Noriyasu Kawamura

Simplified and practical estimation of package cracking during reflow soldering process

A simple method has been developed to estimate cracking of plastic packages during reflow soldering process. A criterion for estimating package cracking has been introduced by comparing the mold resin fracture toughness and the maximum value of the stress intensity factor in the package. The criterion equation for package cracking was made to an empirical form. The real-time measurement of package deformation is effective for providing information on the difference of the fracture mode. The difference of the delamination in the package causes the different shape of the deformation curve. The absorption characteristic of the package obeys Ficks law even for thinner packages.<<ETX>>

Mechanical fatigue test method for chip/underfill delamination in flip-chip packages

Underfill resin between Si chips and printed circuit boards is useful for improving the reliability of flip-chip packages. Generally, thermal cycle tests (TCTs) are applied to electronic packages under development in order to prove their reliability. At the early stage of development, however, a more effective test method is desirable, because TCTs are time-consuming. A new mechanical fatigue test for the underfill resin in flip-chip packages, namely the four points support test method, is proposed in this paper. The validity of the mechanical test method could be verified from the results of stress analyses and experiments. Considering the chip/underfill delamination statistically based on the assumption of Markov process, it was shown that the delamination probability during cyclic loads could be estimated with equations of the displacement range and number of cycles.

Adhesion integrity evaluation of plastic encapsulated semiconductor package

Plastic encapsulated semiconductor packages may crack if interval delamination occurs during the reflow soldering process, and adhesion integrity evaluation of plastic packages is becoming increasingly important. A new method of estimating the adhesion integrity between epoxy molding resin and silicon chips for plastic packages is proposed. First, three-dimensional stress analysis was carried out to obtain stress distributions in quad flat packages (QFPs) during the reflow soldering process. Next, resin adhesion strength tests were carried out. The shearing load acting on a resin cube molded on a silicon chip was measured, and the maximum shearing load at breakage was obtained. Finally, it was proposed that the stress distributions near the adhesion resin edge of the resin in the test specimen at breakage be used as the criterion for adhesion integrity evaluation of the packages. The adhesion integrity for some QFPs was predicted by comparing the analytical stress distribution in the package and the criterion calculated from the results of the adhesion strength test. The results of reliability tests on the package showed that the new criterion allowed effective evaluation.

Development of moisture-proof thin and large QFP with copper lead frame

A novel thin and large Quad Flat Package (QFP) with copper lead frame was developed having a dimension of 28 mm-squared body size and 1.4 mm package thickness. The package accommodated as large as a 12 mm-squared die and, without dry pack, could accomplish the JEDEC LEVEL 1 condition in terms of volume production assurance. Along with the package development, a new simulation methodology was established which could predict the occurrence of delamination and cracking and be applied to similar package designs. In the course of the current study, we initially investigated the dependence of adhesion strength of Epoxy Molding Compound (EMC) on the copper film oxide thickness and, applying this result, obtained the upper limit of the oxide thickness below which sufficient adhesion strength was ensured. Upon this underlying relationship, for the sake of the oxide growth suppression under the threshold, we developed a new packaging process controlling such parameters as heating temperature, heating time and oxygen concentration of an ambient. Since the contact area between EMC and die pad has the smallest adhesion strength among all interfaces inside the package, we adopted split or window die pad designs aiming to hold the area as small as possible. The removal of organic contaminants from the rear side of die by virtue of Ultra-Violet (UV) irradiation enhanced the adhesion strength between EMC and the surface. Also the selections of appropriate EMC and die attach material were conducted from several candidate ones.

Mixed-mode fracture behavior of silica particulate filled epoxide resin

In the present paper, fracture behavior of the epoxide resin, containing 70 wt% silica particles, have been studied at room temperature and high temperature, that is, 250 C. The three-point and four-point bending tests were performed to obtain Mode I and mixed-mode fracture toughness, respectively. The specimen fractured in a brittle manner under room temperature conditions, and the results obeyed the maximum hoop stress criterion. On the other hand, the results at high temperature showed a relatively low dependency on Mode II stress intensity factors. Fracture surface were observed in a scanning laser microscope. Finite element analysis, taking into account the microstructure of the material, have been conducted in order to investigate the influence of temperature on mixed-mode fracture behavior. Local fracture criteria were considered for the silica particles and the epoxide resin. The material constants in criteria were estimated from the bending strength and the Mode I fracture toughness of the material. Good agreement between experimental and numerical results was obtained for the fracture boundary curves on the Mode I and Mode II stress intensity diagram.

