Chris Chang

Effects of underfill material properties on the reliability of solder bumped flip chip on board with imperfect underfill encapsulants

Three different types of underfill imperfections were considered; i.e., (1) interfacial delamination between the underfill encapsulant and the solder mask on the PCB (crack initiated at the tip of underfill fillet), (2) interfacial delamination between the chip and the underfill encapsulant (crack initiated at the chip corner), and (3) the same as (2) but without the underfill fillet. Five different combinations of coefficient of thermal expansion (CTE) and Youngs modulus with the aforementioned delaminations were investigated. A fracture mechanics approach was employed for computational analysis. The strain energy release rate at the crack tip and the maximum accumulated equivalent plastic strain in the solder bumps of all cases were evaluated as indices of reliability. Besides, mechanical shear tests were performed to characterize the shear strength at the underfill-solder mask interface and the underfill-chip passivation interface. The main objective of the present study is to achieve a better understanding in the thermo-mechanical behavior of flip chip on board (FCOB) assemblies with imperfect underfill encapsulants.

An overview of microvia technology

There are many advantages of microvia: it requires a much smaller pad, which saves the board size and weight; with microvia, more chips can be placed in less space or a smaller PCB, which results in a low cost; and with microvia, electrical performance improves due to a shorter pathway. Basically, there are five major processes for microvia formation: NC drilling; laser via fabrication including CO2 laser, YAG laser, and excimer; photo‐defined vias, wet or dry; etch via fabrications including chemical (wet) etching and plasma (dry) etching; and conductive ink formed vias, wet or dry. This paper will discuss the materials and processes of these five major microvia formation methods. At the end, eight key manufacturers from Japan will be briefly illustrated for their research status and current capability of producing smallest microvia.

Solder joint crack propagation analysis of wafer-level chip scale package on printed circuit board assemblies

The solder-joint reliability of a low-cost wafer-level chip scale package (WLCSP) on printed circuit board (PCB) under thermal fatigue is studied. The solder joints are subjected to thermal cycling and their crack lengths at different thermal cycles are measured. Also, the stress intensity factors at the crack tip of different crack lengths in the corner solder joint are determined by fracture mechanics with finite element method. Furthermore, an empirical equation for predicting the thermal-fatigue life of flip chip solder joints is proposed.

Effects of underfill encapsulant on the mechanical and electrical performance of a functional flip chip device

The curing conditions and material properties such as the coefficient of thermal expansion, glass transition temperature, Youngs modulus, and moisture content of four different underfill encapsulants with different size and content of filler and epoxy are measured. The effects of these underfills on the flow rate, mechanical performance, and electrical performance of a solder-bumped functional flip chip on an organic substrate are studied.

TMA, DMA, DSC AND TGA OF LEAD FREE SOLDERS

Most of the electronics packaging materials, especially solders, are temperature dependent. Their temperature‐dependent material properties can be obtained by TMA (thermal mechanical analysis), DMA (dynamic mechanical analysis), DSC (differential scanning calorimetry), and TGA (thermogravimetric analysis). In this study, the thermal coefficient of expansion (TCE), storage modulus, moisture uptake, and melting point of two lead free solders, 96.5wt%Sn‐3.5wt%Ag and 42wt%Sn‐58wt%Bi provided from two different vendors, are measured by, respectively, TMA, DMA, TGA, and DSC. For comparison purpose, the 63wt%Sn‐37wt%Pb solder is also considered.

Identification of individuals by trait prediction using whole-genome sequencing data

Significance By associating deidentified genomic data with phenotypic measurements of the contributor, this work challenges current conceptions of genomic privacy. It has significant ethical and legal implications on personal privacy, the adequacy of informed consent, the viability and value of deidentification of data, the potential for police profiling, and more. We invite commentary and deliberation on the implications of these findings for research in genomics, investigatory practices, and the broader legal and ethical implications for society. Although some scholars and commentators have addressed the implications of DNA phenotyping, this work suggests that a deeper analysis is warranted. Prediction of human physical traits and demographic information from genomic data challenges privacy and data deidentification in personalized medicine. To explore the current capabilities of phenotype-based genomic identification, we applied whole-genome sequencing, detailed phenotyping, and statistical modeling to predict biometric traits in a cohort of 1,061 participants of diverse ancestry. Individually, for a large fraction of the traits, their predictive accuracy beyond ancestry and demographic information is limited. However, we have developed a maximum entropy algorithm that integrates multiple predictions to determine which genomic samples and phenotype measurements originate from the same person. Using this algorithm, we have reidentified an average of >8 of 10 held-out individuals in an ethnically mixed cohort and an average of 5 of either 10 African Americans or 10 Europeans. This work challenges current conceptions of personal privacy and may have far-reaching ethical and legal implications.

Characterization of underfill materials for functional solder bumped flip chips on board applications

The curing conditions and material properties such as the TCE (thermal coefficient of expansion), T/sub g/ (glass transition temperature), flexural storage modulus, tangent delta, and moisture content of nine different underfill materials from three different vendors are measured. Their flow rate and the effect of moisture content on mechanical (shear) strength in solder bumped flip chips on organic substrate are also determined experimentally. Furthermore, their effects on the electrical performance (voltage) of functional flip chip devices on organic substrate are measured. Finally, a simple methodology is presented for the selection of underfills from the measurement results of these nine different underfill materials.

SMT COMPATIBLE NO-FLOW UNDERFILL FOR SOLDER BUMPED FLIP CHIP ON LOW-COST SUBSTRATES

The design, materials, process, reliability, and applications of no-flow underfills for solder bumped flip chip on low-cost substrates are presented in this study. Two different no-flow underfills are considered. One is epoxy-based no-clean flux (liquid-like) material and the other is epoxy-based non-conductive (film-like) material.

Real-time popcorn analysis of plastic ball grid array packages during solder reflow

The time-history deformation of plastic ball grid array (PBGA) packages during solder reflow on printed circuit boards (PCB) is measured by the electrical-resistance strain gauge method. Two groups of experiments are performed, one after 24 hours at 125/spl deg/C (to bake out the moisture from the package) and the other after 24 hours at 125/spl deg/C, then 85/spl deg/C/85RH for 168 hours (to impose a controlled moisture content into the package). The results show that for the PBGAs with moisture content, popcorning occurs not at the peak solder-reflow temperature, but at an earlier stage.

Failure analysis of solder bumped flip chip on low-cost substrates

Failure analyses of 63wt%Sn-37wt%Pb solder bumped flip chip assemblies with underfill encapsulant are presented in this study. Emphasis is placed on solder flowed-out, nonuniform underfill and voids, and delaminations. X-ray, scanning acoustic microscope, and tomographic acoustic micro-imaging techniques are used to analyze the failure samples. Also, cross sections are examined for a better understanding of the failure mechanisms. Furthermore, nonlinear, temperature-dependent, and fracture-mechanics finite element analyses are used to determine the effects of underfill-void sizes on the flip chip solder joint reliability.