A pcb will etch more rapidly along the edges of the board and next to the areas covered with resist. Along the edges of the board you have the side edge of the copper exposed as well as the flat copper face, so that partially explains that phenomenon. There are other forces at work, and these other forces also cause the higher rate of etching that you notice near traces. 1) the etchant gets depleted more rapidly when there is more copper surface exposed. The partially depleted etchant is still active, but does not eat away the copper as fast as fresher, less-depleted etchant. This effect is even more noticeable when the etchant is moving. The edge of the board that is first touched by the moving etchant will etch much faster than any other part of the board. This is one reason why it is a good idea to rotate a board several times during the etching process. 2) Sharp edges will etch faster than smoothly curved edges. Small curves (such as pads) will etch faster than large curves. This is partially accounted for by the tendency for ionization to occur at sharp edges. There will be a tendency for undercutting to occur, especially along traces, since the more rapid etching along the edge of the trace exposes the side to etchant action EARLY in the process, and etching of the sides of the traces continues as the larger copper areas are being etched. 3) VERY small holes usually take longer to etch than a medium size hole. A small hole does not get a good flow of fresh etchant into it, as the depleted etchant will tend to linger at the site. This can be overcome to some extent by agitation of the etchant. Making the etchant flow will increase the etching rate, but you should then rotate the board at about one or two minute intervals. You can minimize the problem of undercutting by ensuring that there are no large exposed areas of copper to be etched away. Use these areas instead as part of a ground plane (it is not a good idea to have isolated floating islands of copper. Ground them.) If the board is large, you can make these ground areas "hatched" instead of solid. This can help minimize warping in large boards. Fr. Tom McGahee ----- Original Message ----- From: Jinx To: Sent: Monday, July 24, 2000 8:14 AM Subject: Re: [EE]: PCBs using toner transfer paper > Just a point of curiosity that I got reminded of today - > > Can anyone offer an explanation of why etchant will take copper > off a board firstly immediately next to the traces and then work > its way to the open spaces ? Noticed it again when etching a board > this afternoon. As I've said B4, I float the PCB upside-down on > the FeCl and can see the pattern develop. All the traces and pads > appear with a thin gap around them, then nothing seems to happen > for a minute or two, then the open spaces just clear as the last thin > coat of copper falls off. I've tried to reason it out but can't explain it. > Does this happen just with an unagitated etching (like mine) or does > it also happen in agitated tanks ? > > Just one of those nutty little things I wonder about > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu