Лако-красочные материалы — производство

Materials for use as anisotropically conductive adhesives must satisfy requirements even more stringent than those defined previously for isotropically conductive adhesives. No specifications, however, have been defined specifically for these materials. When used for flip-chip applications, the adhesive not only serves as a physical and electrical interconnec­tion between the device and the substrate, but also serves as the environmental protection and passivation layer. This fact, combined with high adhesive concentrations, makes the ionic contamination levels of these materials more critical than for isotropic conductive adhesives. In addition, the processing of these materials has a greater influence on joint reliability as the anisotropic electrical properties develop only after heat and pressure are applied to the joint.

Numerous geometrical factors of the specific interconnection will also influence anisotropic adhesive formulation and processing, including lead planarity, IC pad metal­lization, and IC test patterns. The planarity of the leads on the substrate and/or device and the compliance of the conductive particles will determine if anisotropically conductive adhesives can be used in a particular application. For systems with large disparities between lead height, no electrical interconnection will be formed, as shown in Fig. 5. Fine-pitch IC packages for surface-mounted applications, such as the plastic quad flat pack (PQFP), often use gullwing leads that offer much compliance to the joint. Even if the

leads are not initially coplanar, sufficient force can be exerted during assembly and cure to achieve electrical interconnection for all leads. Interconnecting flexible circuits and TAB packages to glass or rigid PWBs is relatively straightforward, whereas interconnecting bare silicon ICs to rigid substrates poses the greatest challenge.

Some of the requirements imposed on an anisotropically conductive interconnection used for flip-chip applications arise from the IC chip design. The metallization of the IC chip is usually aluminum or gold. Gold is preferred for the formation of reliable inter­connections because an aluminum surface is coated with an insulating oxide that can be difficult for the conductive particles to penetrate (especially metal-plated polymer spheres). Most features on an IC, except for the bond pads, are passivated with an inorganic film such as Si3N4. However, some test patterns and grids located in the ‘‘streets’’ between chips are not electrically insulated. These metal features can cause electrical shorts between adjacent conductors via metal particle bridging. To prevent these problems, an additional insulating layer can be applied to the chip, or the bond pads can be bumped, raising the level of chip surface.

Data describing the reliability of joints assembled with anisotropically conductive adhesives are incomplete. Several papers have been published, but usually the sample size investigated is small, the accelerated stress tests are not standardized, and the results are highly dependent on device type (e. g., flexible circuit to rigid PWB, surface-mounted components, and flip-chip assembles). Further work is required in this area.