Hyoung-Joon Kim

Effects of Cu/Al intermetallic compound (IMC) on copper wire and aluminum pad bondability

Copper wire bonding is an alternative interconnection technology that serves as a viable, and cost saving alternative to gold wire bonding. Its excellent mechanical and electrical characteristics attract the high-speed, power management devices and fine-pitch applications. Copper wire bonding can be a potentially alternative interconnection technology along with flip chip interconnection. However, the growth of Cu/Al intermetallic compound (IMC) at the copper wire and aluminum interface can induce a mechanical failure and increase a potential contact resistance. In this study, the copper wire bonded chip samples were annealed at the temperature range from 150/spl deg/C to 300/spl deg/C for 2 to 250 h, respectively. The formation of Cu/Al IMC was observed and the activation energy of Cu/Al IMC growth was obtained from an Arrhenius plot (ln (growth rate) versus 1/T). The obtained activation energy was 26Kcal/mol and the behavior of IMC growth was very sensitive to the annealing temperature. To investigate the effects of IMC formation on the copper wire bondability on Al pad, ball shear tests were performed on annealed samples. For as-bonded samples, ball shear strength ranged from 240-260gf, and ball shear strength changed as a function of annealing times. For annealed samples, fracture mode changed from adhesive failure at Cu/Al interface to IMC layer or Cu wire itself. The IMC growth and the diffusion rate of aluminum and copper were closely related to failure mode changes. Micro-XRD was performed on fractured pads and balls to identify the phases of IMC and their effects on the ball bonding strength. From XRD results, it was confirmed that the major IMC was /spl gamma/-Cu/sub 9/Al/sub 4/ and it provided a strong bondability.

Ultrasonic Bonding Using Anisotropic Conductive Films (ACFs) for Flip Chip Interconnection

In this paper, a novel anisotropic conductive film (ACF) flip chip bonding method using ultrasonic vibration for flip chip interconnection is demonstrated. The curing and bonding behaviors of ACFs by ultrasonic vibration were investigated using a 40-kHz ultrasonic bonder with longitudinal vibration. In situ temperature of the ACF layer during ultrasonic (U/S) bonding was measured to investigate the effects of substrate materials and substrate temperature. Curing of the ACFs by ultrasonic vibration was investigated by dynamic scanning calorimetry (DSC) analysis in comparison with isothermal curing. Die adhesion strength of U/S-bonded specimens was compared with that of thermo-compression (T/C) bonded specimens. The temperature of the ACF layer during U/S bonding was significantly affected by the type of substrate materials rather than by the substrate heating temperature. With room the temperature U/S bonding process, the temperature of the ACF layer increased up to 300degC within 2 s on FR-4 substrates and 250degC within 4 s on glass substrates. ACFs were fully cured within 3 s by ultrasonic vibration, because the ACF temperature exceeded 300degC within 3 s. Die adhesion strengths of U/S-bonded specimens were as high as those of T/C bonded specimens both on FR-4 and glass substrates. In summary, U/S bonding of ACF significantly reduces the ACF bonding times to several seconds, and also makes bonding possible at room temperature compared with T/C bonding which requires tens of seconds for bonding time and a bonding temperature of more than 180degC.

Internal stresses evolution of non conductive pastes (NCPs) and underfill materials for flip chip applications

In this paper, effects of internal stress on material properties such as CTE, modulus, and glass transition temperature (T/sub g/) of various NCPs and underfill materials for flip chip applications are discussed. And a simple method for the internal stress estimation using material properties is introduced. Usually, internal stresses are generated inside NCPs and underfill materials during a curing process. And these internal stresses affect thermomechanical properties of cured NCPs and underfill materials. Using thermo-mechanical analyzer (TMA), dimensional changes of all materials were measured. Dimensional changes after 1st heating cycle rapidly increased near T/sub g/, however, it was not observed after the 2nd and 3rd heating cycles. This difference between the 1st and 2nd cycles is due to excess free volume built during curing processes. Also, using dynamic mechanical analyzer (DMA), modulus and T/sub g/ were measured. The modulus of the 1st cycle was smaller than that of the 2nd cycle for all materials. Based on these differences in the 1st and 2nd cycles, internal stresses of various materials were theoretically estimated. The estimated stresses build-up of the 1st cycle was different from those of the 2nd cycle for all materials. It is considered that internal stresses generated during a curing process alter the stress state inside materials.

