S. W. Ricky Lee

Impact of Ni Concentration on the Intermetallic Compound Formation and Brittle Fracture Strength of Sn-Cu-Ni (SCN) Lead-free Solder Joints

Abstract Cu 6 Sn 5 and Cu 3 Sn are common intermetallic compounds (IMCs) found in Sn–Ag–Cu (SAC) lead-free solder joints with OSP pad finish. People typically attributed the brittle failure to excessive growth of IMCs at the interface between the solder joint and the copper pad. However, the respective role of Cu 6 Sn 5 and Cu 3 Sn played in the interfacial fracture still remains unclear. In the present study, various amounts of Ni were doped in the Sn–Cu based solder. The different effects of Ni concentration on the growth rate of (Cu, Ni) 6 Sn 5 /Cu 6 Sn 5 and Cu 3 Sn were characterized and compared. The results of characterization were used to evaluate different growth rates of (Cu, Ni) 6 Sn 5 and Cu 3 Sn under thermal aging. The thicknesses of (Cu, Ni) 6 Sn 5 /Cu 6 Sn 5 and Cu 3 Sn after different thermal aging periods were measured. High speed ball pull/shear tests were also performed. The correlation between interfacial fracture strength and IMC layer thicknesses was established.

Process development and prototyping for the assembly of LED arrays on flexible printed circuit tape for general solid state lighting

The objective of the present study is to develop a robust high-throughput assembly process of light emitting diode arrays for wide area general lighting. Topics to be covered in this paper include the conceptual design, the flexible printed circuit substrate, flip chip assembly, yellow phosphor coating, and encapsulation. All these are integrated into one reel-to-reel assembly process. A prototype with the proposed process has been developed. A sample strip of packaged LED array has been produced as well to validate the developed assembly process.

Enhancement of thermal conductivity of die attach adhesives (DAAs) using nanomaterials for high brightness light-emitting diode (HBLED)

This research investigates novel nanomaterials filled conductive adhesives for application in High Brightness LEDs packaging. Low cost, green, and large scale synthesis as well as functionalization methods were developed to achieve various types of nano-fillers, including silver nanorod, nickel nanochain and nanosized silver particles. These nano-fillers can be used together with the conventional larger silver fillers as “nano-additives”, which can fill the gaps and form thermal and electrical “shortcuts” among the conventional large particles to create a continuous, direct, and multichannel heat dissipation pathway. As a result, the thermal conductivity of the whole DAA system was increased significantly. The morphologies, compositions, and interaction between the nanomaterials and the polymers were carefully studied using a series of techniques including Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and X-Ray Diffraction (XRD) etc. The thermal conductivity of the developed DAAs was analyzed by thermal conductivity measurement system.

Effects of corner and edgebond epoxy adhesives on board level solder joint reliability of BGA mezzanine connectors

In this paper, the button shear tests of seven kinds of epoxy used for corner/edge bonding of BGA mezzanine connectors are performed firstly for determining adhesive strength between the epoxies and each relevant surface material. The effect of corner/edge bonding epoxy on the board level solder joint reliability of BGA connectors is also investigated including mechanical drop and 4-point bending tests. In addition, the pre-conditioning effect of thermal aging (1000 hours at 125°C) and accelerated temperature cycling (ATC, −40~125°C, 500 cycles) on the reliability of connectors with corner/edge bonding epoxy is discussed. The failure modes and fracture strengths of the button shear and boardlevel tests are cross-referenced for comparison and assessed for correlation. The results from the present study not only contribute to the characterization and selection of corner/edge bonding epoxies for BGA connectors, but also improve the understanding of the corresponding relationship between simple button shear and the more complex and costly boardlevel reliability tests.

Material characterization of corner and edgebond epoxy adhesives for the improvement of board-level solder joint reliability

In this paper, the material characteristics of six epoxies used for corner/edge-bonding are analyzed and compared to board-level mechanical reliability test. An innovative material characterization approach, button shear testing, provided an expedient method of determining adhesive strength between the epoxies and each relevant surface material. The board-level test included mechanical shock test. A number of SMT test devices are evaluated, including 1849 CBGA packages, and 190 ball count mezzanine connectors. The failure modes and fracture strengths of the material characterization tests and board-level tests are cross-referenced for comparison, and assessed for correlation. The results from the present study not only contribute to the characterization and selection methods of corner/edge-bonding epoxies, but also improve understanding of the corresponding relationship between material characterization testing and the more complex and costly board-level reliability tests.

