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. >

Encapsulants used in flip-chip packages

In this paper the rate of moisture ingress into enclosures was studied theoretically and experimentally. A simple quasi-steady state (QSS) model was developed enabling one to calculate easily time constants for moisture diffusion through plastic walls, rubber gaskets and openings. The characteristic time constant is /spl tau/=VL/AP+L/sup 2//2D where P is permeability, D diffusion constant, V volume of the enclosure, L wall thickness and A surface area. The model clarifies the relative importance of moisture permeability vs. diffusion constant of wall materials and is applicable to both large enclosures and microelectronic packages. For thin and/or nonabsorbing walls the first term (a function of P) predominates while with thick and/or absorbing walls the second term (which depends on D) prevails. For openings, /spl tau/=VL/AD, it was shown that the QSS model is practically equivalent to but simpler than full transient solutions of the Fick's second law. The attenuation of variations of atmospheric humidity by packaging was also modeled with the QSS model. The inside air humidity changes with the same frequency as the ambient but its amplitude is attenuated by a factor f=cos /spl phi/=1/(1+/spl omega//sup 2//spl tau//sup 2/)/sup 1/2/ and is phase shifted by an angle /spl phi/=arccos [1/(1+/spl omega//sup 2//spl tau//sup 2/)/sup 1/2/] where /spl omega/ is the angular frequency of humidity changes. Therefore the protective value of packaging will be different in different geographical areas. >

Moisture ingress into nonhermetic enclosures and packages. A quasi-steady state model for diffusion and attenuation of ambient humidity variations

Bio-electrodes have traditionally been made of materials such as metal and silicon that are much stiffer than the tissue from which they record or stimulate. This difference in mechanical compliance can cause incomplete or ineffective contact with the tissue. The electrode stiffness has also been hypothesized to cause chronic low-grade injury and scar-tissue encapsulation, reducing stimulation and recording efficiency. As an initial step to resolve these issues with electrode performance, we have developed and characterized electrically-functional, low-Young’s modulus, microcable-shaped neuroelectrodes and demonstrated electrophysiological recording functionality. The microcable geometry gives the electrodes a similar footprint to traditional wire and microwire neuroelectrodes, while reducing the difference in Young’s modulus from nervous tissue by orders of magnitude. The electrodes are composed of PDMS and thin-film gold, affording them a high-level of compliance that is well suited for in vivo applications. The composite Young’s modulus of the electrode was experimentally determined to be 1.81 ± 0.01 MPa. By incorporating a high-tear-strength silicone, Sylgard 186, the load at failure was increased by 92%, relative to that of the commonly used Sylgard 184. The microcable electrodes were also electromechanically tested, with measurable conductivity (220 kΩ) at an average 8% strain (n = 2) after the application of 200% strain. Electrophysiological recording is demonstrated by wrapping the electrode around a peripheral nerve, utilizing the compliance and string-like profile of the electrode for effective recording in nerve tissue.

Highly-compliant, microcable neuroelectrodes fabricated from thin-film gold and PDMS.

Theory of aluminum metallization corrosion in microelectronics

Poly (trivinyl-trimethyl-cyclotrisiloxane) or polyV(3)D(3) is a promising insulating thin film known for its potential application in neural probe fabrication. However, its time-consuming synthesis rate renders it impractical for manufacturing standards. Previously, the growth mechanism of polyV(3)D(3) was shown to be affected by significant steric barriers. This article describes the synthesis of a copolymer of polyV(3)D(3) via initiated chemical vapor deposition (iCVD) using V(3)D(3) as the monomer, hexavinyl disiloxane (HVDS) as a spacer, and tert-butyl peroxide (TBP) as the initiator to obtain nearly a 4-fold increase in deposition rate. The film formation kinetics is limited by the adsorption of the reactive species on the surface of the substrate with an activation energy of -41.5 kJ/mol with respect to substrate temperature. The films deposited are insoluble in polar and non polar solvents due to their extremely crosslinked structure. They have excellent adhesion to silicon substrates and their adhesion properties are retained after soaking in a variety of solvents. Spectroscopic evidence shows that the films do not vary in structure after boiling in DI water for 1 hour, illustrating hydrolytic stability. PolyV(3)D(3)-HVDS has a bulk resistivity of 5.6 (±1) × 10(14) Ω-cm, which is comparable to that of parylene-C; the insulating thin film currently used in neuroprosthetic devices.

Incorporation of Linear Spacer Molecules in Vapor Deposited Silicone Polymer Thin Films

Electrochemical failure of microelectronics is related to surface impurity levels, to temperature, and to relative humidity (RH). Contamination-by-design experiments were performed in order to investigate the interplay among these factors. Under dry conditions (0% RH), leakage currents were found to be small ( approximately 1-10 pA) and insensitive to surface contamination levels, implying that electrochemical IC failure becomes vanishingly small under dry conditions. At 100% RH, steady-state leakage currents were large ( approximately 10-100 mu A) and roughly proportional to surface loadings. Individual chemical compounds were found to exhibit step increases of leakage current at critical RH values corresponding to solid-to-saturated solution transitions. Aqueous droplets, or vacuoles, were seen visually at surface sites occupied by solid deposits. Variable-temperature studies, made on contaminated specimens at constant external water vapor partial pressure, exhibited a step decrease of leakage current at temperatures corresponding to the critical RH. Sucrose, a nonelectrolyte, showed leakage current steps similar to those observed with NaCl and CaCl/sub 2/. Vacuole formation is considered in some detail. >

Polymer encapsulants for microelectronics: mechanisms for protection and failure

Novel Coatings That Maintain Low Surface-Water Concentrations

This research relates electrochemical failure of encapsulated microelectronics to surface moisture and to surface impurities. It draws heavily on “osmotic blistering”, a phenomenon known to produce corrosive failure of coated/painted metals [1]. Leakage currents were measured on aluminum comb specimens as a function of relative humidity (RH) and temperature. We studied both bare combs and combs encapsulated with polysiloxane. 9.05 Vdc bias was used. We performed “contamination-by-design” experiments by deliberately introducing known amounts of NaCl, CaCl2 and sucrose onto the comb surface. Results were compared with corresponding data taken on well-cleaned specimens.

Polymer Encapsulated Microelectronics: Mechanisms of Protection and Failure

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Polymer encapsulants for microelectronics: mechanisms for protection and failure

Novel Coatings That Maintain Low Surface-Water Concentrations

Polymer Encapsulated Microelectronics: Mechanisms of Protection and Failure