Yongwon Choi

Enhanced Thermal Conductivity of Epoxy–Graphene Composites by Using Non‐Oxidized Graphene Flakes with Non‐Covalent Functionalization

Homogeneous distribution of graphene flakes in a polymer matrix, still preserving intrinsic material properties, is key to successful composite applications. A novel approach is presented to disperse non-oxidized graphene flakes with non-covalent functionalization of 1-pyrenebutyric acid and to fabricate nanocomposites with outstanding thermal conductivity (∼1.53 W/mK) and mechanical properties (∼1.03 GPa).

High performance encapsulant for light-emitting diodes (LEDs) by a sol–gel derived hydrogen siloxane hybrid

The hydrosilylation reaction is a dominant curing method for the light-emitting diode (LED) silicone encapsulant, which requires characteristics of high transparency, high refractive index, and thermal stability against the LED junction temperature. In this research, we synthesized a hydrogen containing oligosiloxane resin through non-hydrolytic sol–gel condensation. Methyldiethoxysilane (MDES) and diphenylsilanediol (DPSD) were used as precursors and a non-hydrolytic sol–gel condensation reaction was performed under acidic conditions. The synthesized hydrogen oligosiloxane resin was mixed with a phenyl and vinyl containing oligosiloxane (PVO) resin, which was prepared by a sol–gel condensation between vinyltrimethoxysilane (VTMS) and the DPSD. This blended solution was cured at 150 °C for 4 hours to complete the hydrosilylation reaction. The cured material (phenyl hybrimer) shows low shrinkage during the curing reaction and provides excellent transparency (∼90% at 450 nm) and thermal stability (440 °C) with a high refractive index (n = 1.58 at 632.8 nm).

Thermally resistant UV-curable epoxy–siloxane hybrid materials for light emitting diode (LED) encapsulation

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.

Wafer level packages (WLPs) using B-stage non-conductive films (NCFs) for highly reliable 3D-TSV micro-bump interconnection

Higher packaging density demands to populate more circuits or chips on smaller substrate areas. 3-D stacking technologies have been developed for smaller package size and higher electronic performances. Among 3-D packaging technologies, the through silicon via (TSV) technology that uses Cu pillar/Sn-Ag micro-bumps to vertically interconnect between chips is the most advanced state-of-art packaging method. However, conventional flux and underfill process for bonding using Cu pillar/Sn-Ag micro-bumps has problems such as process complexity, flux residues and voids trapping. In this study, non-conductive films (NCFs) have been introduced to simplify the bonding processes and avoid flux residues and voids trapping. In addition, wafer level packages (WLPs) introduced using NCFs for 3D-TSV micro-bump interconnection have been investigated. At first, wafer level NCFs lamination was conducted without voids and bubbles formation on a wafer. And the effect of liquid type epoxy on the adhesion and elongation properties of NCFs laminated on a wafer was also investigated to optimize the wafer dicing process using laminated NCFs. After NCF laminated Cu/Sn-Ag bumped wafer was diced into a single chip, singulated chips were bonded on substrate chips using a flip chip bonder. The electrical properties of WLP packages using NCFs were evaluated and compared with conventional single chip packages. As a result, WLPs using NCFs showed the same electrical interconnection properties with conventional single chip packages.

Wafer-Level Packages Using B-Stage Nonconductive Films for Cu Pillar/Sn–Ag Microbump Interconnection

The 3-D stacking technologies have been developed, because higher packaging density demands to populate more circuits or chips on smaller substrate areas. Among 3-D packaging technologies, the through silicon via (TSV) technology that uses Cu pillar/Sn-Ag microbumps to vertically interconnect between chips is the most advanced state-of-the-art packaging method. However, the conventional reflow process with flux and underfill for bonding using Cu pillar/Sn-Ag microbumps has problems, such as process complexity, flux residues entrapment, and voids trapping. In this paper, the B-stage nonconductive films (NCFs) have been introduced to simplify the bonding processes and avoid flux residues entrapment and voids trapping. In addition, wafer-level packages (WLPs) using NCFs for the 3-D-TSV microbump interconnection have also been investigated. At first, the wafer-level NCFs lamination was conducted without voids and bubbles formation on a wafer. And the effect of epoxy resin types on the adhesion and elongation properties of NCFs laminated on a wafer was also investigated to optimize the wafer dicing process using laminated NCFs. After NCF-laminated Cu/Sn-Ag bumped wafer was diced into a single chip, singulated chips were bonded on substrate chips using a flip chip bonder. The electrical properties and reliabilities of the WLP packages using NCFs were evaluated and compared with the conventional single flip chip packages. As a result, the WLPs using the B-stage NCFs showed the same electrical interconnection properties as those of the conventional single flip chip packages.

