John M. Lauffer

Printable electronics: towards materials development and device fabrication

Purpose – There has been increasing interest in the development of printable electronics to meet the growing demand for low‐cost, large‐area, miniaturized, flexible and lightweight devices. The purpose of this paper is to discuss the electronic applications of novel printable materials.Design/methodology/approach – The paper addresses the utilization of polymer nanocomposites as it relates to printable and flexible technology for electronic packaging. Printable technology such as screen‐printing, ink‐jet printing, and microcontact printing provides a fully additive, non‐contacting deposition method that is suitable for flexible production.Findings – A variety of printable nanomaterials for electronic packaging have been developed. This includes nanocapacitors and resistors as embedded passives, nanolaser materials, optical materials, etc. Materials can provide high‐capacitance densities, ranging from 5 to 25 nF/in2, depending on composition, particle size, and film thickness. The electrical properties of ...

Fabrication, integration and reliability of nanocomposite based embedded capacitors in microelectronics packaging

We have developed a variety of barium titanate (BaTiO3)–epoxy polymer nanocomposite based thin film capacitors. In particular, we highlight recent developments on high capacitance, large area, thin film passives, their integration in printed wire board (PWB) substrates and the reliability of the embedded capacitors. A variety of nanocomposite thin films ranging from 2 microns to 25 microns thick were processed on PWB substrates by liquid coating or printing processes. SEM micrographs showed uniform particle distribution in the coatings. The electrical performance of composites was characterized by dielectric constant (Dk), capacitance and dissipation factor (loss) measurements. Nanocomposites provided high capacitance density (10–100 nF inch−2) and low loss (0.02–0.04) at 1 MHz. The manufacturability of these films and their reliability has been tested using large area (13 inch × 18 inch or 19.5 inch × 24 inch) test vehicles. Reliability of the test vehicles was ascertained by IR reflow, thermal cycling, pressure cooker test (PCT) and solder shock. Capacitors were stable after PCT and solder shock. Capacitance change was less than 5% after IR reflow (assembly) preconditioning (3×, 245 °C) and 1400 cycles deep thermal cycle (DTC).

Influence of Nanoparticles, Low Melting Point (LMP) Fillers, and Conducting Polymers on Electrical, Mechanical, and Reliability Performance of Micro-Filled Conducting Adhesives for Z-Axis Interconnections

This paper discusses micro-filled epoxy-based conducting adhesives modified with nanoparticles, conducting polymers, and low melting point (LMP) fillers for z-axis interconnections, especially as they relate to package level fabrication, integration, and reliability. A variety of conducting adhesives with particle sizes ranging from 80 nm to 15 mum were incorporated as interconnects in printed wiring board (PWB) or laminated chip carrier (LCC) substrates. SEM and optical microscopy were used to investigate the micro-structure, and conducting and sintering mechanisms. Volume resistivity of modified adhesives is in the range of 10-5 to 10-6 ohm-cm. Adhesives formulated with a conducting polymer exhibited tensile strength with Goulds JTC-treated Cu ges 3800 PSI, and as low as 1800 PSI for a conducting polymer-LMP based system. There was no delamination of conductive joints after 3X IR-reflow, pressure cooker test (PCT), and solder shock. Among all, the conducting polymer modified micro-filled adhesives showed the highest mechanical strength. The paper also describes a combinatorial approach to the synthesis of LMP coated particles. Several conductive adhesives were used in a z-axis interconnect construction for a laminate chip carrier and printed wiring board (PWB). The present process allows fabrication of z-interconnect conductive joints having diameters in the range of 55-300 microns. The processes and materials used to achieve smaller feature dimensions, satisfy stringent registration requirements, and achieve robust electrical interconnections are discussed.

High capacitance, large area, thin film, nanocomposite based embedded capacitors

This paper discusses thin film technology based on barium titanate (BaTiO3)-epoxy polymer nanocomposites. In particular, we highlight recent developments on high capacitance, large area, thin film passives, their integration in PWB substrates and the reliability of the embedded capacitors. A variety of nanocomposite thin films ranging from 2 microns to 25 microns thick were processed on PWB substrates by liquid coating or printing processes. SEM micrographs showed uniform particle distribution in the coatings. The electrical performance of composites was characterized by dielectric constant (Dk), capacitance and dissipation factor (loss) measurements. Nanocomposites resulted in high capacitance density (10-100 nF/inch2) and low loss (0.02-0.04) at 1 MHz. The manufacturability of these films and their reliability has been tested using large area (13 inch times 18 inch or 19.5 inch times24 inch) test vehicles. Reliability of the test vehicles was ascertained by IR-reflow, thermal cycling, PCT (pressure cooker test) and solder shock. Capacitors were stable after PCT and solder shock. Capacitance change was less than 5% after IR reflow (assembly) preconditioning (3X, 245 degC) and 1400 cycles DTC (deep thermal cycle)

