Chemical Aspects of Printed Wiring Printed wiring, as we know it today, had its real beginning in the proximity fuse of World War 11. Since then, as a replacement for conventional point-to-point wiring, its use has grown by leaps and bounds. It is estimated that approximately 60,000,000 printed wiring boards, of average size 4 X 6 inches, will be produced in 1959. In the past symposia have dealt with printed wiring-with what might be termed the unit operations of printed wiring production (etching, plating, soldering, etc.) without emphasis on the chemical nature of the processes that occur in these operations. There have also been symposia dealing with applications of printed wiring, both general and particular, by and large for electronic engineers and production men. The chemist, as a chemist, has been on the outside, looking in. It is hoped that this symposium is a significant reversal of the trend. It is the first to deal specifically with chemical aspects of printed wiring. Its purpose is to direct attention to a previously neglected facet of the art, to provide a common meeting ground for dissemination of new knowledge and exchange of ideas by those interested in the chemistry associated with the art, and to stimulate research directed toward understanding the chemical processes involved both in producing a printed wiring board and the changes in its behavior in use. Only with understanding will come the ability to translate design ideas rapidly into reliable printed circuits.
LOUISH. SH-ARPE Bell T e l e p h o n e Laboratories, I n c . ?rIurra>- Hill, N. J.
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E. B. SAUBESTRE' Sylvania Research Laboratories, Division of Sylvania Electric Products Inc., Bayside, N. Y
Chemical Problems in Printed Wiring The most pressing problem is development of accelerated testing procedures for predicting the performance of a board
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CONKECTION with wider usage of printed wiring, many chemical problems have arisen, Xvhich can be broadly subdivided into nvo groups: those associated with producing printed circuits, and those associated with electrical performance of boards in the field. This discussion is confined to a general presentation of the many problems facing chemists in this field, and does not include chemical problems in development and manufacture of laminates. solders. and fluxes.
Production of Printed Circuits
Stop-Offs (Resists). \\'hen unwanted copper is etched away from a copper-clad Present address. Enthone, Inc., Nc\v Havrn 8, Conn.
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laminate, a stop-off. or resist? must he first applied to protect the pattern. T h e techniques are conventional and include photo-resist and silk screening. Conventional resist materials d o not completely meet the detailed requirements of printed wiring. This is surprising, because these materials are satisfactory in photoengraving and other uses where even finer detail is required. T h e main differences seem to lie in the roughness. unevenness, and absorptive properties of the laminates. The most serious problem is pinholing, which often causes rejection of the printed wiring board. Pinholing involves four variables: resist composition, methods of application, development or setting, and cleaning and etching. Misuse of any of these four parameters will lead to trouble-for example. inadequate cleaning or a film of resist that is too thin will lead to pinholes. Development work has centered about the cause of pinholes when all four variables are seemingly under control. Developing conditions must be defined more carefully than in the past, and choice of etch influences spread of pinholes. Thus, \Then the resist is changed, both the etch and conditions must be re-evaluated. But because of technical and economic rea-
sons: this may not he simple. Therefore, one conclusion is inevitable: Further development work is required in the composition of resist materials. Solder resists can be used to avoid pinholing. I n this method, a reverse stopoff is applied, then solder is plated onto the laminate. If the organic stop-off contains pinholes. small spots of solder plate in unwanted areas are the only damage ; these are readily brushed away ivhen the reverse stop-off is removed. The unwanted copper is then removed by etching in a medium which \vi11 not attack solder. If the copper was properly cleaned prior to solder plating, little if any pinholing should occur in the pattern. Etching. Many materials etch copper at economically feasible rates. However, to prevent attack or contamination of plastic laminates, attack or pinholing of resists. and subsequent contamination of boards from residues difficult to rinse off, the etchants of interest are largely limited to chlorides (ferric or cupric), nitric acid, chromic-sulfuric acid, and peroxysulfates. CHLORIDES.Use of ferric and cupric chloride etches is the most satisfactory commercial method-these etches stabilize and complex the cuprous ion and VOL. 5 1 , NO. 3
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