A UV-curable epoxy–siloxane hybrid material (epoxy hybrimer) was fabricated by photo-cationic polymerization of a sol–gel derived cyclo-aliphatic epoxy oligosiloxane (CAEO) blended with oxetane cross-linker in the presence of an onium salt. Antioxidants for fabrication of the UV-curable epoxy hybrimer with high thermal resistance against yellowing were incorporated in the UV-curable epoxy hybrimer. The UV-curable epoxy hybrimer with the antioxidants showed high thermal resistance without yellowing during 120 °C thermal aging. High thermal resistance of the UV-curable epoxy hybrimer was similar and higher compared to those of commercial thermally curable silicone and UV-curable epoxy LED encapsulants, respectively. The thermally resistant UV-curable epoxy hybrimer was successfully encapsulated on a LED without any cracking or delamination, and maintained a flat surface on the LED without distortion of the designed flat shape. Before/after thermal and blue light aging, the performance of the LED encapsulated by the UV-curable epoxy hybrimer was not changed. On the basis of its excellent properties as a LED encapsulant, the UV-curable epoxy hybrimer can be utilized as a UV-curable LED encapsulant for white LEDs.

In this paper, a chip-in-flex (CIF) assembly that has an excellent bending performance, including a minimum bending radius without a chip fracture and the capacity to withstand dynamic bending, is developed. Chip-on-flex (COF) and CIF assemblies are fabricated using anisotropic conductive films (ACFs) as interconnection materials. The COF package is composed of 40-μm-thin silicon chips, ACF, and flexible substrates. The CIF package is fabricated by attaching a cover adhesive film and a polyimide film on the COF package to encapsulate the silicon chip. Through static bending tests, the optimal thickness of the cover adhesive film is established. The optimized CIF assembly allows a minimum bending radius of 4 mm without a chip fracture, while the chip in the COF assembly fractures at a bending radius of 10 mm. A finite-element analysis of the static bending test is performed to understand the internal stress state of the assemblies. A bending reliability test of the CIF package is also conducted at a bending radius of 7.5 mm for 160k cycles, by measuring the daisy-chain resistance during the test. The effect of the elastic modulus of the ACF resin on the fatigue endurance is investigated through the bending fatigue test. The higher modulus of the ACF resin resulted in excellent fatigue reliability with stable ACF joints showing neither delamination nor resin crazing after 160k cycles of bending.

Anisotropic conductive films (ACFs) have been widely used as an interconnection adhesive material due to its light weight and simple and low-temperature assembly processes. However, because of the higher demands for further miniaturization, short-circuit problem of ACFs interconnection has been a major issue, when it comes to very fine pitch assembly. Also, due to the fast development of wearable devices, demands for flexible packaging as well as flexible interconnection methods such as flex-on-flex (FOF) are growing more, where the bending characteristics are important. Nanofiber incorporated ACFs were previously introduced by our research group. The nanofiber incorporated ACFs showed excellent conductive particle movement suppression capability that not only prevents short circuit but also improves the conductive ball capture rate, which eventually improves the joint reliability of fine pitch FOF assembly. In order to maximize the conductive particle movement suppression capability of the nanofiber, nanofiber was oriented using a drum-type receiver. In addition, the bending reliability of FOF assembly using nanofiber/solder ACFs with different nanofiber orientations has been also investigated. As a result, parallel nanofiber/solder ACFs showed excellent joint properties as well as bending reliability due to the stable metallurgical solder joint formation and high conductive particle movement suppression capability, and this new type of ACF technology will provide a promising solution for future flexible electronic packaging.

Although cationic epoxy was optimized for low-melting SnBi58 solder ACF joints with the lowest coefficient of thermal expansion (CTE) in terms of reliability, cationic epoxy also showed a faster curing property than any other types of adhesives. In fact, solder joint shapes at 250 °C bonding are very different from those shapes at 200 °C bonding. In this paper, four adhesive film types were investigated in terms of Sn-3Ag-0.5Cu solder ACF joint shapes on the electrical performances and reliability in a pressure cooker test (PCT). Thermal stability of adhesive films was tested to be first. Resin curing speeds were measured in a 250 °C isothermal mode differential scanning calorimetry, resin viscosities were checked by a parallel-plate rheometer, and adhesive thermomechanical properties, such as modulus and CTE, were characterized. Then, four different types of ACF resins containing the same weight percentages of Sn-3Ag-0.5Cu solders were assembled by the same thermocompression bonding parameter (250 °C 10s 2MPa on bump) on a 500-μm-pitch flex-on-board (FOB) application, and different solder joint morphologies were verified. Various bonded solder ACFs joints were compared in terms of solder wetting areas, electrical performances by a four-point-probe method, and the reliability of PCT (121 °C 100% humidity 2atm) for 120 h. This paper aims at optimizing the best adhesive film candidate for SAC305 solder ACF joints of FOB application.

In this paper, joint morphologies and failure mechanisms of anisotropic conductive films (ACFs) during a power handling capability test for flex-on-board applications were investigated in terms of four conductive particles: gold-coated polymer ball, Ni metal ball, SnBi58, and Sn-3Ag-0.5Cu particles. It was found that all the polymer ball and Ni ball joints could not handle the ACFs' joint temperature of 200 °C presumably due to polymer resin expansion and physical contacts loss, and their corresponding contact resistance also became open circuit after the power test. In contrast, the failure mechanism of SnBi58 solder joint was that when temperature was over 138 °C and Sn element was quickly consumed by Sn-Ni intermetallic compounds resulting in brittle Bi-rich phase remaining, more severe crack occurred along the Bi-rich phase of SnBi58 joints. Sn-3Ag-0.5Cu solder ACF joints due to a higher solder melting point were the best among the four conductive particles in terms of the little crack formation in its joint morphology and the morphology and the remaining stable electrical resistance even after the highest power test to 300 °C, when FR4 printed circuit board burned. Beyond the FR-4 decomposition at 300 °C, ACF joints were burned due to epoxy resin decomposition from 330 °C.