Electrical conductivity and reliability of nano- and micro-filled conducting adhesives for z-axis interconnections

This paper discusses epoxy-based conducting adhesives for z-axis interconnections. Recent work on adhesives formulated using controlled-sized particles to fill small diameter holes is highlighted, particularly with respect to their integration in laminate chip carrier substrates, and the reliability of the electrically conductive joints formed between the adhesive and metal surfaces. A variety of conductive adhesives with particle sizes ranging from 80 nm to 15 mum were laminated into printed wiring board substrates. SEM and optical microscopy were used to investigate the micro-structures, conducting mechanism and path. Volume resistivity of Cu, Ag and low melting point (LMP) alloy based paste were 5 times 10-4 ohm-cm, 5 times 10-5 ohm-cm, and 2 times 10-5 ohm-cm, respectively. Volume resistivity decreased with increasing curing temperature. The mechanical strength of the various adhesives was characterized by 90 degree peel test and measurement of tensile strength. Adhesives exhibited peel strength with Goulds JTC-treated Cu as high as 2.75 lbs/inch for silver, and as low as 1.00 lb/inch for LMP alloy. Similarly, tensile strength for silver, Cu and LMP alloy was 3370, 2056 and 600 psi, respectively. Reliability of the adhesives was ascertained by IR-reflow, thermal cycling, pressure cooker test (PCT), and solder shock. Change in tensile strength of adhesives was within 10 % after 1000 cycles of deep thermal cycling (DTC) between -55 degC and 125 degC. There was no delamination for silver, copper and LMP alloy samples after 3X IR-reflow, PCT, and solder shock. Among all, silver-based adhesives showed the lowest volume resistivity and highest mechanical strength. It was found that with increasing curing temperature, the volume resistivity of the silver-tilled paste decreased due to sintering of metal particles. Sinterability of silver adhesive was further evaluated using high temperature/pressure lamination, and shows a continuous metallic network when laminated at 365 degC. As a case study, an example of silver-filled conductive adhesives as a z-axis interconnect construction for a flip-chip plastic ball grid array package with a 150 mum die pad pitch is given. This effort is an integrated approach centering on three interrelated fronts: (1) materials development and characterization; (2) fabrication, and (3) integration at the device level

Resistance drift in aluminum to gold ultrasonic wire bonds

It is noted that the literature contains many instances of reliability problems associated with the aluminum wire to gold bond system. The authors, in evaluating the potentials of this technology, experienced similar problems and initiated some fundamental studies to enhance their knowledge base. The system of interest consisted of 0.002-in. diameter Al-1% wire ultrasonically bonded to Ni-Au electroplated Cu printed circuit pads. The reliability problems observed manifested themselves as resistance drifting associated with thermal aging of the sample. It was found that plating thickness, plating current density, and bath agitation strongly influenced the resistance drift phenomenon. A simple four-wire resistance test was developed to monitor for out-of-control gold plating. >

Laser Micromachining of Barium Titanate

This paper discusses laser micromachining of barium titanate (BaTiO3)-polymer nanocomposite thin films. In particular, recent developments on high-capacitance, large-area, thin, flexible, embedded capacitors are highlighted. A variety of barium titanate (BaTiO3)-epoxy polymer nanocomposite-based flexible/rollable thin films ranging from 2 to 25 mum thick were processed on large-area substrates (330 mm times 470 mm, or 495 mm times 607 mm) by liquid coating processes. The electrical performance of composites was characterized by dielectric constant (Dk), capacitance, and dissipation factor (loss) measurements. Nanocomposites provided high capacitance density (10-100 nF/in2) and low loss (0.02-0.04) at 1 MHz. Scanning electron microscopy (SEM) micrographs showed uniform particle distribution in the coatings. Uniform mixing of nanoparticles in the epoxy matrix results in high dielectric (> 3 times 107 V/m) and mechanical strengths (> 3700 PSI). Reliability of the capacitor was ascertained by thermal cycling. Capacitance change was less than 5% after baking at 140degC for 4 h, and 1100 cycles from -55degC to 125degC (deep thermal cycle). A frequency-tripled Nd:YAG laser operating at a wavelength of 355 nm was used for the micromachining study. The micromachining was used to generate arrays of variable-thickness capacitors from the nanocomposites. The resultant thickness of the capacitors depends on the number of laser pulses applied.

