Лако-красочные материалы — производство

1. Particle Specifications

The goal in formulating anisotropically conductive adhesives is to maximize particle con­centration without compromising electrical isolation in the X-Y plane. Higher particle loadings increase the probability that an electrical interconnection will be made (especially for relatively small contact areas) and decrease contact resistance. Typical concentrations range from 5 to 15 vol % (30 to 60 wt % based on pure silver particles). The size of the particles usually ranges from 10 to 20 pm in diameter. Smaller particles offer the best results for very fine pitch applications.

To lower the probability of conduction in the X-Y plane (i. e., reduce the short-range percolation coherence length X), particles are used with an aspect ratio as close to 1 as possible. In contrast, isotropically conductive systems use flakes with high aspect ratios as fillers. Particle size distributions are minimized so that each particle can potentially serve as an electrical bridge between substrate and device.

Numerous types of particles are used in anisotropic adhesive formulations. Silver has high electrical conductivity and good resistance to corrosion; however, electromigration may cause problems in some applications. Nickel is a lower-cost alternative, but corrosion has been reported during accelerated aging tests. Solder particles offer the opportunity to form fusible linkages [53]. Gold offers the best properties, but the cost may be prohibitive for large-volume applications. Plated glass or polymer particles provide a lower-cost solution. Using a particle with a polymer core offers additional advantages, including low aspect ratio, good particle size uniformity, and compliance, that can help accommo­date nonplanar surfaces.

The current-carrying capabilities of different particles will influence particle selection and concentration for a particular application. Measurements of several different materi­als have been reported [16] that exhibit a range of almost two orders of magnitude, as shown in Table 5.