Janet K. Lumpp

Electrical and mechanical characterization of carbon nanotube filled conductive adhesive

Electronic assemblies rely heavily on soldering to attach components to the interconnect wiring on printed circuit boards and other types of substrates. In response to environmental legislation, the lead-tin alloys commonly used for soldering are being replaced with lead-free alloys and electrically conductive adhesives. Isotropic conductive adhesives (ICA) filled with metal particles are an alternative to solder reflow processing. ICA materials, however, require up to 80 wt% metal filler to achieve minimum electrical resistivity and the high loading content substantially degrades the mechanical properties of the polymer matrix. Carbon nanotubes (CNTs), a new material based on C60 fullerenes (Bucky Balls) and graphite rolls have many novel properties. Replacing the metal particles with carbon nanotubes in ICA compositions has the potential benefits of being lead free, low process temperature, corrosion resistant, high electrical conductivity, high mechanical strength and lightweight. In this paper, new conductive adhesives were formulated by adding multiwall nanotubes (MWNT) as filler to epoxy. Different loadings of CNTs and mixing methods were used to make the new conductive adhesives. Contact resistance, volume resistivity, high frequency performance, thermal conductivity and mechanical properties were measured and compared with metal filled ICA and traditional solder paste

Excimer laser machining and metallization of vias in aluminum nitride

Laser machining of ceramics is used extensively in the microelectronics industry for scribing and via hole drilling. Scribing involves laser ablation of a groove or row of holes that form perforation lines to separate a large substrate into individual circuits. Via machining is generally followed by a metallization step to create three-dimensional (3-D) interconnections in a multilayer circuit board. Aluminum nitride (AlN) is a desirable substrate material for high power, high frequency applications because of its high thermal conductivity and low thermal expansion coefficient than Al/sub 2/O/sub 3/. In this paper, an excimer laser is used to machine high aspect ratio, straight walled via holes in aluminum nitride with or without a metallization layer deposited on the via walls. Via diameters range between 60 and 300 /spl mu/m through substrates 635 /spl mu/m thick. Through hole machining can cause damage to the back surface of the substrate, however, attachment of a second substrate or metal sheet will prevent damage. Ablation of the attached metal backing with subsequent redeposition on the via walls produces a metallized via with a resistance of less than 1 /spl Omega/ per via. Single and multilayer via structures are described. Substrate damage at through hole exits results from shock wave propagation and reflection in the substrate. The attached backing material reduces reflection of the shock wave at the back surface of the substrate to prevent damage. Shock wave analysis, via cross sections, and resistance measurements are discussed.

Aspect Ratio and Loading Effects of Multiwall Carbon Nanotubes in Epoxy for Electrically Conductive Adhesives

Isotropic conductive adhesives (ICAs) filled with metal particles are commercially available as alternatives to solder joining in electronic packaging. Replacing metal fillers with multiwall carbon nanotubes (MWCNTs) offers the potential benefits of being corrosion resistant, high strength and lightweight. Traditional metal filled ICAs require high metal loading to ensure electrical conductivity, which may cause problems with respect to reliability and strength to weight ratio. The ultra-high aspect ratio and surface area of multiwall carbon nanotubes induce a low percolation threshold of less than 0.25 wt% in epoxy. MWCNTs dispersed in epoxy increase the thermal diffusivity of the polymer by a factor of 2 to 3 and decrease the volume resitivity to less than 10 Ω cm for loadings up to 12 wt%. Lap shear strength decreases with increased MWCNT loading. The dependences of electrical, thermal and mechanical properties on loading and aspect ratio of MWCNTs are reported, and the electrical conduction mechanism is discussed.

Permeability spectra of hole arrays defined on single layer Permalloy thin films

Although considerable work has been done investigating the properties of arrays of magnetic elements, there have been few investigations on the reverse geometry, i.e., an array of nonmagnetic regions defined within a magnetic thin film. The 10 Hz BH loops, 10–500 MHz permeability spectra, and domain patterns of homogeneous, single layer 100 nm radio frequency (rf) sputtered Ni81Fe19 thin films with arrays of 23-, 50-, and 100-μm-diam holes defined by laser ablation were measured. The holes were defined in a grid along the hard and easy axes of the sample. Letting (x,y) represent, respectively, the hole spacing parallel to the easy and hard axes, the point to point spacing of the ablated circular regions was varied from (5, 2 mm) to (0.1, 0.1 mm).