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This paper discusses laser micromachining of barium titanate (BaTiO3)-polymer nanocomposites and sol-gel thin films. In particular, recent developments on high capacitance, large area, and thin flexible embedded capacitors are highlighted. A variety of flexible nanocomposite thin films ranging from 2 microns to 25 microns thick were processed on copper or organic substrates by large area (13 inch times 18.5 inch, or 19.5 inch times 24 inch) liquid coating processes. SEM micrographs showed uniform particle distribution in the coatings. Nanocomposites resulted in high capacitance density (10-100 nF/inch2) and low loss (0.02-0.04) at 1 MHz. The remarkably increased flexibility of the nanocomposite is due to uniform mixing of nanoparticles in the polymer matrix, resulting in an improved polymer-ceramic interface. BaTiO3-epoxy polymer nanocomposites modified with nanomaterials were also fabricated and were investigated with SEM analysis. Capacitance density of nanomaterial-modified films was increased up to 500 nF/inch2, about 5-10 times higher than BaTiO3-epoxy nanocomposites. A frequency-tripled Nd:YAG laser operating at a wavelength of 355 nm was used for the micromachining study. The micromachining was used to generate arrays of variable-thickness capacitors from the nanocomposites. The resultant thickness of the capacitors depends on the number laser pulses applied. Laser micromachining was also used to make discrete capacitors from a capacitance layer. In the case of sol-gel thin films, micromachining results in various surface morphologies. It can make a sharp step, cavity-based wavy structure, or can make individual capacitors by complete ablation. Altogether, this is a new direction for development of multifunctional embedded capacitors.

-Epoxy Nanocomposite-Based Flexible/Rollable Capacitors: New Approach for Making Library of Capacitors

This paper examines the use of nanocomposites or materials in the area of printing technology. A variety of printable nanomaterials for advanced organic packaging have been developed. This includes nano capacitors and resistors as embedded passives, nano magnetic materials, multifunctional materials, etc. Nanocomposites can provide high capacitance densities, ranging from 5 nf/inch2 to 25 nF/inch2, depending on composition, particle size and film thickness. The electrical properties of capacitors fabricated from BaTiO3-epoxy nanocomposites showed a stable capacitance and low loss over a temperature range from 25degC to 100degC. A variety of printable discrete resistors with different sheet resistances, ranging from 1 ohm to 120 Mohm, processed on large panels (19.5 inches times 24 inches) have been fabricated. Low resistivity nanocomposites, with volume resistivity in the range of 10-4 ohm-cm to 10-6 ohm-cm depending on composition, particle size, and loading can be used as conductive joints for high frequency and high density interconnect applications. Thermosetting polymers modified with ceramics can produce low k dielectrics with k value in the range between 5.41 and 3.59. Similarly, low loss dielectric materials can be produced form mixing epoxy with silica or other low loss fillers. Reliability of the nanocomposites was ascertained by IR-reflow, thermal cycling, pressure cooker test (PCT), and solder shock. Change in capacitance after 3X IR-reflow and after 1000 cycles of deep thermal cycling (DTC) between -55degC and 125degC was within 5%. Most of the nanocomposites in the test vehicle were stable after IR-reflow, PCT, and solder shock.

Laser Micromachining of Nanocomposite-Based Flexible Embedded Capacitors

This paper will describe a method to assist system designers of High Speed Systems in the selection of PWB design/build attributes. The attributes that will be covered are; dielectric material, circuit trace width and thickness, PWB thickness, copper surface roughness, and Plated Through Hole (PTH) stub length. Changes to these attributes are weighed against the cost and the risk of each change to arrive at an optimal system design. Manufacturability limitations that must be considered include board size, drill and plate aspect ratio capabilities, minimum spacing and internal layer registration capabilities. The system requirements may drive design attributes such as low loss dielectrics, subcomposites, microvias, buried vias, and/or back drilling. Such complex board designs will limit the number of capable suppliers that can be quoted so unnecessary complexity needs to be avoided.