Jin-Hyoung Park

Studies on the thermal cycling reliability of fine pitch Cu/SnAg double-bump flip chip assemblies on organic substrates: Experimental results and numerical analysis

A thick Cu column based double-bump flip-chip structure is one of the promising alternatives for fine pitch flip-chip applications. In this study, the thermal cycling (T/C) reliability of Cu/SnAg double-bump flip-chip assemblies was firstly investigated and the failure mechanism was analyzed through correlation of T/C test and the finite element analysis (FEA) results. After 1000 thermal cycles, the T/C failure site was the Cu column/Si chip interface, where was identified via a FEA as the location of the maximum stress concentration during thermal cycling. During thermal cycling, the Al pad and Ti layer between the Si chip and Cu column bumps were displaced due to thermo-mechanical stress. Based on the low cycle fatigue model, the accumulation of equivalent plastic strain resulted in thermal fatigue deformation of the Cu column bumps, and ultimately reduced the thermal cycling lifetime. In addition, the normal plastic strain of the y-direction, 822, was determined to be compressive and was a dominant component in relation to the plastic deformation of Cu/SnAg double-bumps. As the number of thermal cycles increased, normal plastic strains in the perpendicular direction to the Si chip were accumulated on the Cu column bumps at the chip edge in the low temperature region. Thus it was found that displacement failure of the Al pad and Ti layer, the main T/C failure mode of the Cu/SnAg flip-chip assembly, occurred at the Si chip/Cu column interface by compressive normal deformation during thermal cycling. Next, the effect of Cu column height was investigated for the enhancement T/C reliability. As results of T/C test for 60 um and 85 um Cu column heights, flip chip assemblies with thicker Cu column height showed better T/C reliability. In the real time moire interferomerry, shear strain and normal strain of the x-direction was almost same regardless of Cu column height. On the other hand, the normal strain of y-direction (perpendicular direction to the Si chip) at Si chip/Cu column interface for 85 um-thick Cu samples shows significantly reduced value compared with 60 um-thick Cu samples. This relaxation of the normal plastic strain of the y-direction is the origin that thicker Cu column height guarantees better T/C reliability.

A Study of Hygrothermal Behavior of ACF Flip Chip Packages With Moiré Interferometry

A primary factor of anisotropic conductive film (ACF) package failure is delamination between the chip and the adhesive at the edge of the chip. This delamination is mainly affected by the thermal shear strain at the edge of the chip. This shear strain was measured on various electronic ACF package specimens by micro-Moire interferometry with a phase shifting method. In order to find the effect of moisture, the reliability performance of an adhesive flip-chip in the moisture environment was investigated. The failure modes were found to be interfacial delamination and bump/pad opening which may eventually lead to total loss of electrical contact. Different geometric size specimens in terms of interconnections were discussed in the context of the significance of mismatch in coefficient of moisture expansion (CME) between the adhesive and other components in the package, which induces hygroscopic swelling stress. The effect of moisture diffusion in the package and the CME mismatch were also evaluated by using the Moire interferometry. From Moire measurement results, we could also obtain the stress intensity factor K. Through an analysis of deformations induced by thermal and moisture environments, a damage model for an adhesive flip-chip package is proposed.

Estimation of Measurement Uncertainty in Evaluation of Tensile Properties

Estimation of tensile properties measurement uncertainty of material was carried out. Sources of uncertainty affecting the measurement of tensile properties were classified and analyzed. The models for uncertainty evaluation of measurands to be determined from tensile test, such as elastic modulus, yield strength and tensile strength, were suggested and derived from the mathematical relations, corresponding to the respective measurands, and the measuring quantities by calculating each sensitivity coefficient of the quantities. Based on these models, the uncertainty of the tensile properties was evaluated from the experimental data of SUS316LN determined according to ISO 6892.

High resolution AFM Moiré technique for the detection of defects in nano structure

Moire interferometry is a useful technique to assess the reliability of electronic package because Moire interferometry can measure the whole-field and real-time deformations. The optical limitation of Moire interferometry makes ambiguous the shear strain of a micro area. An atomic force microscope (AFM) is used to measure the profile of a micro site. High resolution of AFM can apply to the Moire technique. AFM Moire technique is useful to measure the strain of a small area. Also, AFM Moire technique is used to detect of defects in nano structure. In this research, the method to accurately measure the deformation of a small area by using AFM Moire is proposed. A phase-shifting method is applied to improve the resolution of AFM Moire.

