The "Tuf-Plate Hole" for Printed Wiring

EXAMINATIONS of the parameters effecting the reliability of thru connections indicate the following major factors: compatibility of the metal used in the hole with the surface metal, the condition of the laminate in the vicinity of the hole wall, and the relative stresses introduced by environmental cycling. Although advances have been made in all methods of side-to-side connection, most users are convinced that the electrolytic deposit of copper offers the most sound solution. Photocircuits Corporation, therefore, has expended considerable time to the improvement of the plated-thru-hole, and has successfully developed the "Tuf-Plate Hole." This trademark covers the process as well as the product in which the individual parameters of the hole are produced, controlled, and inspected. The manufacturing process can be divided into three maj or operations: 1) Prefabrication--New tooling which is being employed to produce a clean hole free from burrs and delamination is the beginning of the "Tuf-Plate Hole." With existing laminates, hole preparation requires additional treatment to produce the type of surface which will accept the sensitizer and prevent high current density points for hole size control and uniform plating. 2) Sensitizing--The sensitizing step has long been a matter of great concern and secrecy. This operation has improved to a point where the metal can be applied without degradation of the hole wall or interlaminate bond between the copper surface and plastic. Photocircuits has found that adherent copper reduction provides the most acceptable solution to this problem. Process changes and associated problems during the evolution of this sensitizer were discussed. 3) Plating--Plating the prepared boards employs a technique of controlled current density in a plating solution which provides the throwing and leveling characteristics essential to the "Tuf-Plate Hole." Controls must be applied to this process to insure the maximum hole-to-surface plating ratio, deposition of fine grain copper, and adhesion to the base copper. Checks on these controls are provided by ductility tests, hardness measurements, and observations through the use of cross sections of the boards. Over-all observation of the effectiveness of the process is made through tests designed to compare the board with the conditions encountered in operation. Photocircuits tests include heat-shock, high-current burn out, and flexural cycling for intermittence. Additional environmental tests of high- and low-temperature endurance and cycling have been completed with success. In order to substantiate the qualities of the copper plating, vibration tests as well as the procedures outlined above have been made with the test boards without additional surface platings. These results indicate that the user may select the finished plating for the storage or soldering requirements without regard to the added mechanical strength. Reliability as represented by strength alone is not complete. Degradation of material thru process contamination will destroy a printed-wiring board's function electrically. Photocircuits Corporation has used this parameter in the development and use of all process solutions required for the "Tuf-Plate Hole." Final etching of the wiring has been changed to eliminate the problems previously confronted in storage life and electrolite contamination. The results of the manufacturing procedures and tests were provided by the author. Photomicrographic displays of process sections were presented to illustrate problems and their solutions.