Don Ball, Process Engineer, Chemcut Corporation, State College, PA, USA
The gradual but growing recognition of photo-chemical machining as an alternative to stamping for the production of small, burr-free, stress-free parts and the inevitable trend towards ever smaller and more complex designs has brought the problem of undercut, the ratio of sideways etch to down-ward etch, to the forefront once again. I say this once again because this problem arises every few years with each new generation of design specifications that call for more features to be crowded into less and less available real estate. For instance, the demand in lead frames 10 years ago was for 208 pins with an 8 to 9 mil pitch. Today the demand is for 256 pins with a 5.3 mil pitch in the same space. This, along with an influx of new personnel, has again focused attention on the problem of undercut. (I assume that undercut is a problem since, in over thirty years of experience in the PCM business,
I cannot recall anyone asking how to get more undercut.) Many things have been tried over the years to control or reduce undercut, some based on testing and experience and others based more on gut feelings and intuition. This article will cover the major factors that affect undercut along with test results showing how they affect undercut.