Soon Bok Lee

A Comparative Study of The Fatigue Behavior of SnAgCu and SnPb Solder Joints

Fatigue behaviors of 63Sn37Pb and two types of lead-free solder joints were compared using pseudo-power cycling testing method, which provides more realistic load cycling to solder joints than chamber cycling method does. Pseudo-power cycling test was performed in various temperature ranges to evaluate the shear strain effect. A nonlinear finite element model was used to simulate the thermally induced visco-plastic deformation of solder joint in BGA packages. The results revealed that lead free solder joints have a good fatigue property in the low temperature condition, where a small strain was induced. In the high temperature condition where a large strain was induced, however, lead contained solder joints have a longer fatigue life.

Developing phase-shifting micro moire interferometry using phase shifter with rough resolution and by shifting specimen grating

In this research, phase shifting method is used to modify PEMI (portable engineering moire interferometer) into micro moire interferometer which can measure displacement field with highly improved sensitivity. Apart from existing micro moire technique, a low cost and less precise translation stage with rough resolution (10 /spl mu/m resolution) is adapted for the phase shifter. Moreover, specimen grating instead of reference grating is phase shifted and again the cost is outstandingly reduced. Least square algorithm and other image processing schemes, such as FFT filtering and fringe enhancement technique, are applied to minimize the errors induced by lowering the cost. To verify the newly constructed micro moire technique, displacement fields of flip chip package with ACA interconnection and wafer level chip size package were measured. In case of flip chip package, microscopic local displacement fields such as ACA layer and underfilled area were obtained with increased sensitivity. For chip size package, displacement field of a solder ball with 400 /spl mu/m diameter was acquired with predominantly elevated sensitivity of 26 nm per fringe.

Measurement of Mechanical Properties of Electroplated Nickel Thin Film

Electroplated nickel manufactured via the LIGA process, offers the possibility of stronger structure and connectors in a micro electro mechanical systems (MEMS). In this study, the mechanical properties of electroplated Nickel thin film were characterized using two methods; tension test and nano-indentation test. In tension test, a linear guided motor was used as actuator and the applied force was measured using a load cell. Strain was measured with a dual microscope that obtains the displacement of two separated zone by the tracking process of the image captured with CCD camera. In indentation test, elastic modulus was measured using a CSM(continuous stiffness measurement) module. Two types of specimen were prepared in the same wafer and tested after four months of aging, which reduces the variation of properties caused by fabrication condition and aging effect. The tension specimen is 15 µm thick and 300 µm wide. The indentation specimen is also 15 µm thick. Youngs modulus were measured by two different testing methods and compared quantitatively.

Chip Warpage Damage Model for ACA Film Type Electronic Packages

The use of anisotropically conductive adhesives (ACA) 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. Despite numerous benefits, ACA film type packages bare several reliability problems. The most critical issue among them is their electrical performance deterioration upon consecutive thermal cycles attributed to gradual delamination growth through chip and adhesive film interface induced by CTE mismatch driven shear and peel stresses. In this study, warpage of the chip is monitored by real time moiré interferometer during –50oC to +125oC temperature range. Moreover, reduction in chip warpage due to increase in delamination length is obtained as in function of thermal fatigue cycles. Finally, a new model to predict damage level of ACA package and remained life is proposed and developed.

Low Cycle Fatigue Behavior of 429EM Ferritic Stainless Steel at Elevated Temperatures

In recent, ferritic stainless steels are widely used in high temperature structure because of their high resistance in thermal fatigue and low prices. Tensile and low cycle fatigue(LCF) tests on 429EM stainless steel were performed at several temperatures from room temperature to 600°C. Elastic modulus, yield stress and ultimate tensile strength(UTS) decreased with increasing temperature. Considerable cyclic hardening occurred at 200°C and 400°C. 475°C embrittlement observed could not explain this phenomenon but dynamic strain aging(DSA) observed from 200°C to 500°C could explain the hardening mechanism at 200°C and 400°C. And it was observed that plastic strain energy density(PSED) was useful to predict fatigue life when large cyclic hardening occurred. Fatigue life using PSED over elastic modulus could be well predicted within 2X scatter band at various temperatures.