Flux function added solder anisotropic conductive films (ACFs) for high power and fine pitch assemblies

In this study, flux function added solder ACF was proposed in order to be used not only with the ultrasonic bonding method which can break solder using ultrasonic vibration oxides but also with conventional thermo-compression bonding method. Since the solder oxides cannot be broken only with conventional thermo-compression method, we had to remove the oxide chemically. The test vehicles were consist of polyimide based flexible substrates and FR-4 organic rigid boards which have 300 μm pitch Cu patterns with electroless nickel immersion gold (ENIG) surface finish. Solder ACFs 50 μm thick were epoxy based adhesive films containing flux functional additive. The films contain 5-15 μm diameter SAC305 (96.5Sn-3.0Ag-0.5Cu) solder particles which are used as conductive particles and 5 μm diameter Au coated Ni particles which are used as spacers. According to the experimental results, the flux functional additive did not change the curing reaction of the solder ACF. Moreover, the addition of flux functional additive caused significant improvement of electrical properties of solder ACF joints such as power handling capability and reliability. Therefore, using a flux function added solder ACFs can be used for various FOB and FOF assemblies and can provide an alternative interconnection for high power and fine pitch assemblies.

3D-TSV vertical interconnection method using Cu/SnAg double bumps and B-stage non-conductive adhesives (NCAs)

In this study, the chip to chip eutectic solder bonding method using NCAs for TSV stacking was investigated as an alternative 3D-TSV interconnection method. The non-conductive polymer adhesive was applied at TSV wafers as a film format before eutectic solder bonding resulting in no extra underfill process. The electrical interconnections between micro-sized bumps for TSVs of the stacked chips were investigated. The electrical interconnection through the arrays of the bumps between two chips showed no change even after the reliability tests which meant that vertical interconnection by one step metal/polymer hybrid bonding was rapid as well as stable.

A novel double layer NCF for highly reliable micro-bump interconnection

40 μm pitch Cu-pillar/Sn-Ag bump to Ni pad thermo-compression bonding was performed using non-conductive films (NCFs) with different curing agents. The joint morphology of Cu-pillar/Sn-Ag bump/Ni pad was dependent on the viscosity and curing speed of NCFs. Imidazole NCF with high viscosity and fast curing speed showed no solder wetting on both Cu-pillar and Ni pad. However, anhydride NCF with low viscosity and slow curing speed showed both solder wetting on Cu-pillar and Ni pad. Double layer NCF consisting of imidazole NCF as top layer and anhydride NCF as bottom layer was designed and optimized to eliminate solder wetting on Cu-pillar. Joints bonded using double layer NCF showed ideal joint shape with no solder wetting on Cu pillar and good wetting on pad. Thermal aging at 150 C was performed and IMC growth rate of Cu6Sn5 and Cu3Sn was slower for double layer NCF compared to single layer NCF due to reduced reacting area for Cu and Sn.

Analysis of 3D TSV Vertical Interconnection Using Pre-applied Nonconductive Films

Much research has been carried out to realize through-silicon via (TSV) technology for three-dimensional (3D) chip stacking packaging. A vertical chip interconnection method using Cu/Sn-Ag bumps and nonconductive films (NCFs) is one of the most promising approaches for 3D TSV vertical interconnection. In this work, the relationship between the viscosity of pre-applied NCFs and loading forces was investigated to predict the gap change between a TSV chip and a substrate chip. Existing theories of squeeze flow are adapted to predict the gap change of a real TSV chip and a substrate chip during TSV bonding using a simplified model. The real gaps measured during bonding of test dies were matched to check the validity of the prediction model. Considering the thixotropy of NCFs, the prediction well matched the real gap changes between bumped TSV chips and substrate chips during bonding.

Development of anhydride-based NCFs for Cu/Sn-Ag eutectic bonding and process optimization for fine pitch TSV chip stacking

Methyl tetrahydrophthalic anhydride (MeTHPA), and Methyl hexahydrophthalic anhydride (MeHHPA) were added as curing agent for bisphenol A/phenoxy resin-based non-conductive film (NCF). Only NCF with MeTHPA showed thermal stability and void-less interface at 250°C, and addition of 0.5 wt% of latent curing accelerator showed minimum NCF cure at pre-bonding stage (120°C) and full NCF cure after main-bonding stage (250°C). The formulated NCF were applied to TSV chip on substrate bonding with Cu/Sn-Ag double bump to Cu pad joint structure. Film thickness and bonding pressure were optimized at 15 μm and 2.4 MPa to form state-of-art joint structure with optimal fillet shape. The joint structures were observed to be stable even after three reliability tests: high temperature storage test (HTST) of 200 hours, pressure cooker test (PCT) of 24 hours, and thermal cycle (T/C) of 200 cycles. Anhydride-based NCF showed stable joints due to flux-ability of anhydride; however, conventional dicyan diamide (DICY) based NCF showed broken joints after T/C due to lack of flux-ability.