Acid traps - Page 1
Forum section: electronics / general

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 02, 2022, 06:27:07 pm
content: I don't know if I should ask here or in general. But are acid traps still a concern?I don't really understand in any case. An acid trap has always been explained to me as the tendency for a liquid etching solution to stay "trapped" in acute angles of copper because of surface tension I suppose.But... aren't boards etched in solution? So where's the "surface" for any tension?And even if this effect happens when the board is pulled out of the solution, is the acid now more powerful because it is in the air and is now etching at 1000% its rate in solution? In other words, the time the board has the acid trap is quite small, no?And boards are etched with alkaline solutions, at least for inner layers from what I understand. But yes indeed, chloride is still used for outer layers.I thought boards these days are first imaged and exposed and etched on a very thin layer of copper which is then plated up to the required thickness. Is this where a trap effect occurs? But isn't this still in solution, probably agitated or pumped?In any case, an acute angle is rare and I tend to avoid them anyways, but, is a 90 degree corner like a trace coming out of a rectangular pad an issue?Isn't it more of the PCB manufacturer's job to prepare the Gerbers to their process rather than me trying to cosmetically beautify a layout with teardrops that may or may not be actually useful for manufacturing but take up a lot of memory and slow down my tool and generate mountains of DRCs?

author: nctnico
author_rank: Super Contributor
timestamp: February 02, 2022, 06:56:58 pm
content: Acid traps aren't a concern. Something from a forgotten past.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 02, 2022, 06:57:57 pm
content: That's what I thought. But ....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2022, 07:04:20 pm
content: Right, the surface is when the board is pulled out, rinsed, and acid remains behind in pores, concavities, etc. Etching rate in those pores need not be very much at all, after all it'll consume the etching capacity of that tiny amount of solution very rapidly; the problem is trapping salts in the pores of the metal, under soldermask, etc., to eventually absorb enough moisture and oxygen to cause breakout and continuing corrosion problems. Chlorides are pretty nasty with copper, it doesn't take much. And they don't evaporate on heating, the water does but there's no guarantee the deposit remains dry forever. (Hydrous cupric or ferric chlorides do decompose a bit on heating, but some chloride is always left.)Probably, the etching is better controlled than in olden days, leaving a smoother surface with less porosity? A more thorough rinse/wash step might also be relevant. Say, using a neutral (non-oxidizing) complexing solution to dissolve residual copper chloride/oxide.And yeah, if nothing else, might as well avoid them [acute angles] because they're ugly.Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 02, 2022, 07:19:57 pm
content: So I was kinda right, but the problem is the junk that persists in these traps after etching. Gotcha.Now. Haven't manufacturers solved this issue?I really wonder about a simple 8 mil trace coming out of a simple rectangular pad, that's two 90 degree corners on each side of the trace. Is this enough of a problem?If it were, I just don't see the graveyard of boards with "acid trap" written on the obituary.35 years on and my C64s still work. To me, and my experience doesn't include the whole world or the entire range of PCBs and EE, but "Acid trap" is like Beetlejuice or Slenderman, something to scare the kids. No?

author: thm_w
author_rank: Super Contributor
timestamp: February 03, 2022, 01:43:13 am
content: Its mostly to scare the kids into keeping with convention.https://www.nwengineeringllc.com/article/top-emc-design-myths-to-avoid-in-your-pcb-layout.phpBut, free_electron is saying there could be issues <4mil with 1oz+ designs:https://www.eevblog.com/forum/eda/no-!-you-don_t-need-to-worry-about-90-degrees-corner-at-pcb-trace/and also with flex PCBs (which makes sense, tons of niche layout tricks are needed on flex PCBs).If you are using standard JLC 5mil+ clearances, there should never be a problem.

author: thinkfat
author_rank: Unknown
timestamp: February 03, 2022, 03:28:22 am
content: Acute angles are ugly. That's my routing mantra.

author: Bassman59
author_rank: Super Contributor
timestamp: February 04, 2022, 08:28:29 pm
content: Quote from: thinkfat on February 03, 2022, 03:28:22 am... but the electrons don't fly off the ends of the trace!