Study on the characteristics of various dopants in Sn-1Ag-0.8Cu solder

Recently, the Sn-Ag-Cu system alloy has been widely used in the packaging (PKG) industry a replacement for conventional SnPb solders. The Sn-Ag-Cu system alloy was divided into two types, high Ag solder and low Ag solder composition, according to Ag content. Generally, high Ag solder, as Sn-3∼4Ag-0.5Cu, showed good thermal cycle (TC) reliability because of excellent creep resistance and thermal fatigue reliability, but it showed poor drop impact reliability compared to SnPb solder or low Ag solder. Meanwhile, low Ag solder as Sn-0.3∼1.2Ag-0.5Cu showed better drop shock reliability than high Ag solder, but it showed poorer TC reliability. These results mean that just a change in Ag content in the SAC alloy cannot simultaneously improve TC and drop impact reliability.

Effects of Thermal Cycling on Material Properties of Nonconductive Pastes (NCPs) and the Relationship Between Material Properties and Warpage Behavior During Thermal Cycling

In this paper, the effects of thermal cycling on material properties such as coefficient of thermal expansion (CTE), modulus, and glass transition temperature (Tg) of nonconductive pastes (NCPs) for flip chip applications were investigated. Using a thermomechanical analyzer, the dimensional changes of NCPs and an underfill material were measured. The dimensional changes of all materials during the first cycle rapidly increased near Tg. However, the rapid increase of dimensional change near Tg was not observed during the second and third cycles. Furthermore, using a dynamic mechanical analyzer, the modulus and Tg were measured. The modulus in the first cycle was smaller than that in the second cycle for all materials. After the first cycle, the modulus curves followed the second cycle curve. Next, the warpage behavior of the flip chip assemblies was observed using the Twyman-Green interferometry method to investigate how material property changes affect warpage behavior during thermal cycling (T/C) and it was found that the warpage of the flip chip assembly decreased after the first cycle. However, after the first cycle, the amount of warpage was constant for the following five cycles. As a result, it was verified that the material properties of NCPs and the underfill material change after the first thermal cycle, and the material property changes are closely related to the warpage hysteresis behavior during T/C. Finally, warpage hysteresis was understood as shear strain.

Bubbles formation in rigid-flexible substrates bonding using anisotropic conductive films (ACFs) and their effects on ACFs joints reliability

Rigid-flexible (R-F) substrates bonding technology using ACFs becomes more important as an alternative to connectors and rigid/flex substrates. ACF interconnection is a well-established technology but the quality of ACF interconnection is very dependent on bonding process variables. Especially, formation of process bubbles, which are entrapped inside the ACF layer during bonding processes, is strongly influenced by bonding process variables. These bubbles can reduce mechanical property, such as adhesion strength in ACFs joints, and induce moisture penetration and entrapment location during reliability tests in humid environments. Therefore, trapped bubbles also can reduce the reliability of ACFs interconnection joints. Bonding process variables, such as bonding temperature, bonding pressure and flexible substrate types, are controlled in order to investigate their effects on bubbles formation. According to the results, bubbles formation is closely related to these three factors. The ratio of bubble area increased as the bonding temperature increased. Moreover, same tendency was appeared against the bonding pressure change at fixed bonding temperature conditions. Two different flexible substrates, which have different surface roughness and energy, were used and bubbles formed only at flexible substrates with larger roughness and lower surface energy. Furthermore, a reliability test in humid environment, such as a PCT (pressure cooker test), has been performed to investigate the effect of process bubbles on ACFs interconnection joints reliability. According to the results of PCT, process bubbles acted as important moisture penetration and entrapment sites. Therefore, the daisy resistance of the samples which have process bubbles increased abruptly and finally, all bubble formed samples were failed after PCT

A Novel Anisotropic Conductive Film (ACF) Bonding Method Using Vertical Ultrasonic Vibration

