Conductive particle-filled adhesives have been widely used for flex-to-rigid board interconnections in many consumer electronics, such as calculators and palmcorders. Most of the applications were in coarse pitch interconnections where the adjacent conductor's distance is greater than 0.4 mm. The success of coarse pitch applications has increased the interest to use such adhesives in fine pitch applications, such as flip-chip on board interconnection. Since these materials contain metallic particles to conduct currents in the z-direction (i.e., perpendicular to the plane of circuit board), their propensity for metal migration is a concern. Therefore we have applied accelerated temperature, humidity and bias (THB) tests to a group of materials designed for fine pitch applications. The accelerated life test conditions were 85-degrees-C/85%RH at three different voltages: 10 V, 50 V, and 100 V. The studies were focused on the samples' time-to-failure as well as the associated conduction and failure mechanisms. The test results showed significant metal migrations, and we are proposing enhanced electric field streSseS (10(2) to 10(4) V/MM) as the driving force for failures.

Conductive adhesives have been used in the electronics industry for several years to attach chips to package lead frames in the semiconductor industry and for general interconnection of components to flexible circuits for various consumer products. Generally, these materials conduct equally in all directions. To obtain pad isolation, the adhesives are screen printed to the pattern of the circuit pads. In the last few years, a new class of adhesives that are conductive in a single direction have been developed. These are referred to as Anisotropic Conductive Adhesive Films (ACAF). These anisotropically conductive adhesives provide electrical as well as mechanical interconnections for fine pitch applications. The conductivity of ACAF materials is only in the Z-direction (perpendicular to the plane of the board) while electrical isolation is maintained in the X-Y plane. Currently, at least 15 ACAF materials are commercially available. We have developed a methodology for evaluating these materials for their mechanical and electrical properties and interconnection use in the 8 to 15 mil pitch range. In addition, we characterized the materials as to their physical properties and cure characteristics. This paper details our findings with a comparison of physical form to assembly/cure and final electrical properties. We include in this study data from scanning electron microscopy, thermal analysis of the ACAFs, and cure and assembly studies on mixed substrate test vehicles. Information on initial electrical testing and long term reliability testing is also given.< >

A family of anisotropically conducting metal-filled polymer composites that are representative of ACPF (anisotropically conducting polymer film) has been developed. These composites are collectively referred to as elastomeric conductive polymer interconnection (ECPI) materials. The ECPI materials are metal-polymer composites with conduction isolated to the thin (or Z) direction, as are all ACPF materials. These materials offer unique advantages over more conventional technologies used for module attachment, high density connectors, socketing, and device testing. ECPI materials are well suited to interconnecting pad-grid arrays and, because of their electrical properties and mechanical compliance, they can accommodate short-range surface variations of several mils and transmit high-frequency signals without distortion, and their through-contact resistance is low. In general, ECPI needs only the application of pressure to give interconnection, but a group of design criteria was developed to assure optimal performance. The ECPI structure is discussed, the electrical and mechanical properties of typical ECPI interconnections are described, and design guidelines for using ECPI in connector and testing applications are summarized

In this paper, we present the most severe under-damped case of surge test on high K multilayer ceramic capacitors (MLC's), in which the surge current is determined by the inductance of electrical connection and a small current limiting resistor (0.5-OMEGA). At a time scale less than 20-mu-s upon applying a voltage, V(o) (0-275 V), the surge current is oscillatory, with the peak current increasing with V(o). The voltage across the MLC is also oscillatory with the peak voltage exceeding twice V(o). The impact of surge current, voltage, and power on relaxor and barium titanate MLC under test is discussed. While both types of MLC can sustain similar current and power density, the failure of the relaxor MLC at a lower applied voltage (175 V) is related to the lower breakdown strength of the relaxor dielectrics. For 50-V rated MLC, the peak transient voltage should not exceed 125 V. Thus both relaxor and barium titanate MLC will perform satisfactorily under normal surge conditions.

Observations on sectioned and polished multilayer ceramic capacitors, which were subjected to controlled current-surge test conditions, are presented. Capacitors from several vendors were examined in situ under an optical microscope while current pulses of varying magnitude were applied at a constant voltage. Subsequently, some samples were further examined by scanning electron microscopy (SEM). The failure mechanism appeared to be the heat-induced local melting of internal electrodes, leading to blow-out or charring of the capacitor. In less severe cases, local melting and crack formation in the surrounding ceramic was observed. The primary change in capacitor properties was in the degradation of the insulation resistance (IR). In severe cases, this also led to an increase in the dissipation factor (DF).<>

Electrical resistivity studies were performed on multilayer ceramic capacitors (MLC) based on lead magnesium niobate and containing dopants of lead titanate, lead zinc niobate, and lead cobalt niobate. The results showed that lead titanate and/or lead zinc niobate had no effect on the electrical resistivity while lead cobalt niobate decreased the resistivity. In samples without lead cobalt niobate, the authors observed a conduction mechanism with an activation energy of ~1 eV, which is commonly observed in barium titanate based dielectrics. This is attributed to ionic conduction by the motion of oxygen vacancies. The increase in conductivity (or decrease in resistivity) resulting from the addition of lead cobalt niobate was attributed to electronic conduction through charge hopping among the cations. This conduction mechanism was characterized by an activation energy of ~0.5 eV. The transition between electronic and ionic conduction was a function of temperature and the concentration of lead cobalt niobate. Since the activation energy associated with the long-term failure was previously determined by a matrix of temperature and voltage accelerated-life tests to be ~1 eV, the authors conclude that conduction through charge hopping is not affecting the long-term reliability of these devices

The partial discharge characteristics of barium titanate multi-layer ceramic capacitors were investigated under 60 Hz voltage excitation. For capacitor specimens which were specifically fabricated for this study, the partial discharge characteristics were found to depend strongly on such para, meters as dielectric film thickness, relative permittivity, porosity and void size distribution.

Electric stress distribution within dielectric cavities are studied using the finite-element numerical technique. The results indicate that the electric stress is enhanced within a gas-filled cavity embedded in the dielectric. The amount of stress enchancement and the electric field distribution within the dielectric are significantly influenced by the dielectric permittivity, the dimensions of the cavity, the number of cavities in series within the dielectric gap, the cavity orientation, the dielectric thickness (electrode spacing), the contour of the edge of the electrodes, and the presence of floating electrodes near electrode edges. The effect of various parameters on the stress distribution within a dielectric is important in the design, fabrication, and testing of devices made of multi-layered dielectric/electrode film structures.

Pd characteristics for MLC capacitors with different relative permittivities, porosity, and void size distributions were evaluated. The results show that: 1. Pd is a strong function of and increases with the increase of the applied field. 2. Higher dielectric constant, which gives a bigger field enhancement, also enhances the Pd activity. 3. The source of Pd is the ionization discharge across the voids in the bulk as well as those adjacent to the electrodes. Knowing the void size distribution, the Pd characteristics are generally predictable. 4. The cause of low breakdown for a dielectric with high porosity is that the regions of thin dielectric associated with porosity cannot withstand a high field as the voltage across a void drops significantly by the partial discharge.