High‐density wiring technology in multiwire boards

Abstract

AbstractThis paper considers the recent requirement for high‐density packaging technology and discusses the high‐density wiring technique for the Multiwire board. A proposal is made for the wiring rule as well as the fundamental manufacturing processes, which can realize twice higher wiring density compared with the traditional method. The Multiwire wiring employing the insulated wires as conductor can realize the wire crossings on the same plane. However, the technique has the following problem. When thin wire is used and the wiring pitch is reduced to realize a high‐density wiring, the strength of the wire is reduced, its adhesion to the board is degraded, and an interference with the adjacent wire due to stylus is produced. Then there are dangers of producing such defects as wire breakage, floating wire, position shift and flaws in a wire insulator. Those phenomena place a limit on the high‐density implementation by reduciong of the wiring pitch. From such a viewpoint, the diagonal wiring is newly utilized positively, leading to the wire breakage prevention at three or four wire crossovers which has a high breakage probability. Based on the estimation of the number of terminals and the terminal pitch of the LSI package in the future, the required wiring density is calculated. The industrially practical accuracy is assigned to the manufacturing processes, and considering those situations, the high‐density wiring rules are discussed. As a result, the following wiring rules are derived. The through‐holes are located by a 2.54‐mm grid and at the center, and eight passings (five orthogonal plus three diagonal) in a 2.54‐mm grid can be made as the highest wiring density. In addition, some intermediate wiring rules for five, six and seven passings are also presented. An example is shown in which the new wiring rule is applied to the product based on those investigations.