Carbon Nanotube Filled Conductive Adhesives for Aerospace Applications

Polymer matrix composites with multi-wall carbon nanotube (MWNT) filler improve the thermo-mechanical and electrical properties of the host material. The high electrical conductivity of MWNT induces a low percolation threshold in the polymer to produce electrically conductive epoxies at loadings of less than 10 wt% MWNT. Applications of these conductive adhesives address issues such as lead-free environmental legislation, metallic corrosion, tin whiskers, and lightweight electronic assemblies. MWNT filled adhesives are electrically comparable to lead-tin alloys commonly used for soldering and commercial metal filled electrically conductive adhesives. Aerospace applications also require increased strength to weigh ratio, heat dissipation and static charge dissipation. We are currently investigating the dependence of electrical and thermo-mechanical properties on loading, aspect ratio of the carbon nanotubes and various sources of carbon nanotubes. Measurements include DC resistivity, AC parameters, lap shear, laser flash thermal analysis, and volume resistivity. Commercially available isotropic conductive adhesives require up to 80 wt% metal filler to achieve minimum electrical resistivity, however the high loading content degrades the mechanical properties of the polymer matrix. Replacing the metal particles with MWNTs in ICA compositions has the potential benefits of being lead free, low process temperature, corrosion resistant, high electrical conductivity, high mechanical strength and lightweight. Preliminary results show that thermal diffusivity increases with MWNT loading compared to pure epoxy and mechanical strength is improved when compared with metal filled adhesives. Accelerated aging tests in 85C/85%RH are also underway to determine the reliability of the bonds. Carbon nanotubes filled epoxies are suitable for pressure dispensing, spin coating, stencil printing and extrusion for fabrication of structural and electronic assemblies.

Mechanical Properties of CuO

INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS AND DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES 2

Excimer laser machining and metallization of vias in aluminium nitride

Abstract Aluminium nitride (AlN) is a high thermal conductivity, high electrical resistivity, low dielectric constant ceramic with a small thermal coefficient of expansion mismatch to silicon. Machining and metallization processes for fired AlN are needed for prototype and small scale fabrication of power hybrid substrates and multichip module packages. Ablation has been investigated with a KrF (248 nm) laser to determine the dependence of ablation rate on ambient pressure, fluence and hole depth. Ablation rate is constant with increasing depth, and increases with increasing fluence and decreasing pressure. Depth control of less than 0.2 μm pulse−1 can be achieved and vias can be metallized by ablating a metal sheet attached to the substrate. The attached sheet also prevents fracture of the substrate by absorbing shock waves generated during ablation.

The soft magnetic properties of stripes fabricated using laser ablation of multilayer thin films

We examine the low frequency BH loop and complex permeability spectra of multilayer permalloy films laser processed to define samples consisting of stripes parallel to the as‐deposited hard axis. We begin by examining different laser energy densities for groove definition in films of different thickness. We observe that for well coupled multilayer films with thin, 15 nm, spacer layers the demagnetizing field, which is a function of groove depth, is able to reorient the easy and hard axes from the initial orientation. As the groove becomes more clearly defined the hard axis permeability increases, while the easy axis permeability decreases. The role of stripe width and ambient processing atmosphere are also investigated.

Electrically Conductive Carbon Nanotube Adhesives on Lead Free Printed Circuit Board Surface Finishes

Electrically conductive adhesives are attractive alternatives to solder and die attach materials in electronic assemblies particularly in the lead free era. Compared to metal filled conductive adhesives, multiwall carbon nanotube (MWCNT) filled adhesives are lightweight, corrosion resistant, high strength and resistant to metal migration. Previous studies of MWCNT filled epoxies on bare copper printed circuit boards identified contact resistance as a challenge for surface mount components where pressure was required during curing to lower the average contact resistance. Currently, we are repeating contact resistance, volume resistivity, lap shear and die shear testing of CNT filled epoxy on printed circuit boards with various surface finishes on the copper pads. The surface finishes under investigation are immersion tin, immersion silver and electroless nickel-immersion gold (ENIG).

The evaluation of solder and circuit board materials for small satellite solar cell arrays

Body mounted germanium substrate solar cell arrays form the faces of many small satellite designs to provide the primary power source on orbit.1,2 High efficiency cells are made affordable for University scale satellite programs as triangular devices trimmed from wafer scale solar cells. The smaller cells allow the array designs to pack tightly around antenna mounts and payload instruments. We are investigating the reliability of solar cells attached to FR-4 and carbon core laminate printed circuit boards. FR-4 circuit boards have significantly higher thermal expansion coefficients and lower thermal conductivities than germanium; whereas carbon core laminates can be selected to more closely match the device substrate. We are also comparing various solder pastes and cleaning processes for array assembly. Storage, vacuum exposure, thermal cycling and vibration testing will be used to compare the survivability and performance of the solar arrays.