ornitz@kodak.UUCP (10/30/87)
In article <8710300607.AA17884@ucbvax.Berkeley.EDU> hoffman@pitt.CSNET (Bob Hoffman) writes: > >I, for one, would like to know the exact process by which plated-through >holes are made on double-sided boards, "nasty" chemicals and all. They >can't be any worse than the stuff they're playing with in the Chemistry >Building next door... > > ---Bob. > The following is from memory so it may contain a few holes. :-) You may start with either a bare board or a double sided one. The holes are first drilled using silcon carbide bits for smooth edges and close tolerances (more on this later). The drilled board is soaked in a catalyst solution (I believe an organometallic salt of palladium but I am not sure) to sensitize the holes for the next step which is the electroless plating of copper. A very controlled thickness of electroless copper is produced in the holes (and over the entire board). The board is then electroplated to produce a hard copper finish and build up the copper thickness in the holes. Note that both plating steps reduce the diameter of the holes. This is a critical step in the process and is why the close tolerances are needed. Conventional resist is then applied and the board is plated with solder where the traces should be. After solder plating and resist removal, the board is etched to remove the copper not protected by the solder. Finally, a solder mask is generally applied. As you can see, the basic process is not too complicated. However, a number of cleaning steps in between the above have been left out. Also, a fair degree of experience is needed to control the various plating steps. Dimensional control of one to two mils is needed even on crude boards. I would suggest that the production of plated-through holes is beyond the capabilities of even the most dedicated home experimenter. I generally pay a commercial PC house about $250-$500 for set-up charges for a typical board about 6x6". After the initial set-up, the price per board is generally $10 to $25. With these prices, it is not worth the hassle to even try to produce my own plated-through holes. If you are careful in your design, you can often eliminate the need for most plated-through holes. Use wire-wrap sockets (or even better - machine pins) so the sockets can be lifted enough from the board to solder both sides; also it is often possible to extend the traces next to inaccessible components so a wire jumper can be used to make the through connection. For a better discussion of all of this, buy Steve Kasten's book, "Electronic Prototype Construction" (ISBN: 0-672-21895-X, Howard W. Sams, pub.) for $18 or so. Several net users have commented favorably on this book. I am prejudiced because Steve is a close personal friend, but I feel the book is well written and very practical. I can also pass on questions to Steve and send replies back over the net. Barry WA4VZQ ----------------- | ___ ________ | | | / / | | Dr. Barry L. Ornitz UUCP:...!rochester!kodak!ornitz | | / / | | Eastman Kodak Company | |< < K O D A K| | Eastman Chemicals Division Research Laboratories | | \ \ | | P. O. Box 1972 | |__\ \________| | Kingsport, TN 37662 615/229-4904 | | -----------------