Woo-Ram Myung

Effects of sintering conditions on microstructure and characteristics of screen-printed Ag thin film

Direct printing techniques can provide a shorter manufacturing time, lower cost, and greater environmental friendliness compared to conventional photolithography. The electrical and mechanical characteristics of directly printed films are determined by their microstructure. This research focuses on the microstructural evolution of silver (Ag) thin films screen-printed onto a silicon (Si) wafer passivated with SiO2. To investigate the effect of heat treatment on the microstructure of the thin film, various sintering conditions were used: temperatures of 150°C, 200°C, 250°C, and 300°C, atmospheres of air, vacuum, N2, and Ar and rates of increase of temperature of 10°C/min, 20°C/min, 30°C/min, 40°C/min, and 50°C/min. Each parameter plays an important role in determining the density and microstructural evolution, which affect the densification of and void distribution inside the thin film. The electrical resistivity of the Ag thin film was measured by a four-point probe method. The oxygen concentration profiles on the Ag film surface were investigated using Auger electron spectroscopy (AES). The film sintered in air showed the highest oxygen concentration. At higher sintering temperatures, the film showed a denser microstructural evolution and void growth resulting in a lower electrical resistivity of 2.88 µΩ·cm.

Effects of Plasma Polymerized Acrylic Acid Film on the Adhesion of Ag Tracks Screen-Printed on Polyimide

The adhesion of conductive patterns printed on polymer substrates is an indispensible issue for the commercialization of printable and flexible electronic devices. Plasma treatment has been widely used to improve the interfacial adhesion between a metal and a flexible polymer substrate. This study aims to investigate the influence of a polymerized acrylic acid layer coated by atmospheric-pressure plasma (APP) on the adhesion of a screen-printed silver (Ag)/polyimide (PI) system. The acidic oxygen-containing functional groups were incorporated onto a PI film by plasma polymerization of acrylic acid and, on it, the conductive tracks were constructed with a Ag nanopaste via screen printing. The Ag tracks were sintered at various temperatures ranging from 150 to 300°C for 30 min in air. The adhesion was evaluated by a roll-type 90% peel test. The peel strength of the screen-printed Ag/PI system with the acrylic acid film approximately quadrupled. To understand this adhesion enhancement, field emission scanning microscopy (FE-SEM), atomic force microscopy (AFM), contact angle analyzer, and X-ray photoelectron spectroscopy (XPS) were utilized. It was confirmed from these analyses that a hydrophilic film was formed due to the plasma polymerization process, and the carbon-oxygen (C–0)and carbonyl (C=0) bonds increased at the interfacial surface. Under the optimized conditions, a maximum adhesion of 245.5 N/m was obtained, and the stronger adhesion with the acrylic acid coating influenced the improvement in the flexibility of the film.

Evaluation of the Bondability of the Epoxy-Enhanced Sn-58Bi Solder with ENIG and ENEPIG Surface Finishes

The effect of different surface finishes, electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG), on the mechanical properties of Sn-58Bi bumps made with solder paste enhanced with epoxy were investigated. The microstructure and fracture surfaces were observed with scanning electron microscopy, and the compositions of the IMC and solder were measured using energy dispersive spectrometry and an electron probe micro-analyzer (EPMA). To evaluate the mechanical properties, low-speed shear tests and board-level drop tests were performed. The result of the shear tests showed that the bonding strength of the epoxy-enhanced Sn-58Bi solder bumps was higher than that of Sn-58Bi solder for all surface finishes, because of the epoxy surrounding the solder, and the fracture surfaces of epoxy-enhanced Sn-58Bi indicated ductile fracture in the solder joint. However, the result of the drop tests showed that samples with the ENIG and ENEPIG surface finishes had lower drop numbers compared to the sample without these surface finishes. The lower performance resulted from insufficient ejection of epoxy from the ENIG and ENEPIG surface finishes during reflow, which reduced the interfacial bonding area.

Microstructures and Drop Impact Test of SAC305, Sn58%Bi and Epoxy Sn58%Bi SolderJoint on the OSP Surface Finished PCB Substrate

Lead free Sn3.0%Ag0.5%Cu (SAC305) solder and low temperature Sn58%Bi solder have been widely used to replace lead based solder alloys. Because Sn58%Bi solder has poor ductility and shock absorbance ability, previous researches have tried to improve its mechanical properties by adding additional elements, reinforcements, carbon nano tube (CNT) and polymer. The bonding strength and drop impact reliability of SAC305 solder, Sn58%Bi and epoxy contained Sn58%Bi solder (epoxy Sn58%Bi solder) assembled on the OSP surface finished PCB substrate were investigated using low speed shear and board drop impact tests. After soldering, Cu6Sn5 intermetallic compound (IMC) was formed in the solders and OSP surface finished PCB substrate joints. Bonding strength and drop reliability of epoxy Sn58%Bi solder had superior mechanical properties than those of SAC305 solder and Sn58%Bi solder. The crack in the solder joint of SAC305 after board drop impact testing takes place within the IMC layer. However, the crack at the solder joint of the Sn58%Bi after board drop impact testing occurred on the interface between IMC layer and Sn58%Bi solder and the crack in the solder joint of the epoxy Sn58%Bi presented within the solder, respectively.

