Zhaonian Cheng

A Study of CFD Simulation for On-chip Cooling with 2D CNT Micro-fin Array

The 2D CFD simulations have been carried out for a series of CNT micro-fin cooling architectures based on two dimensional fin array models in this paper. The influence of micro-fin structures, fluid speeds, heating powers and effective thermal conductivity on cooling effects have been obtained in the case studies, and the 2D fin array simulation results have been compared with our previous 1D fin array simulation results. The CFD simulations indicate that the fluid speed is the key factor of heat transfer, and the heat generated on chip is removed by liquid mass flowing in the channels of micro-fin architectures. The pressure drop between inlet and outlet of the cooling device is an important cause limiting the fluid speed, and the excessive pressure drop will destroy the carbon nanotube fin arrays. The simulation results in present work also indicate that the cooling capability of 2D carbon nanotube fin array is more efficient than that of 1D carbon nanotube fin array, that is, 2D fin array can reduce the thermal resistance of the cooler although 2D fin array causes more pressure drop in the architecture than 1D fin array.

Computational Fluid Dynamics Simulation for On-chip Cooling with Carbon Nanotube Micro-fin Architectures

The 2D CFD simulations have been carried out for a series of CNT micro-fin cooling architectures in this paper. The influence of micro-fin structures, fluid speeds and heating powers on cooling effects have been obtained in the case studies, and the numerical results have been compared with our experiment data. The CFD simulations indicate that the fluid speed is the key factor of heat transfer, and the heat generated on chip is removed by liquid mass flowing in the channels of micro-fin architectures. The pressure drop between inlet and outlet of the cooling device is an important cause limiting the fluid speed, and the excessive pressure drop will destroy the carbon nanotube fin arrays. The harmfulness condition of nanotube array is an unavoidable limitation for cooling capability of carbon nanotube micro-fin structures, and consequently it is necessary to study the harmful situations of carbon nanotube. The simulation results in present work also indicate that the cooling capability of 2D carbon nanotube fin array is more efficient than that of 1D carbon nanotube fin array.

Influences of filler geometry and content on effective thermal conductivity of thermal conductive adhesive

Thermal interface materials (TIMs), including thermal conductive adhesives (TCAs), grease, phase change materials (PCMs) and thermal pads, are widely used in thermal management for electronics system. In this work, a bimodal thermal conductive adhesive (b-TCA) composed of epoxy matrix and silver fillers were developed. The silver fillers consist of silver flakes and silver particles. The specimens have been made with the same filler loading fraction, possessing different ratios of silver flake portion to silver particle one to study the filler geometry effects. The thermal conductivity in vertical and in-plane direction were tested, also the electrical resistance in in-plane direction has been measured. Meanwhile, the numerical simulation was carried out in addition to the experiment. An improved cubic cell model (CCM) was developed to calculate the effective thermal conductivity of the considered b-TCA. Both the experiment and the simulation show the anisotropic thermal conductive property in the b-TCA, but this anisotropy decreased with the increase of silver particle portion in the total filler. And the electrical conductivity also decreased with the increase of silver particles.

Adhesion behavior between epoxy molding compound and different leadframes in plastic packaging

Adhesion has been identified as one of the key elements in solving failure problems in electronic packaging. Understanding and improving the adhesion between epoxy molding compound (EMC) and leadframes is thought to be a key step towards improving package performance. The objective of this work was to study the effect on adhesion behavior and delamination of plastic packages using various metal coated leadframes that had been subjected to preconditions. In this work, the effects of moisture absorption testing and reflow process on the interface adhesion strength of the EMC/metal leadframes were studied. Three kinds of metal coated leadframes were used: copper, silver coated copper and nickel/palladium/gold coated copper. Adhesion strength was measured using the tab pull test. These data were correlated to some extent with the findings of package delamination observed by the C-mode Scanning Acoustic Microscope (C-SAM). Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) were employed to characterize the surface of EMC and metal leadframes after separation along the vertical plane.

Reliability Investigation for Encapsulated Isotropic Conductive Adhesives Flip Chip Interconnection

Isotropic conductive adhesives (ICA) are gaining more and more application interests in electronic manufacturing, however, their failure mechanism have not been fully understood. This paper presents reliability investigations on encapsulated ICA flip chip interconnection. Experimental work included product lifetime measurement, cross section observation, and whole module warpage scanning. Results revealed the chip-size effect on ICA lifetime was obvious, and the warpage measurement implied that normal stress in ICA rather than shear stress play an important role for ICA reliability. A theoretical analysis was conducted with finite element method (FEM) simulation. Visco-elastic models for adhesives and underfill materials were employed, and the comparison with elastic model was made. Calculated stresses trend fitted well with the experimental lifetime measurement, thus a lifetime relationship similar as Coffin-Manson formula was established to predict the thermal fatigue life of encapsulated ICA flip chip. Furthermore, the influences of underfill properties on the ICA reliability were discussed.

Coffin‐Manson constant determination for a Sn‐8Zn‐3Bi lead‐free solder joint

Purpose – To determine the Coffin‐Manson (CM) equation constants for fatigue life estimation of Sn‐8Zn‐3Bi solder joints, since Sn‐8Zn‐3Bi solder has a melting temperature of around 199°C which is close to that of the conventional Sn‐Pb solder which has previously been used in the electronics assembly industry.Design/methodology/approach – Three dimensional finite element (FE) simulation analysis was used for comparison with the experimentally measured data and to determine the CM constants. Low cycle fatigue tests and FE simulations were carried out for these lead‐free solder joints, and eutectic Sn‐37Pb solder was used as a reference.Findings – The CM equation for Sn‐8Zn‐3Bi solder joints was fitted to the lifetimes measured and the shear strains simulated. The constants were determined to be 0.0294 for C, the proportional constant, and for the fatigue exponent, β, −2.833.Originality/value – The CM equation can now be used to predict the reliability of Sn‐8Zn‐3Bi solder joints in electronics assembly an...

Reliability analysis of embedded chip technique with design of experiment methods

This paper presents reliability and failure analysis of embedded components on the liquid crystalline substrate. First, a demonstration module for embedded technology is fabricated. Then it is put into thermal cycling to find out the most obvious failure. 3D FEM analysis is employed to investigate the failure mode. A design of experiment method is conducted to obtain the optimal structure of embedded chip components.

Studies on microstructure of epoxy molding compound (EMC)-Leadframe interface after environmental aging

The Leadframe-Epoxy molding compound (EMC) interface is known to be one of the weakest interfaces in an electronic packaging exhibiting delamination during reliability test. Interfaces of EMC and leadframes with different metal coatings exhibit different failure mode behavior after environmental aging because of the different adhesion strengths. In this paper, the interface microstructure of EMC-leadframes with different metal coatings was studied using Scanning Electron Microscopy (SEM). The leadframes used in this study were copper, copper with Ag coating, Ni coating and Ni/Pd/Au coating. The results of tab pull testing showed the adhesion strength of the EMC/copper leadframe interface was strongest while that of the EMC/leadframe with Ni/Pd/Au coating interface was weakest. Little delamination appeared on the EMC/Cu sample, including the samples after the moisture and reflow treatment. Delamination appeared on the interface of EMC/leadframe with Ag coating after treatment, and serious delamination was found on the interface of EMC/leadframe with Ni coating. The SEM micrographs showed that there were some microcracks between the filler of EMC and the leadframe.

A study of the heat transfer characteristics of the micro-channel heat sink

Micro-channels are generally regarded as an effective method for the heat transfer in electronic products, and much effort has been put into improving their capacities. At the same time, carbon nanotubes have shown great potential in the field of heat transfer. This paper focuses on the microchannel heat sinks combined with carbon nanotubes. A series of 3D models had been created, and the heat transfer characteristics were studied. Two kinds of numerical model were carried out by using Fluent. One model contained the whole micro-channel cooler structure, and the other was a simplified model with only one channel taken into simulation. A comparison of results by the two models indicates a 19.5% variation in the maximum temperature on the fins. Furthermore, five micro-channel cooler structures were constructed with different fin widths. The simulation results showed that with the decrease in the fin width, heat transfer ability was improved. This is attributed to the anisotropic thermal conductivity of the fin arrays.

Coffin‐Manson equation of Sn‐4.0Ag‐0.5Cu solder joint

Purpose: The purpose of this paper is to fit Coffin-Manson equation of Sn-4.0Ag-0.5Cu lead free solder joint by using results of solders joint reliability test and finite element analysis. Also to present a novel device for solder joint reliability test. Design/methodology/approach: Two-points bending test of Sn-4.0Ag-0.5Cu lead free solder joint was carried out at three deflection levels by using a special bending tester that can control displacement exactly by a cam system. The failure criterion was defined as resistance of solder joint getting 10 percent increase. The X-section was done for all failure samples to observe crack initiation and propagation in solder joint. Finite element analysis was presented with ANSYS for obtaining shear strain range, analyzing distribution of stress and strain and supporting experimental results. Findings: The experimental results indicate that the fatigue life decreased obviously with the displacement increased. By using optical microscope and SEM photographs, two kinds of failure mode were found in solder joint. The majority failure mode took place at the bottom corner of solder joint under the termination of resistor initially, and propagated into the solder matrix. Another failure mode was delamination. It appeared at the interface between the termination of resistor and its ceramic body. The distribution status of stress and strain in solder joint and the calculation results of shear strain range at different deflection levels were obtained from simulation result. The Coffin-Manson equation of Sn-4.0Ag-0.5Cu lead free solder joint was fitted by combining experimental data with result of finite element analysis. Originality/value: This paper presents Coffin-Manson equation of Sn-4.0Ag-0.5Cu solder joint with two-points bending test. An effective and economical device was designed and applied.