In-Plane Deformation Measurement of Thin Packages Using an Atomic Force Microscope Moiré Method With a Pseudo-Phase-Shifting Technique

High-resolution microscope Moiré methods have recently been used to measure small deformations in specimens occasionally due to some restrictions on the use of optical measurement techniques. The atomic force microscope (AFM) Moiré method, a type of high-resolution microscope Moiré method, is usually adopted to measure the in-plane deformation of electronic packages due to the simple process of specimen preparation associated with this process. The sensitivity of the AFM Moiré method is determined by the frequency of the specimen and the reference grating. The latter is controlled by varying the pitch of the AFM scanning lines. Therefore, a high-frequency reference grating is easily achieved by decreasing the pitch. On the other hand, it is difficult to form a high-frequency specimen grating on the surface of a thin package. Alternatively, a phase-shifting technique can be employed with the Moiré method to enhance the sensitivity using a specimen grating with a relatively low grating frequency. However, some obstacles exist when doing this, including the hysteretic behavior that arises in the specimen stage actuator of the AFM system and areas mismatch among the phase-shifted images. In this paper, a pseudo-phase-shifting technique is proposed to overcome the obstacles of the AFM system. An image-decomposition algorithm that obtains phase-shifted images is also presented. For an application of the AFM Moiré method, in-plane deformations of a chip-on-flex package with thin thickness were investigated using the proposed technique. The effects of the specimen grating on the specimen were also evaluated through a finite element analysis.

Effects of the functional groups of nonconductive films (NCFs) on materials properties and reliability of NCF flip-chip-on-organic boards

Nonconductive films (NCFs) are one of the conducting polymer adhesives alternatives for flip-chip interconnection. NCFs containing no conductive particles have functions of adhesion, insulation, and encapsulation. The most important issue in NCFs-bonded flip-chip-on-board (FCOB) assemblies is thermal cycling reliability. Thermo-mechanical properties such as glass transition temperature (Tg), modulus (E), and thermal expansion coefficient (CTE) of cured NCFs significantly affect to the thermal cycling reliability of NCFs bonded FCOB assembly. In this paper, we have mainly focused on the improvement of thermo-mechanical properties of NCFs by controlling the number of functional groups of NCFs resin. The functionality modified NCFs-bonded FCOB assembly showed significantly enhanced reliability under thermal cycling test environment (-40 /spl deg/C /spl sim/ 150 /spl deg/C, 1000 cycles). To compare the reliability of conventional and modified NCFs-bonded FCOB assemblies after thermal cycling test electrical analysis and scanning acoustic microscopy (SAM) investigation were performed. Thermal deformations of each NCFs-bonded FCOB assembly under thermal cycling environment were also investigated and quantitatively compared using high sensitivity Twyman-Green interferometry. According to experimental results, the functional groups of NCFs have great effects on thermomechanical properties of cured NCFs, the thermal deformation, and thermal cycling reliability of NCFs-bonded FCOB assemblies.

Reliability Evaluation for Flip-Chip Electronic Packages under High Temperature and Moisture Condition using Moiré

The use of anisotropically conductive Film (ACF) for the direct interconnection of flipped silicon chips to printed circuits (flip chip packaging) offers numerous advantages such as reduced thickness, improved environmental compatibility, lowered assembly process temperature, increased metallization options, cut downed cost, and decreased equipment needs. Its wide application to glass displays through chip on glass (COG) process has accelerated interest in using this technology on rigid/flexible substrates or even on paper substrates as novel ACFs are being developed. A primary factor of the ACF package failure is delamination between the chip and the adhesive at the edge of the chip. This delamination is mainly affected by the shear strain at the edge of the chip. Therefore, the shear strain of each specimen was measured using portable engineering Moire interferometry (PEMI) experiments. PEMI experiments cannot acquire many data points of the shear strain. Accordingly, a phase-shifting method was utilized to obtain many exact data values of the shear strain. In the study of moisture effect, the reliability performance of the adhesive flip chip in the moisture environment test conditions was investigated. The failure modes were found to be interfacial delamination and bump/pad opening which may eventually lead to total loss of electrical contact. Different geometric size specimens in the interconnections were discussed in the context of the significance of mismatch in coefficient of moisture expansion (CME) between adhesive and other components in the package, which induces a hygroscopic swelling stress. The effect of moisture diffusion in the package and the CME mismatch were also evaluated by using the Moire interferometry. In this study, it is concluded that hygroscopic swelling assisted by loss of adhesion strength upon moisture absorption is responsible for the moisture-induced failures in these adhesive flip chip interconnects. Therefore, fully moisture induced ACF specimen and dried specimen are performed for calculating the interfacial fracture toughness. From Moire results, we can also get the stress intensity factor K. Through the analysis of the deformations by thermal and moisture environment, a damage model for adhesive flip chip package is proposed.

