Excimer Laser Drill for Multilayer Printed Circuit Boards: From Advanced Development to Factory Floor

Extract

A novel excimer laser process has been developed for generating cylindrical via holes with an aspect ratio of about one. The fabrication process is being successfully run on a production line for a highly miniaturized printed circuit board used for the multichip module in the new Siemens 7500 H 90 mainframe computer. The process is outstanding in terms of reliability and reproducibility. To the best of our knowledge, this is the first that that excimer lasers have been put into large-scale use in an industrial environment.

Since signal delay times for chips have decreased much more rapidly than delay times for packaging, the computing speed of high-speed computers is restricted by the packaging techniques used. Therefore, further development of packaging technology became a prime objective for those developing high-performance computers. Packaging delay times had to be reduced drastically to keep up with increasingly shorter chip delay times. This, in effect, meant that a greater packaging density had to be implemented.

A novel planar packaging technique has lead to considerable progress in solving this problem. This technique has been described in detail elsewhere. A key component in this technology is a multichip module, which can take in each of 16 areas, either an LSI module with 320 leads or 9 MSI modules with 52 leads as “bare” ICs. This means that a micro-wiring printed circuit board of this kind can accomodate between 16 (LSI) and 144 (MSI) chips. This article describes how these printed circuit boards are manufactured.

As the specifications (Table I) show, blind vias 80 μm in diameter at a pitch of 0.5 mm have to be made in a 16-layer printed circuit board. It is intended that these blind vias will provide the through-contact for neighboring layers. The excimer laser plays a major role in this process.