Robert W. Kay

Ultra-Fine Pitch Stencil Printing for a Low Cost and Low Temperature Flip-Chip Assembly Process

This paper presents the results of a packaging process based on the stencil printing of isotropic conductive adhesives (ICAs) that form the interconnections of flip-chip bonded electronic packages. Ultra-fine pitch (sub-100-mum), low temperature (100degC), and low cost flip-chip assembly is demonstrated. The article details recent advances in electroformed stencil manufacturing that use microengineering techniques to enable stencil fabrication at apertures sizes down to 20mum and pitches as small as 30mum. The current state of the art for stencil printing of ICAs and solder paste is limited between 150-mum and 200-mum pitch. The ICAs-based interconnects considered in this article have been stencil printed successfully down to 50-mum pitch with consistent printing demonstrated at 90-mum pitch size. The structural integrity or the stencil after framing and printing is also investigated through experimentation and computational modeling. The assembly of a flip-chip package based on copper column bumped die and ICA deposits stencil printed at sub-100-mum pitch is described. Computational fluid dynamics modeling of the print performance provides an indicator on the optimum print parameters. Finally, an organic light emitting diode display chip is packaged using this assembly process

Sub process challenges in ultra fine pitch stencil printing of type-6 and type-7 Pb-free solder pastes for flip chip assembly applications

Purpose – The study investigates the sub process behaviour in stencil printing of type‐6 and type‐7 particle size distribution (PSD) Pb‐free solder pastes to assess their printing limits.Design/methodology/approach – Two solder pastes were used in a design of experiments approach to find optimal printing parametersFindings – Solder paste printing has been achieved to ultimately produce 30 μm deposits at 60 μm pitch for full area array patterns using a type‐7 Pb‐free solder paste. For a type‐6 PSD solder paste, full area array printing was limited to 50 μm deposits at 110 μm pitch. However, for peripheral printing patterns, 50 μm deposits at 90 μm pitch were obtained. The disparities in the behaviour of the two paste types at different geometries can be attributed to differences in the sub‐processes of the stencil printing. The paste release of the type‐6 paste from the stencil apertures at fine pitch was superior to the type‐7 paste, which may be attributed to the finer particle paste producing an increas...

A review of stencil printing for microelectronic packaging

Purpose – The purpose of this paper is to present a detailed overview of the current stencil printing process for microelectronic packaging.Design/methodology/approach – This paper gives a thorough review of stencil printing for electronic packaging including the current state of the art.Findings – This article explains the different stencil technologies and printing materials. It then examines the various factors that determine the outcome of a successful printing process, including printing parameters, materials, apparatus and squeegees. Relevant technical innovations in the art of stencil printing for microelectronics packaging are examined as each part of the printing process is explained.Originality/value – Stencil printing is currently the cheapest and highest throughput technique to create the mechanical and electrically conductive connections between substrates, bare die, packaged chips and discrete components. As a result, this process is used extensively in the electronic packaging industry and ...

Microstructure formation in a thick polymer by electrostatic-induced lithography

This article demonstrates the manufacturing of microstructures in a thick polymer using electrostatic-induced lithography. Unlike previous work reported elsewhere, it focuses on the fabrication of structures from meso- to micro-scale. The electrostatic-induced lithography technique is proven to work with not only dc voltage but also ac voltage. Microstructures including microchannels, sinusoidal surface profile microstructures, waveguide core, microlens array and binary Fresnel zone plate have been successfully fabricated. The aspect ratio obtained for some samples is up to 4.5:1. The whole fabrication process is fast, cost-effective in terms of the simple experimental setup and no photosensitive material is needed. This process is expected to find applications in microfluidics, photonics or micro-opto-electro-mechanical systems.