Experimental investigation of the failure mechanism of Cu–Sn intermetallic compounds in SAC solder joints

Abstract A detailed experimental study on the fracture mechanism of Cu–Sn intermetallic compounds (IMCs) in the Pb-free solder was presented in this paper. The growth behaviors of the Cu 6 Sn 5 and Cu 3 Sn IMCs were inspected and the respective evolution pattern of their microstructures was investigated. Then, a detailed fractographic analysis on brittle fractured solder joints was conducted after the high speed ball pull test. The fracture locations in the Cu–Sn IMC layers during different periods of aging process were identified. The fracture modes of Cu 6 Sn 5 and Cu 3 Sn were determined as well. Afterwards, the fracture energies of different Cu–Sn IMC materials were directly compared using the Charpy impact test with a specially designed specimen. It was found that the grain boundary of Cu 3 Sn is the weakest link in the Cu–Sn IMC system. Finally, based on these three parts of study, a mechanism to explain the thermal degradation of Cu–Sn IMCs was proposed.

Investigation on the properties and processability of polymeric insulation layers for through silicon via

3D packaging using through silicon via (TSV) technology is becoming important in IC packaging industry. In this paper, the TSV process uses a polymeric liner as insulation material and a buffer for thermo-mechanical stress relaxation. Fourier transform infrared spectroscopy (FTIR), thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), dielectric and contact angle tests are applied to select a suitable dielectric from two kinds of polymers. All the properties show that the linear o-crosel phenolic (LOPF) is suitable for acting as an insulation liner in the TSV process. Then the LOPF liquid is spun on the wafer, followed by soft baking at the temperature of 115 °C, the processed wafer is inspected using optical microscope, step profiler and scanning electron microscope (SEM). All the results indicate that LOPF has good potential to be the insulation layers for TSV.

Investigation of lead-free BGA solder joint reliability under 4-point bending using PWB strain-rate analysis

Board level solder joint reliability was evaluated using a 4-point monotonic bending test on different BGA package assemblies, including a 51×51 mm 2400 CBGA and a 40×40 mm 1517 PBGA. The edgebond epoxy was applied between the package samples and solder-attached printed wiring boards (PWBs). During the bending test, the PWB strain, bending force and daisy-chain resistance were monitored in real time. The daisy-chain resistance and PWB strain-rate during the bending test were cross-referenced for detailed comparison. The test results indicated that the PWB strain-rate showed an obvious change at the moment of package failure. The PWB strain-rate can be a useful monitoring method for detecting the onset of the solder joint failure during the monotonic bending test.

Reliability study of surface mount printed circuit board assemblies with lead‐free solder joints

Purpose – The purpose of this paper is to evaluate the lead‐free solder joint reliability of a variety of surface mount components assembled onto printed circuit boards (PCBs) under a number of different tests.Design/methodology/approach – Lead‐free solder with a composition of Sn96.5‐Ag3.0‐Cu0.5 was used in a surface mount reflow process. Different types of surface mount dummy components with a daisy chain, such as CBGAs, BGAs, PLCCs, CSPs, and QFNs, were assembled onto PCBs. Both the mechanical and thermo‐mechanical reliability of the solder joints were evaluated by several tests. The experiments included package shear, package pull, three‐point bending and accelerated thermal cycling testing for 2,000 cycles. The packages were examined by X‐ray and C‐SAM before the reliability tests were carried out. The maximum load and the corresponding load‐displacement curve were recorded in the mechanical test.Findings – The results from the mechanical tests show the major failure mode is on the copper pad. Weibul...

Through silicon underfill dispensing for 3D die/interposer stacking

This study describes a through-silicon-underfill dispensing that the encapsulant is dispensed through through-silicon-vias (TSVs). The TSVs function as entrances for encapsulant dispensing or paths for fluid flow. Typically, the inflow for TSV dispensing may be flow with a constant speed or free droplets. A model was developed to investigate the filling time and the pressure distribution for the quasi-steady, radial and laminar flow between parallel plates. Compared with free droplets, a constant inflow has shorter filling time at the expense of increasing the fluid pressure. 3D stacking with the same-size interposers forms several planar sidewalls. Encapsulant may flow out from the edges of the sidewalls and form an edge flood failure if the edge flow of an underfill can overcome the surface force. An optimized pattern of TSVs was designed for the underfill of a 3D package to identify the trade-off between the filling efficiency and the lower risk of the edge flood. In each interposer, the TSVs are classified into two groups: the central group is dispensed by a constant inflow whereas the outer group is dispensed by free droplets; the inflow of free droplets eliminates the risk of an edge flood. A four-stack 3D package with the optimized TSV pattern was developed for validation. Edge dispensing was used to fill the gaps at the bottom levels as much as possible. Subsequently, the remaining gaps were encapsulated by TSV dispensing (with both a constant inflow and an inflow of free droplets). Inspections by scanning acoustic microscopy and cross-sectioning verified that the combined underfill could result in a void-free encapsulation and suitable fillets.