Experimental Investigation on the Proper Fatigue Parameter of Cyclically Non-Stabilized Materials

Low-cycle fatigue tests were carried out in air in a wide temperature range from room temperature to 650oC to investigate the role of temperature on the low-cycle fatigue behavior of two types of stainless steels, cold-worked (CW) 316L austenitic stainless steel and 429 EM ferritic stainless steel. CW 316L stainless steel underwent additional hardening at room temperature and in 250-600oC: plasticity-induced martensite transformation at room temperature and dynamic strain aging in 250-600oC. As for 429 EM stainless steel, it underwent remarkable hardening in 200-400oC due to dynamic strain aging, resulting in a continuous increase in cyclic peak stress until failure. Three fatigue parameters, such as stress amplitude, plastic strain amplitude and plastic strain energy density, were evaluated. The results revealed that plastic strain energy density is nearly invariant through a whole life and, thus, recommended as a proper fatigue parameter for cyclically non-stabilized materials.

Thermal Stress and Fatigue Analysis of Exhaust Manifold

In this study, we investigated the reliability assessment of exhaust manifold used in thermomechanical condition. Overlay model proposed by Besseling[1] was modified to consider the strain range dependence on elastic limit. By combining geometrical relation in hysteresis loop and temperature dependence of elastic limit with isothermal overlay model, temperature dependent cyclic plasticity model was proposed. Continuous damage model based on isothermal fatigue data was generalized for non-isothermal condition. Finite element analysis and life prediction of exhaust manifold were performed under severe operating conditions.

Definition of Damage Parameter in Low-Cycle Fatigue of Gray Cast Iron

Gray cast iron shows large asymmetrical features by the graphite flake when tensile and compressive stresses are applied. The plastic strain rage which is used in low-cycle fatigue life prediction by many researchers is hardly defined and gives very different values by the Standards in this case. From the results of this study, it is not reliable to use the plastic strain range as a low-cycle fatigue damage parameter. Therefore, the plastic strain energy density which is uniquely defined was suggested as a damage parameter and it showed good correlation in low-cycle fatigue in gray cast iron.

Effect of High Glass Transition Temperature on Reliability of Non-Conductive Film (NCF)

Among many factors that influence the reliability of a flip-chip assembly using NCF interconnections, the most effective parameters are often the coefficient of thermal expansion (CTE), the modulus (E), and the glass transition temperatures (Tg). Of these factors, the effect of Tg on thermal deformation and device reliability is significant; however, it has not been shown clearly what effect Tg has on the reliability of NCF. The Tg of a conventional NCF material is approximately 110°C. In this study, a new high Tg NCF material that has a 140oC Tg is proposed. The thermal behaviors of the conventional and new NCFs between -40oC to 150oC are observed using an optical method. Twyman-Green interferometry and the moiré interferometry method are used to measure the thermal micro-deformations. The Twyman-Green interferometry measurement technique is applied to verify the stress-free state. The stress-free temperatures of the conventional and new Tg NCF materials are approximately 100oC and 120oC respectively. A shear strain at a part of the NCF chip edge is measured by moiré interferometry. Additionally, a method to accurately measure the residual warpage and shear strain at room temperature is proposed. Through the analysis of the relationship between the warpage and the shear strain, the effect of the high-Tg NCF material on the reliability is studied.

Thermomechanical fatigue behavior of the ferritic stainless steel

A thermomechanical fatigue (TMF) life prediction model for ferritic stainless steel, used in exhaust manifold of automobile, was developed based on Tomkins’ two-dimensional crack propagation model. Low-cycle fatigue (LCF) and TMF tests were carried out in a wide temperature range from 200 to 650°C. New concept of plastic strain range on TMF was proposed. Effective stress concept was introduced to get a reasonable stress range in TMF hysteresis loop. The proposed model predicted TMF life within 2X scatter band. The experimental results reveal that TMF life is about 10% of isothermal fatigue life.