A synthetic criterion for level-1 crack-free package-proposal of a superior package structure

A synthetic criterion to provide guidance for the high-reliability structure design for the level-1 crack-free package is introduced. The criterion reflects a consideration of both delamination and package cracking. According to the criterion, to prevent delamination is the most effective means to realize the level-1 crack-free package for large chips (ex. 15 mm square). Therefore, a new package structure, a chip side support (CSS) structure which eliminates a die-pad is proposed and reliability tests are carried out. The CSS package realizes the level-1 crack-free package without moisture-proof packing.

A highly reliable press packed IGBT

A newly developed press packed reverse conducting IGBT (RCIGBT), the ST1000EX21, having ratings of 2500 V and 1000 A, has successfully been introduced in high-reliability application areas. A multiple-chip press packed RCIGBT structure, containing IGBT chips and fast recovery diode (FRD) chips, has been achieved by using basic experimental results and a stress analysis using the finite element structure analysis program ABAQUS. Excellent electrical characteristics, especially a robust turn-off capability, such as Ic = 5000 A, Vcp = 2800 V at Tj = 125 °C, have been obtained. High reliability, withstanding a thermal cycling (fatigue) test of more than 50,000 cycles and a high-temperature voltage blocking test for 2000 hours, has been confirmed. The device is now available and is being successfully used for transportation systems and other applications that require high reliability and long-term stability. Voltage and current ratings in IGBTs and IEGTs (Injection Enhanced Gate Transistors) will be raised in the future.

TESTING METHOD FOR MEASURING IMPACT STRENGTH OF BGA SOLDER JOINTS ON ELECTRONIC PACKAGE

A pendulum-impact testing machine was developed to measure the impact strength of ball-grid-array (BGA) solder joints between an electronic package and a circuit board. Ball solders were connected to daisy-chain between a dummy electronic package and a circuit board. The upper side of the package was directly bonded to a load cell. The rear side of the circuit board was also bonded to an aluminum alloy block fixed on a base. A pendulum made of aluminum alloy was collided into the load cell to apply tensile impact to the solder joints through the load cell. The history of the impact load could be controlled by raising the angle of the pendulum. The fracture of a BGA solder joint was detected by measuring the resistance of the daisy-chain circuit on the board. Therefore, the impact strengths of the solder joints at electrical disconnection and mechanical breaking of all joints could be determined. The experimental results showed that this method is useful for measuring the impact strength of BGA solder joints.

Evaluation of Drop Impact Load for Thin Cone-Shaped Structures

A method to predict impact load caused by drop impact of thin cone-shaped structures is investigated. The Hertzian contact theory and the spherical shell theory are applied for modeling the contact stiffness of the impact tip of the cone. Experiments and finite element simulations are performed for several cones to evaluate the accuracy of impact load prediction. It is shown that appropriate choice of contact stiffness model depending on the curvature and thickness of impact tip of the cone is necessary in order to predict the impact load accurately.

Multidisciplinary Reliability Design of Electronics Packaging

In the electronics packaging design, the growing complexities in conjunction with decrease in design margins are the underlying elements driving the need for a new reliability design method applicable to trade-off problems. In this work, multidisciplinary reliability analysis based on a probabilistic and multivariate statistical method has been performed for CPU module packaging in order to reveal the reliability relationship among packaging solutions and to identify the statistical trade-off mechanism among multi-objectives. It was verified that the applied method involving RSM (Response Surface Modeling) / FORM (First Order Reliability Method) / SEM (Structural Equation Modeling) can assess the packaging solutions with respect to reliability indices and failure mode correlation, and grasp the trade-off mechanism among multi-failure modes in packaging design space.