Evaluation of thermal deformation behavior in electronic package using UV moire interferometry

In recent years, moire interferometry method has been used extensively in the electronics industry to determine thermal strains caused by temperature changes in microelectronics devices. The size of a flip-chip package has getting smaller. To measure thermal deformations of these small flip-chips, instruments with much higher resolution are required. Many researchers have tried to achieve this resolution enhancement of moire interferometry by using a phase-shifting method. But the phase-shifting method has physical limitations. Fundamentally to get higher resolution, the laser source should be changed. In this article, we constructed a moire interferometry system by utilizing an ultraviolet laser (/spl lambda/=325nm). This UV system realizes higher resolution than He-Ne (/spl lambda/=633nm) system. And we apply this system to evaluate thermal deformation in flip-chip package.

Warpage Behavior and Life Prediction of a Chip-on-Flex Package Under a Thermal Cycling Condition

Flip-chip assembly has been widely adapted to various electronic devices due to advantages, such as miniaturization of electronic devices and high density integration. The chip-on-flex (COF) package used in this paper is a flip-chip package with an anisotropic conductive adhesive flim (ACF) interconnection and shows flexible features and reduced thickness compared with chip-on-board (COB) packages. All electronic packages experience temperature variation during service conditions and under environmental changes. Under temperature variation, stresses emerge due to the differences in the coefficient of thermal expansion among components. In order to evaluate the thermomechanical reliability of a COF package, a thermal cycling (TC) test was conducted. A moiré experiment using Twyman/Green interferometry was performed to observe the warpage behavior of the package under a TC condition. Through the experiment, the rate of change of chip warpage with respect to temperature (dw/dT) as a parameter of the thermal damage model was obtained. A finite element analysis (FEA) was also performed to calculate the maximum shear stress at the ACF layer as another parameter of the model. From the experiment and FEA results, the thermal damage model can accurately represent the TC life of the COF package. However, based on observations of different warpage behavior of the COF package compared with a COB package from the moiré experiment, a modified thermal damage model that can predict the TC life of both packages more accurately was proposed.

Studies on the Thermal Cycling Reliability of Fine Pitch Cu/SnAg Double-Bump Flip Chip Assemblies on Organic Substrates: Experimental Results and Numerical Analysis

A thick Cu column based double-bump flip-chip structure is one of the promising alternatives for fine pitch flip-chip applications. In this study, the thermal cycling (T/C) reliability of Cu/SnAg double-bump flip-chip assemblies was firstly investigated and the failure mechanism was analyzed through correlation of T/C test and the finite element analysis (FEA) results. After 1000 thermal cycles, the T/C failure site was the Cu column/Si chip interface, where was identified via a FEA as the location of the maximum stress concentration during thermal cycling. During thermal cycling, the Al pad and Ti layer between the Si chip and Cu column bumps were displaced due to thermo-mechanical stress. Based on the low cycle fatigue model, the accumulation of equivalent plastic strain resulted in thermal fatigue deformation of the Cu column bumps, and ultimately reduced the thermal cycling lifetime. In addition, the normal plastic strain of the y-direction, 822, was determined to be compressive and was a dominant component in relation to the plastic deformation of Cu/SnAg double-bumps. As the number of thermal cycles increased, normal plastic strains in the perpendicular direction to the Si chip were accumulated on the Cu column bumps at the chip edge in the low temperature region. Thus it was found that displacement failure of the Al pad and Ti layer, the main T/C failure mode of the Cu/SnAg flip-chip assembly, occurred at the Si chip/Cu column interface by compressive normal deformation during thermal cycling. Next, the effect of Cu column height was investigated for the enhancement T/C reliability. As results of T/C test for 60 um and 85 um Cu column heights, flip chip assemblies with thicker Cu column height showed better T/C reliability. In the real time moire interferomerry, shear strain and normal strain of the x-direction was almost same regardless of Cu column height. On the other hand, the normal strain of y-direction (perpendicular direction to the Si chip) at Si chip/Cu column interface for 85 um-thick Cu samples shows significantly reduced value compared with 60 um-thick Cu samples. This relaxation of the normal plastic strain of the y-direction is the origin that thicker Cu column height guarantees better T/C reliability.