PHOTODIELECTRIC DRY FILMS FOR ULTRA HIGH DENSITY PACKAGING
Author: William E. Estes Company: DuPont Photopolymers and Elctr Date Published: 4/28/1997
Surface Mount International
Abstract: The wiring density of typical plastic ball grid arrays (PBGA’s) and chip scale packages (CSP’S) can be increased dramatically by using photo defined microvias as opposed to conventional drilling plated through holes. Blind microvias, 3-5 roils (75-125 ~m) in diameter, can be fabricated conveniently and inexpensively using photodielectric dry film (PDDF) technology. This PDDF technology utilizes readily available printed wiring board (PWB) fabrication processes that are ideally suited for cost effective manufacture of the surface mount technology (SMT) packages. Hence, PDDF properties and processes must be tailored to meet subsequent methods that are used to attach the silicon chip to its package. Thermosonic wire bonding is the dominant process used in connecting these chips, and it requires demanding thermal and mechanical properties in the non-reinforced PDDF polymer systems. Two different PDDF chemical compositions and thicknesses from two different scale coaters, various PDDF curing thermal process hierarchies, and variations in wire bonding processes were used to achieve wire bond pulls of 10-11 grams using 25 ~ gold (Au) wire. Thickness and PDDF composition were found to be important variables for constant process conditions.