Olin Lathrop embedinc.com> writes: >=20 > Oli Glaser wrote: > > longer etch times mean (as you probably know) > > more undercut and less accuracy >=20 > Why? The etch time needs to be longer when the process is slower, but > you're implying that lateral etching under the mask then somehow slows do= wn > less than etching the exposed copper from the top. Why would that be? > I am assuming that longer etch time does have more undercut. If it is true= , than diffusion will be a rate-limiting step in the chemical reaction respon= sible for oxidation of copper and it's removal. With fast reaction, diffusion li= mits undercut, with slow reaction diffusion is fast enough to be not important.= =20 Agitation which is often used during enthing, should have limmited effect o= n removal of used up components in the area of undercut, since flow rate in z= ero at the surface. Agitation does help removing copper from flat surfaces, si= nce flow will be more turbulent at the surface (as opposed to corner formed by = PCB and trace) and will provide supply of fresh chemicals. =20 Other aspect is that masks are not perfect, so with the slow process, the exposure to etchant is longer, therefore more time to extent damage at imperfections in the mask, probably by the same mechanism.=20 Please note, this is my potential explanation from chemists point of view, = but not verified by experiments.=20 Sergey Dryga http://beaglerobotics.com --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .