Ultra thin ultrafine-pitch chip-package interconnections for embedded chip last approach

Ever growing demands for portability and functionality have always governed the electronic technology innovations. IC downscaling with Moore s law and system miniaturization with System-On-Package (SOP) paradigm has resulted and will continue to result in ultraminiaturized systems with unprecedented functionality at reduced cost. The trend towards 3D silicon system integration is expected to downscale IC I/O pad pitches from 40µm to 1- 5 µm in future. Device- to- system board interconnections are typically accomplished today with either wire bonding or solders. Both of these are incremental and run into either electrical or mechanical barriers as they are extended to higher density of interconnections. Alternate interconnection approaches such as compliant interconnects typically require lengthy connections and are therefore limited in terms of electrical properties, although expected to meet the mechanical requirements. As supply currents will increase upto 220 A by 2012, the current density will exceed the maximum allowable current density of solders. The intrinsic delay and electromigration in solders are other daunting issues that become critical at nanometer size technology nodes. In addition, formation of intermetallics is also a bottleneck that poses significant mechanical issues. Recently, many research groups have investigated various techniques for copper-copper direct bonding. Typically, bonding is carried out at 400oC for 30 min followed by annealing for 30 min. High thermal budget in such process makes it less attractive for integrated systems because of the associated process incompatibilities. In the present study, copper-copper bonding at ultra fine-pitch using advanced nano-conductive and non-conductive adhesives is evaluated. The proposed copper-copper based interconnects using advanced conductive and non-conductive adhesives will be a new fundamental and comprehensive paradigm to solve all the four barriers: 1) I/O pitch 2) Electrical performance 3) Reliability and 4) Cost. This thesis investigates the mechanical integrity and reliability of copper-copper bonding using advanced adhesives through test vehicle fabrication and reliability testing. Test vehicles were fabricated using low cost electro-deposition techniques and assembled onto glass carrier. Experimental results show that proposed copper-copper bonding using advanced adhesives could potentially meet all the system performance requirements for the emerging micro/nano-systems.M.S.Committee Chair: Prof. Rao R Tummala; Committee Member: Dr. Jack Moon; Committee Member: Dr. P M Ra

Novel fine pitch interconnection methods using metallised polymer spheres

There is an ongoing demand for electronics devices with more functionality while reducing size and cost, for example smart phones and tablet personal computers. This requirement has led to significantly higher integrated circuit input/output densities and therefore the need for off-chip interconnection pitch reduction. Flip-chip processes utilising anisotropic conductive adhesives anisotropic conductive films (ACAs/ACFs) have been successfully applied in liquid crystal display (LCD) interconnection for more than two decades. However the conflict between the need for a high particle density, to ensure sufficient the conductivity, without increasing the probability of short circuits has remained an issue since the initial utilization of ACAs/ACFs for interconnection. But this issue has become even more severe with the challenge of ultra-fine pitch interconnection. This thesis advances a potential solution to this challenge where the conductive particles typically used in ACAs are selectively deposited onto the connections ensuring conductivity without bridging. The research presented in this thesis work has been undertaken to advance the fundamental understanding of the mechanical characteristics of micro-sized metal coated polymer particles (MCPs) and their application in fine or ultra-fine pitch interconnections. This included use of a new technique based on an in-situ nanomechanical system within SEM which was utilised to study MCP fracture and failure when undergoing deformation. Different loading conditions were applied to both uncoated polymer particles and MCPs, and the in-situ system enables their observation throughout compression. The results showed that both the polymer particles and MCP display viscoelastic characteristics with clear strain-rate hardening behaviour, and that the rate of compression therefore influences the initiation of cracks and their propagation direction. Selective particle deposition using electrophoretic deposition (EPD) and magnetic deposition (MD) of Ni/Au-MCPs have been evaluated and a fine or ultra-fine pitch deposition has been demonstrated, followed by a subsequent assembly process. The MCPs were successfully positively charged using metal cations and this charging mechanism was analysed. A new theory has been proposed to explain the assembly mechanism of EPD of Ni/Au coated particles using this metal cation based charging method. The magnetic deposition experiments showed that sufficient magnetostatic interaction force between the magnetized particles and pads enables a highly selective dense deposition of particles. Successful bonding to form conductive interconnections with pre-deposited particles have been demonstrated using a thermocompression flip-chip bonder, which illustrates the applicable capability of EPD of MCPs for fine or ultra-fine pitch interconnection

Mechanics of Non Planar Interfaces in Flip-Chip Interconnects

With the continued proliferation of low cost, portable consumer electronic products with greater functionality, there is increasing demand for electronic packaging that is smaller, lighter and less expensive. Flip chip is an essential enabling technology for these products. The electrical connection between the chip I/O and substrate is achieved using conductive materials, such as solder, conductive epoxy, metallurgy bump (e.g., gold) and anisotropic conductive adhesives. The interconnect regions of flip-chip packages consists of highly dissimilar materials to meet their functional requirements. The mismatches in properties, contact morphology and crystal orientation at those material interfaces make them vulnerable to failure through delamination and crack growth under various loading patterns. This study encompasses contact between deformable bodies, bonding at the asperities and fracture properties at interfaces formed by the interconnects of flip-chip packages. This is achieved through experimentation and modeling at different length scales, to be able to capture the detailed microstructural features and contact mechanics at interfaces typically found in electronic systems

Mechanical and electrical characterisation of anisotropic conductive adhesive particles

This thesis presents research into the mechanical and electrical characterisation of Anisotropic Conductive Adhesive (ACA) particles and their behaviour within typical joints. A new technique has been developed for study of individual ACA particle mechanical and electrical performance when undergoing deformation. A study of the effects of planarity variations on individual electrical joints in real ACA assemblies is presented firstly, followed by the research on the mechanical deformation and electrical tests of individual ACA particles undergoing deformation. In the co-planarity research, experiments introducing deliberate rotation between a chip and substrate were designed and carried out to simulate planarity variations in ACA assemblies. There are two outputs from this part of the research. One is the planarity variation effects on individual electrical joints in ACA assemblies, and the other is the effect of bond thickness on the resistance of a real joint. [Continues.

Experimental and modelling analysis on the performance of anisotropic conductive films as used in electronics packaging

The aim of this research is to understand the failure modes and mechanisms of adhesive materials used to flip-chip bond a silicon die onto a polyimide substrate. The bonding material investigated in this research is called Anisotropic Conductive Film (ACF). This is a promising interconnection material and has gained extensive interest in the electronics packaging industry. Both the experimental and finite element analysis (FEA) methods were used in order to investigate the behaviour of the ACF materials when subjected to certain manufacturing and environmental testing conditions. The manufacturing condition investigated was a subsequent solder reflow process on an ACF flip-chip bonded device. The environmental testing condition investigated was the moisture test. For the manufacturing condition, both experimental and modelling results demonstrate the impact of a subsequent reflow process on the behaviour of the ACF joint. Typical failures observed after this process were cracks at the pad/particle interface. This failure mode was more sever with a higher peak reflow temperature. This was also found using FEA where high tensile stresses were predicted in these regions. FEA modelling was also used to help identify the mechanisms leading to these failures. This is primarily due to the Coefficient of Thermal Expansion (CTE) miss-match in the materials and the elastic/plastic deformation behaviour of the conductive particle. Important design variables that can minimise these failures are the Young’s Modulus and CTE of the adhesive and the height of the hump on the die. For the environmental testing condition, an autoclave test at 121°C, 100%RH and pressure of 2atm was used. More than 85% of the ACF joints failed during the first 24 hours of testing. The failure mode observed was cracking along the interface between the adhesive and substrate and pad. A macro-micro modelling approach was used to help identify the mechanisms leading to these failures. It was found that most of the damage is caused by moisture diffusion and associated swelling. Important design variables that will help minimise this mode of failure are: Coefficient of Moisture Expansion (CME) and Young’s Modulus of the adhesive and the height of the bump on the die

Electrical conduction characteristics of solid metal anisotropic conductive adhesive particles

Anisotropic Conductive Adhesives (ACAs) have been used in fine pitch electronics packaging for over a decade and provide a high density and low temperature bonding method in a range of niche applications. In an ACA assembly, individual particles act as electrical conductors, providing current paths for fine pitch electronics interconnections. This paper presents a model of the electrical conduction characteristics of the solid metal particles used in some ACAs. Conduction through such ACA particles results from contact between the component and substrate pads and the particle, which is deformed by the assembly process. In order to investigate the effect of the extent of particle deformation, or transformation degree, upon the particle resistance, the particle transformation factor is defined. A mathematical model of the electrical resistance of an ACA particle, which is an integral function of the transformation factor and the particle geomehy, has been developed from a physical model of the ACA particle. MuthCAD software has been used to provide solutions for this function. According to these numerical solutions, the greater the particle transformation, the lower the particle resistance will be. In conclusion, it is shown that the ACA particle resistance is determined by the particle transformation and the particle geometries. Finally, the resistance function will explain the conductive mechanism of a deformed metal ACA particle

Performance and Reliability of Polymer-based Sensor Packages at High Temperatures

Electronics is increasingly used in many applications. In addition to new consumer electronics, there is a trend to implement electronics and sensors in many industrial applications, which often requires improved reliability in very demanding conditions. This thesis concerns the effect of high temperature on electronics packages. High temperature tends to accelerate chemical reactions, aging of materials and thereby also failures. Properties of polymer materials, which are often used in electronic packaging, are heavily temperature-dependent. They are, however, very versatile and compatible materials which are readily available and easy to manufacture compared to expensive high-temperature speciality materials such as ceramics. Therefore, using polymers at high temperatures would be highly beneficial. However, it is crucial to understand the effects of high temperature on polymer materials before they are used.This work concentrates on the reliability of polymer-based sensor packages at high temperatures and the changes occurring in their materials at different temperatures. The structures were aged using several high temperature tests including thermal cycling, step stress and thermal storage tests at several temperatures around 200°C. The effects of high temperature on several commercial polymer-based printed circuit boards (PCB) and electrically conductive adhesives (ECA) were analysed from an electrical and mechanical perspective. Moreover, changes in material parameters due to aging were studied to achieve a more profound understanding of the effects of high temperatures.The temperature of 180°C seemed to be low enough not to cause reliability problems in the polymer-based packages studied. Additionally, a good performance in thermal cycling testing up to 180°C was achieved. At 200°C degradation was seen on the surfaces of the polymer materials and their mechanical properties gradually declined. Good electrical performance was nevertheless achieved with suitable material choices. A temperature of 240°C was shown to be too high for extended exposure of the materials studied. With careful material choices relatively good electrical performance was achieved, but FTIR showed dramatic and rapid degradation with most of the polymerbased materials at this temperature. The degradation was much more severe than at 200°C, and the mechanical properties also showed drastic impairment at 240°C.Material selection was shown to be absolutely critical for the reliability of the whole polymer-based package. With poor PCB material interconnections failed much earlier than with more stable PCB materials. Additionally, ECA selection was also important. This thesis showed that polymer-based electronic packages can withstand high temperatures, especially for limited exposure times. However, it is crucial that all materials present are able to withstand the selected temperatures.<br/

Nanowires for 3d silicon interconnection – low temperature compliant nanowire-polymer film for z-axis interconnect

Semiconductor chip packaging has evolved from single chip packaging to 3D heterogeneous system integration using multichip stacking in a single module. One of the key challenges in 3D integration is the high density interconnects that need to be formed between the chips with through-silicon-vias (TSVs) and inter-chip interconnects. Anisotropic Conductive Film (ACF) technology is one of the low-temperature, fine-pitch interconnect method, which has been considered as a potential replacement for solder interconnects in line with continuous scaling of the interconnects in the IC industry. However, the conventional ACF materials are facing challenges to accommodate the reduced pad and pitch size due to the micro-size particles and the particle agglomeration issue. A new interconnect material - Nanowire Anisotropic Conductive Film (NW-ACF), composed of high density copper nanowires of ~ 200 nm diameter and 10-30 µm length that are vertically distributed in a polymeric template, is developed in this work to tackle the constrains of the conventional ACFs and serves as an inter-chip interconnect solution for potential three-dimensional (3D) applications

Development and reliability of a direct access sensor using flip chip on flex technology with anisotropic conductive adhesive

Technological developments in biomedical microsystems are opening up new opportunities to improve healthcare procedures. Swallowable diagnostic sensing capsules are an example of these. In none of the diagnostic sensing capsules, is the sensor’s first level packaging achieved via Flip Chip Over Hole (FCOH) method using Anisotropic Conductive Adhesive (ACA). In a capsule application with direct access sensor (DAS), ACA not only provides the electrical interconnection but simultaneously seals the interconnect area and the underlying electronics. The development showed that the ACA FCOH was a viable option for the DAS interconnection. Adequate adhesive formed a strong joint that withstood a shear stress of 120N/mm2 and a compressive stress of 6N required to secure the final sensor assembly in place before encapsulation. Electrical characterization of the ACA joint in a fluid environment showed that the ACA was saturated with moisture and that the ions in the solution actively contributed to the leakage current, characterized by the varying rate of change of conductance. Long term hygrothermal aging of the ACA joint showed that a thermal strain of 0.004 and a hygroscopic strain of 0.0052 were present and resulted in a fatigue like process. In-vitro tests showed that high temperature and acidity had a deleterious effect of the ACA and its joint. It also showed that the ACA contact joints positioned at around or over 1mm would survive the gastrointestinal (GI) fluids and would be able to provide a reliable contact during the entire 72hr of the GI transit time. A final capsule demonstrator was achieved by successfully integrating the DAS, the battery and the final foldable circuitry into a glycerine capsule. Final capsule soak tests suggested that the silicone encapsulated system could survive the 72hr gut transition