prior to plasma reflow. Each UBM area comprised four layers deposited on a Si-wafer. Sequentially, the deposit comprised 0.4 mm of Al, 4 mm each of Ni and Cu, and 20 nm of Au from bottom to top of the metallization. Two compositions of lead-free solder balls (Sn-3.5 mass%Ag and Sn-3.5 mass%Ag-0.7 mass%Cu) and a lead containing one (Sn-37 mass%Pb) as a reference were selected for the experiment. The solder balls, of 500 mm diameter, were placed on the UBM's and fluxlessly soldered under Ar-10%H 2 plasma (with or without prior plasma cleaning). Additionally, air reflow with flux was also performed for comparison. Experimental results showed that the spreading ratios of the solders by plasma reflow after plasma cleaning were 20-40% higher than those plasma reflowed without cleaning. The shear strengths of the solder balls processed by the plasma-cleaned plasma reflow showed around 58-65 MPa, which is 60-80% and 15-35% higher than that of plasma reflow without plasma cleaning, and that of fluxed air reflow, respectively. From this study, plasma cleaning of UBM's using Ar-10%H 2 gas was shown to be quite effective to improve the bond strength of solder balls.
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