Mechanical Property of SAC305 Ball-Grid Array and Epoxy Sn-58Bi Solder Joints with 85 °C/85% Relative Humidity Environmental Conditions

The ball-grid array (BGA) is widely used to reduce component size and it had advantages such as high I/O pins and fine pitch. Typical Sn-Ag-Cu (SAC) solder alloys are used for formation of BGA because SAC solder has excellent characteristics among lead-free solders. However, the electronic components assembled by SAC solder were easily damaged by heat during manufacture process because SAC solder had high melting point of 220 °C. To prevent these thermal damages, SAC305 BGA component assembled by Sn-58Bi solder paste has been studied because Sn-58Bi solder had low melting point of 139 °C. In generally, Sn-58Bi solder was improved by additional elements or polymer such as epoxy because Sn-58Bi had a brittle property. However, the epoxy Sn-58Bi solder did not guaranteed high environmental reliability such as high-temperature high-humidity (HTHH) test. Thus, we evaluated the shear strength of solder joints assembled by SAC305 BGA components with Sn-58Bi solder paste and epoxy Sn-58Bi solder paste. The shear strength of solder joints was evaluated by die shear test after HTHH test at the 85 °C/85% RH conditions. The Cu6Sn5 intermetallic-compound (IMC) at the interface of solder joints was observed by scanning electron microscope (SEM). The IMC thickness of Sn-58Bi solder joints was smaller than that of epoxy Sn-58Bi solder. The shear strength was improved up to 20% by epoxy addition. The shear strength of epoxy Sn-58Bi solder joints dramatically decreased after HTHH test for 100 h.

Effect of Epoxy on Mechanical Property of SAC305 Solder Joint with Various Surface Finishes Under 3-Point Bend Test

Microstructures and mechanical property of Sn-3.0Ag-0.5Cu (SAC305) and epoxy Sn-3.0Ag-0.5Cu (epoxy SAC) solder joints were investigated with various surface finishes; organic solderability preservative (OSP), electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG). Bending property of solder joints was evaluated by 3-point bend test method. Microstructure and chemical composition of solder joints was characterized by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX), respectively. Epoxy did not effect on intermetallic compound (IMC) morphology. Scalloped shaped Cu6Sn5 IMC was observed at OSP surface finish. Chunky-like shaped and needle-like shaped (Ni,Cu)6Sn5 IMC were observed at the solder/ENIG joint and solder/ENEPIG joint, respectively. The bending cycles of SAC305/OSP joint, SAC305/ENIG joints and SAC305/ENEPIG joints were 720, 440 and 481 cycle numbers. The bending cycles of epoxy SAC and three types surface finished solder joints were over 1000 bending cycles. Under OSP surface finish, bending cycles of epoxy SAC solder was approximately 1.5 times higher than those of SAC305 solder joint. Bending cycles of epoxy SAC solder was over twice times higher than those of SAC305 solder with ENIG and ENEPIG surface finishes. The bending property of epoxy solder joint was enhanced due to epoxy fillet held the solder joint.

Mechanical Reliability of the Epoxy Sn-58wt.%Bi Solder Joints with Different Surface Finishes Under Thermal Shock

The effect of thermal shock on the mechanical reliability of epoxy Sn-58wt.%Bi composite (epoxy Sn-58wt.%Bi) solder joints was investigated with different surface-finished substrates. Sn-58wt.%Bi-based solder has been considered as a promising candidate for low-temperature solder among various lead-free solders. However, Sn-58wt.%Bi solder joints can be easily broken under impact conditions such as mechanical shock, drop tests, and bending tests because of their poor ductility. Therefore, previous researchers have tried to improve the mechanical property of Sn-58wt.%Bi solder by additional elements and mixtures of metal powder and epoxy resin. Epoxy Sn-58wt.%Bi solder paste was fabricated by mixing epoxy resin and Sn-58wt.%Bi solder powder to enhance the mechanical reliability of Sn-58wt.%Bi solder joints. The epoxy Sn-58wt.%Bi solder paste was screen-printed onto various printed circuit board surfaces finished with organic solder preservatives (OSP), electroless nickel immersion gold (ENIG), and electroless nickel electroless palladium immersion gold (ENEPIG). The test components were prepared by a reflow process at a peak temperature of 190°C. The thermal shock test was carried out under the temperature range of − 40 to 125°C to evaluate the reliability of Sn-58wt.%Bi and epoxy Sn-58wt.%Bi solder joints. The OSP-finished sample showed a relatively higher mechanical property than those of ENIG and ENEPIG after thermal shock. The average number of cycles for epoxy Sn-58wt.%Bi solder with the OSP surface finish were 6 times higher than that for Sn-58wt.%Bi solder with the same finish. The microstructures of the solder joints were investigated by scanning electron microscopy, and the composition of the intermetallic compound (IMC) layer was analyzed by using energy dispersive spectrometry. Cu6Sn5 IMC was formed by the reaction between Sn-58wt.%Bi solder and a OSP surface-finished Cu after the reflow process. Ni3Sn4 IMC and (Ni, Pd)3Sn4 IMC were formed at the solder joints between the ENIG and solder, and between ENEPIG surface finish and solders, respectively.