In this study, a novel anisotropic conductive film (ACF) bonding method using vertical ultrasonic vibration was developed and demonstrated in flex-on-board applications. In terms of ultrasonic bonding processes, the relations between ACF temperature and ultrasonic variables were both experimentally and theoretically investigated. Considering the effects ultrasonic variables on ACF temperature, a novel ACF bonding equipment utilizing vertical ultrasonic vibration was newly developed in combination with a vision alignment system and pneumatic actuators without any external heat sources. ACF temperature was successfully controlled with a range from room temperature to above 250°C by adjusting ultrasonic vibration amplitude. The ACF temperature showed rapid heating rates and stable plateau regions within 2 sec with ±10°C temperature uniformity through the bonding area. Ultrasonic bonded ACF joints showed similar peel adhesion strengths and daisy-chain resistances as thermo- compression bonded ACF joints and showed stable resistances during 85°C/85 % RH test and 125°C high temperature storage test for 1000 hours, and -55°C/125°C thermal cycling test for 1000 cycles.

Room temperature ACF bonding process using ultrasonic vibration for chip-on-board and flex-on-board applications

In this study, a novel anisotropic conductive film (ACF) bonding process using ultrasonic vibration was investigated in chip-on-board (COB) and flex-on-board (FOB) applications. The ACF temperature increased as the U/S power increased and the bonding pressure decreased. The ACF temperature was successfully controlled by adjusting both U/S power and bonding pressure. The optimized U/S bonding time was 3 sec at room temperature. The significant meaning of this result is that the ACF bonding process can be remarkably improved by U/S bonding compared with conventional 15 sec T/C bonding at 190degC. Using the optimized U/S bonding parameters, the ACF interconnects showed similar bonding performances as T/C bonding in terms of the daisy-chain resistance and the adhesion strength. The FTIR (Fourier transformation infrared spectroscopy) analysis showed that the cure degree of adhesive resin was achieved 90% at 3 sec. In the reliability tests, the U/S bonded ACF interconnects showed no significant change in electrical resistances during 85degC / 85% RH test and 125degC high temperature storage test for 1000 hours and -55degC -125degC thermal cycling test for 1000 cycles.

Thermal stability at the anisotropic conductive films (ACFs)/ organic solderability preservatives (OSPs) Interface

Failure surface analysis by SEM and XPS showed that OSP layers disappeared after heat treatment above 240degC. Furthermore, FT-IR analyses of OSP layer showed that C=N bonding strength became stronger after heat treatment, and it was considered that Cu-N coordinate bonds that form the OSP layers were broken. Considering that the OSP layer remained after ACF bonding and OSP layer was weakness after heat treatment above 240degC, interface between the ACFs and OSP treated Cu has a weakness for high temperature environment above 240degC. Finally, in application to OSP finished rigid substrates-flexible substrates system, ACFs/OSP finished Cu electrodes interface was major failure site after heat treatment above 240degC.

Effects of metal surface finish on the anisotropic conductive adhesives (ACAs) joints

This work was performed to investigate the effect of final surface finish of Cu electrodes on the adhesion and reliability of anisotropic conductive films (ACFs) joints. Two different materials, electroless Ni/immersion Au (ENIG) and organic solderability preservatives (OSPs), were selected because these materials has been most commonly used as the final surface finish materials in printed circuit board (PCB) industries. However, the effect of OSPs on the adhesion and reliability of ACF joints has not been studied. Therefore, 1) investigation of the adhesion of ACF/OSP joints, and 2) evaluation of the reliability of ACF/OSP joints were performed in this study. Then, the results of ACA/OSP joints were compared to those of ACF/ENIG to confirm the feasibility and reliability of ACF/OSP joints. Adhesion strengths of ACF/OSP joints were also higher than ACF/bare Cu and ACF/ENIG joints. The fractured site of ACF/bare Cu and ACF/ENIG joints was ACF/metal interface but that of ACF/OSP joints was ACF inside. TEM and FT-IR analyses showed that the OSP coating layer on Cu electrode remained even after ACF bonding and it seemed to play as an adhesion promoter