Darbha Suryanarayana

Enhancement of flip-chip fatigue life by encapsulation

Encapsulation of controlled collapse chip connection (C4) joints, using a filled epoxy resin with a matched coefficient of thermal expansion (CTE), has provided a substantial increase in the life of C4 joints in accelerated thermal cycle (ATC) fatigue testing on both low CTE organic and ceramic chip carriers. The C4 joints are encapsulated by dispensing a bead of the resin along an edge of the chip. The encapsulant flows underneath the chip by capillary action and completely fills the gap between the chip and the substrate. Optimization of the filler size distribution and resin rheology to obtain consistent flow under the chip without any bubbles is discussed. The filler size distribution and flow under the chip are shown in cross sections of several different materials, including low alpha emitting encapsulants for memory applications. Encapsulant formulations are tested by videotaping the flow of encapsulant under transparent quartz chips. The formation of bubbles as the encapsulant flows around the C4 joints and irregularities in the surface of the substrate can clearly be seen. Proper C4 encapsulation provides virtually complete coverage around all C4 connections. C4 life testing over various temperature ranges shows a five to ten times improvement for both memory and logic footprints when the C4 joints are encapsulated. The vast improvement in C4 joint reliability provided by encapsulation allows the C4 technology to be extended to much larger chips or to higher service temperature ranges without conventional distance from neutral point (DNP) constraints. >

Encapsulants used in flip-chip packages

Low thermal expansion encapsulants have been used in a wide range of IBM flip-chip products to enhance the fatigue life of solder interconnections and to prevent environmental attack. In order to determine whether an encapsulant can satisfy all requirements of a given product or process, the physical behaviors of the encapsulant must be carefully characterized. Typical material properties of interest are: viscosity and flow coverage, thermal expansion, modulus, glass transition temperature, filler size, alpha activity, adhesion, ionics content, fracture toughness, and electrical properties. The sensitivity of these properties to process variables such as encapsulant dispensing, gelling, curing and post curing exposures to environment and reliability experiments need to be explored. An overall evaluation of all these properties is essential for determining the ultimate performance of the encapsulant on specific products. This paper presents a systematic approach to quantify these encapsulant properties. Various test methods and their relevant technical aspects are discussed. Results achieved on a few selected materials are also presented as examples. >

Flip-chip solder bump fatigue life enhanced by polymer encapsulation

Encapsulation of controlled collapse chip connection (C4) joints, using a filled epoxy resin having a matched coefficient of thermal expansion (CTE), has provided a substantial increase in the life of C4 joints in accelerated thermal cycle (ATC) fatigue testing on both low-CTE organic and ceramic chip carriers. The C4 joints are encapsulated by dispensing a bead of resin along an edge of the chip. The encapsulation flows underneath the chip by capillary action and completely fills the gap between the chip and the substrate. Optimization of the filler size distribution and resin rheology to get consistent flow under the chip without any bubbles is discussed. The filler size distribution and flow under the chip are shown to cross sections of several different materials including low-alpha-emitting encapsulants for memory applications. Novel encapsulant formulations were tested by videotaping the flow of encapsulant under transparent quartz chips. The formation of bubbles as the encapsulant flows around the C4 joints and irregularities in the surface of the substrate can clearly be seen. Proper C4 encapsulation provides virtually complete coverage around all C4 connections. C4 life testing over various temperature ranges show a 5 to 10 times improvement for both memory and logic footprints when the C4 joints are encapsulated. The vast improvement in C4-joint reliability provided by encapsulation allows the C4 technology to be extended to much larger chips or to higher service-temperature ranges without conventional DNP (distance from neural point) constraints.<<ETX>>

Behavior of aluminum nitride ceramic surfaces under hydrothermal oxidation treatments

AlN ceramics were prepared by pressureless sintering of the nitride powders with mixtures containing yttria, magnesia and silicon nitride at 1900 degrees C. To investigate the sensitivity of these ceramics to hydrothermal treatments, samples were treated in oxygen-containing atmospheres for different exposure times and temperatures. During the exposure, a thin layer of oxide develops and coats the ceramic surface. A variety of physical and chemical methods of analysis were used to characterize the resulting materials. Surface roughness, morphology, and weight changes during exposure were followed. The ceramic phases as formed during the sintering step and additional treatments have been characterized using X-ray photoelectron spectroscopy (XPS), and the photoelectron binding energies for the pure and reacted phases are reported. The XPS technique was also used to follow the surface evolution of the solid/gas interface produced by the surface treatments.<<ETX>>

Probing molecular cages in polymeric gels using paramagnetic ions: Internal motion of cupric ion in a cage

The effect of high‐energy irradiation on the aqueous solutions of certain water soluble polymers, namely, poly(vinyl alcohol) and poly(ethylene oxide), brings about physical and chemical changes. The cross links or networks formed in the polymer chains cause gel formation. These gels are considered to be made up of cubic cells or molecular cages with size varying between 0.6 and 1.5 nm. Such cages have been recently probed using the paramagnetic ions of hydrated cupric as well as vanadyl ions and were studied by electron paramagnetic resonance (EPR) and pulsed EPR techniques. The time‐dependent EPR spectra of Cu2+ ions in the cages are analyzed in this study using specific motional models, employing the modified Bloch equations. Computer simulations show that the spin probe’s motion is governed by the segmental motion of the polymer chains.