Light emitting diodes reliability review

Pb-free solders have replaced Pb-containing SnPb solders in the electronic packaging industry due to environmental concerns. Both electromigration (EM) and thermomigration (TM) have serious reliability issues for fine-pitch Pb-free solder bumps in the flip-chip technology used in consumer electronic products. We review the unique features of EM and TM in flip-chip solder bumps, emphasizing the effects of current crowding and Joule heating. In addition, the challenges to a better understanding of EM and TM in Pb-free solders are discussed. For example, the anisotropic nature of Sn microstructure in Pb-free solders can enhance the dissolution rates of Ni and Cu in solders driven by EM and TM.

https://ir.nctu.edu.tw:443/bitstream/11536/6094/1/000280818700021.pdf

Electromigration and Thermomigration in Pb-Free Flip-Chip Solder Joints

We developed a nanocomposite with highly aligned graphite platelets in a copper matrix. Spark plasma sintering ensured an excellent copper–graphite interface for transmitting heat and stress. The resulting composite has superior thermal conductivity (500 W m–1 K–1, 140% of copper), which is in excellent agreement with modeling based on the effective medium approximation. The thermal expansion perpendicular to the graphite platelets drops dramatically from ∼20 ppm K–1 for graphite and copper separately to 2 ppm K–1 for the combined structure. We show that this originates from the layered, highly anisotropic structure of graphite combined with residual stress under ambient conditions, that is, strain-engineering of the thermal expansion. Combining excellent thermal conductivity with ultralow thermal expansion results in ideal materials for heat sinks and other devices for thermal management.

The Origin of High Thermal Conductivity and Ultralow Thermal Expansion in Copper-Graphite Composites.

Solid-state memory is an essential component of the digital age. With advancements in healthcare technology and the Internet of Things (IoT), the demand for ultra-dense, ultra-low-power memory is increasing. In this review, we present a comprehensive perspective on the most notable approaches to the fabrication of physically flexible memory devices. With the future goal of replacing traditional mechanical hard disks with solid-state storage devices, a fully flexible electronic system will need two basic devices: transistors and nonvolatile memory. Transistors are used for logic operations and gating memory arrays, while nonvolatile memory (NVM) devices are required for storing information in the main memory and cache storage. Since the highest density of transistors and storage structures is manifested in memories, the focus of this review is flexible NVM. Flexible NVM components are discussed in terms of their functionality, performance metrics, and reliability aspects, all of which are critical components for NVM technology to be part of mainstream consumer electronics, IoT, and advanced healthcare devices. Finally, flexible NVMs are benchmarked and future prospects are provided.

/pdf/review-on-physically-flexible-nonvolatile-memory-for-43wykav8d2.pdf

Review on Physically Flexible Nonvolatile Memory for Internet of Everything Electronics

As the most common of the intermetallic compounds (IMCs) formed between Sn-based solders and Cu substrates during the packaging of integrated circuits (ICs), Cu 6 Sn 5 is frequently involved in the fabrication of solder joints and plays an important role in the integrity of electronic devices This is especially true for recently developed micro-bumps in 3-dimensional (3D) high-density integrated circuits (ICs), in which the volume fraction of Cu 6 Sn 5 is significantly higher than in conventional ball grid array (BGA) or through hole pin (THP) arrangements Recently, with the use of advanced characterization techniques, significant progress has been made in the understanding of Cu 6 Sn 5 intermetallics in terms of their crystal structure, solidification behaviour, role in interface reactions, thermal expansion and mechanical properties This improved understanding is of fundamental importance for the production of next generation electronic devices, however there is no existing comprehensive summary of this research available Here, we provide a review on the properties of Cu 6 Sn 5 with a focus on: (1) identification of crystal structure and possible phase transformations of Cu 6 Sn 5 in real solder joints; (2) formation of Cu 6 Sn 5 during solidification of commonly used Pb-free alloys and its influence on the final microstructure; (3) the formation and growth texture of interfacial Cu 6 Sn 5 ; (4) thermal expansion and mechanical properties of Cu 6 Sn 5 and the relationship between crystal structure and temperature The effects of selected alloying elements that have remarkable influences on the above properties are also discussed The aim of this paper is to identify the key factors that affect the properties of this important IMC and the relationship between these properties and the integrity of solder joints under various conditions

/pdf/critical-properties-of-cu6sn5-in-electronic-devices-recent-549hfccl1c.pdf

Critical properties of Cu6Sn5 in electronic devices: Recent progress and a review

Fracture strength characterization and failure analysis of silicon dies

A study in flip-chip UBM/bump reliability with effects of SnPb solder composition

This paper aims to investigate electromigration phenomena of under-bump-metallization (UBM) and solder bumps of a flip-chip package under a high temperature operation life test (HTOL). UBM is a thin film Al/Ni(V)/Cu metal stack of 1.5 /spl mu/m, while the bump material consists of Sn/37Pb, Sn/90Pb, and Sn/95Pb solders. Current densities of 2,500 and 5,000 A/cm/sup 2/ and ambient temperatures of 150 to 160/spl deg/C are applied to study their impact on electromigration. It is observed that bump temperature has a more significant influence than current density on bump failures. Owing to its higher melting point characteristics and lower Sn content, high-lead bumps are observed to have an 8 to 12-fold improvement in Mean-Time-To-Failure (MTTF) compared with eutectic Sn/37Pb. Individual bump resistance history is used to evaluate UBM/bump degradation. The measured resistance increase is from bumps with electrical current flowing upward into the UBM/bump interface (cathode), while bumps with opposite current polarity cause only minor resistance change. The identified failure sites and modes reveal structural damage at the region of UBM and UBM/bump interfaces in the form of solder cracking or delamination. The effects of current polarity and crowding are key factors in the observed electromigration behavior of flip-chip packages.

J.D Wu

Papers

Fracture strength characterization and failure analysis of silicon dies

A study in flip-chip UBM/bump reliability with effects of SnPb solder composition

A study in flip-chip UBM/bump reliability with effects of SnPb solder composition

Board level reliability of PBGA using flex substrate

Board level reliability of a stacked CSP subjected to cyclic bending