Hybrid additive manufacturing of 3D electronic systems

A novel hybrid additive manufacturing (AM) technology combining digital light projection (DLP) stereolithography (SL) with 3D micro-dispensing alongside conventional surface mount packaging is presented in this work. This technology overcomes the inherent limitations of individual AM processes and integrates seamlessly with conventional packaging processes to enable the deposition of multiple materials. This facilitates the creation of bespoke end-use products with complex 3D geometry and multi-layer embedded electronic systems. Through a combination of four-point probe measurement and non-contact focus variation microscopy, it was identified that there was no obvious adverse effect of DLP SL embedding process on the electrical conductivity of printed conductors. The resistivity maintained to be less than 4  ×  10−4 Ω centerdot cm before and after DLP SL embedding when cured at 100 °C for 1 h. The mechanical strength of SL specimens with thick polymerized layers was also identified through tensile testing. It was found that the polymerization thickness should be minimised (less than 2 mm) to maximise the bonding strength. As a demonstrator a polymer pyramid with embedded triple-layer 555 LED blinking circuitry was successfully fabricated to prove the technical viability.

Optimization and characterization of Drop-on-Demand inkjet printing process for platinum organometallic inks

Inkjet printing has been extensively used over the past 30 years in the graphic arts and packaging industries. This technology involves dispensing accurately positioned droplets of ink onto a substrate, which then solidifies through the evaporation of the constituent solvent, the cross-linking of a polymer or through crystallization. The mask-less, flexible, rapid and low cost nature of inkjet printing, combined with the development of a range of functional inks, has led to the adoption of this technology in system manufacturing. The SMART Microsystems research project underway at the Institute for Integrated Systems is investigating the use of this technique in the rapid customization of CMOS foundry wafers for More-than-Moore applications. This paper presents results obtained during the development and optimization of a drop-on-demand inkjet printing process for initial batches of platinum organometallic inks. Drop-on-Demand (DOD) inkjet printing works by inducing a transient pressure pulse in the ink reservoir through electrical excitation of either a thermal or piezoelectric element. The correct implementation of this excitation signal is necessary to produce a pressure pulse capable of reproducibly and reliably generating a series of droplets. The effects of system parameters on the formation of these droplets are investigated. Methods used to characterize droplet ejection are also described.

Dynamically controlled deposition of colloidal nanoparticle suspension in evaporating drops using laser radiation

Dynamic control of the distribution of polystyrene suspended nanoparticles in evaporating droplets is investigated using a 2.9 μm high power laser. Under laser radiation a droplet is locally heated and fluid flows are induced that overcome the capillary flow, and thus a reversal of the coffee-stain effect is observed. Suspension particles are accumulated in a localised area, one order of magnitude smaller than the original droplet size. By scanning the laser beam over the droplet, particles can be deposited in an arbitrary pattern. This finding raises the possibility for direct laser writing of suspended particles through a liquid layer. Furthermore, a highly uniform coating is possible by manipulating the laser beam diameter and exposure time. The effect is expected to be universally applicable to aqueous solutions independent of solutes (either particles or molecules) and deposited substrates.

Tattoo Antenna Temporary Transfers Operating On-Skin TATTOOS

This paper discusses the development of RFID logo antennas based on the logos of Loughborough University and the University of Kent which can be tattooed directly onto the skins surface. Hence, this paper uses aesthetic principles to create functional wearable technology. Simulations of possible designs for the tattoo tags have been carried out to optimize their performance. Prototypes of the tag designs were fabricated and read range measurements with the transfer tattoos on a volunteers arm were carried out to test the performance. Measured read ranges of approximately 0.5i¾?m have been achieved with the tag only10i¾?µm from the body.

Computational modelling for reliable flip-chip packaging at sub-100 μm pitch using isotropic conductive adhesives

Abstract This paper presents the assembly process using next generation electroformed stencils and Isotropic Conductive Adhesives (ICAs) as interconnection material. The utilisation of ICAs in flip-chip assembly process is investigated as an alternative to the lead and lead-free solder alloys and aims to ensure a low temperature ( T

On the Use of Silver Nanoparticles for Direct Micropatterning on Polyimide Substrates

This paper proposes a direct micropatterning process based on the growth of photoreduced silver nanoparticles onto polyimide substrates. The silver nanoparticles are found to have sufficient catalytic efficiency for subsequent electroless plating. Characterization of the process indicates that UV energy dose and heat treatment have to be traded off against photo and thermal degradation of the polymer substrate. Factors affecting the adhesion of the final electroless metal deposit are also discussed.