Gold plated PCB or alloy PCB? - Page 1
Forum section: electronics / projects

author: OM222O
author_rank: Frequent Contributor
timestamp: February 18, 2019, 02:27:12 am
content: Hello guys, I know this might be a weird one but I just watched this video from Cody's lab where he coats copper with different materials and bakes them in an oven to get bronze and brass:It is a very thin coating, just like the ENIG which should be about 1 mil (standard PCB has about 1.4mil copper thickness), so I was wondering if gold plated PCBs actually become uniform alloys after going through the re flow oven and if yes, how much would this change the conductivity of the traces? (gold is a slightly worse conductor)Again I'm not sure what temperature and for how long they were cooked in the oven to form alloys so this might be totally irrelevant so sorry for being a noob in advance!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 18, 2019, 03:04:02 am
content: Nope -- with good reason.1. The thickness of coating he applied, was on the order of microns -- I guess you could actually figure it out from the time*temp it was baked at (if we had this detail), the color achieved (the brass looks pretty light, maybe in the 30-40% Zn range at the optical surface*), and the diffusion rate of Zn and Cu.*Which is in the top, oh, not quite 100nm or so. That's still thousands of atoms, so the concentration within that layer can still vary quite a bit! The reflected color would correspond to, roughly, the average composition in that layer, I think.2. ENIG, as the name suggests, isn't just gold. The nickel is put there as a diffusion barrier -- for whatever reason (I don't know, actually; I should see if there's research on why), nickel diffuses very slowly in neighboring metals (Cu, Fe, etc.). This makes it fantastic as a barrier or buffer layer, between a base metal and plating which diffuse together too readily otherwise (e.g, Sn, Zn, Ag, Pd, Au and others in Cu). It's also nearly inert, making it great on top of base metals that are too reactive to plate directly (e.g., Zn, Al) -- assuming you can get the nickel on there successfully in the first place, of course.Incidentally, Sn and Cu diffuse reasonably well (as also seen in the video, sort of -- the result is a bit patchy), it's just that we can afford to put on a heavy layer, and we slather it with more during soldering anyway, so it's not much of a problem. The interface layer does contain brittle intermetallics, so you do want to avoid cooking it for too long (at soldering temperatures, that'd be... hours+?) which would result in that layer growing in thickness, and therefore weakening the material, and the solder joint.3. So because of the barrier layer, the gold dissolves quite easily into the solder, at least at typical thicknesses and soldering temp profiles. You do want to have enough solder and time to ensure it's dissolved, otherwise the Sn-Au intermetallics are particularly weak.Nickel does dissolve in molten solder, but not nearly as fast, so it would take quite a long time to expose the copper directly to solder. Which I suppose is nice because it would eat through the copper foil in...about as much time (~hours)??, assuming there's enough solder present and it didn't have much Cu dissolved in it to begin with of course.Metallurgy is cool stuff, chemistry with shiny solids and liquids (and rarely gasses*).*Well, not the shiny part (gasses are mostly clear), but, notable low boiling points include Zn, Mg, Mn, alkali metals and others. Interesting to note there are a number of important alloys that are handled near or above the boiling point of their main addition -- namely, brass (and most bronzes) having zinc (boils 907C) but poured at ~1050C, and wear-resistant steels having manganese (2061C) but poured at ~1700C. Magnesium (boils 1091C) is also used in small amounts with ductile iron (~1500C), and notable amounts (and also zinc) with aluminum (poured around 700C). All of these have notable vapor pressures when molten; but just like alcohol in cooking, it doesn't actually evaporate all that quickly, so you can quite easily finish a process without changing the alloy much. Keep that in mind next time you make, say, a pumpkin pie with a couple shots of bourbon mixed in -- a slice of that is enough to feel a slight buzz.Tim

author: OM222O
author_rank: Frequent Contributor
timestamp: February 18, 2019, 03:19:18 am
content: Yeah I need to up my game when it comes to metallurgy! Thanks for the detailed answer, I really had no ideaJust one question remains: If we want to make sure that the gold is actually dissolved in tin, why do we even bother with ENIG to begin with? wouldn't just a nickel coating do the same job at a lower cost? (I know it's not much gold at all, but still!) what is the gold doing for us? AFAIK nickel is just as good as gold when it comes to rust resistance, which is the only reason that comes to mind for using gold(except for aesthetics but that can't be the only reason ... I'm pretty sure I'm missing something again)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 18, 2019, 04:55:17 am
content: Good catch -- and easy answer: nickel is pretty inert by itself, but it also manages to passivate (oxidize) in air. Nickel oxide is a bitch to solder on!...Which I experience each and every time I solder down a BNC connector. Why oh why do they all make them nickel plated only? There are few with tin plated solder ends, and they're stupid expensive, gah.Another one -- what about silver plating? That's used, with or without nickel barrier (preferably with), but not as often, and it's about as good as OSP (organic solderability preservative -- basically an organic adsorbent that sticks to copper a little better than most other things) as far as shelf life, in terms of time between fabbing and soldering PCBs.Lastly, tin plate and HASL. Tin plate is about as good as HASL, I think? I forget exactly how/why it's less common... Tin does tarnish over time, so a thin layer might end up with comparable shelf life to OSP or silver? Otherwise, there's HASL, which is pretty good for shelf life and solderability but can be uneven, making it less attractive for fine pitch no-leads (QFNs, LGAs, BGAs..).Tim

author: OM222O
author_rank: Frequent Contributor
timestamp: February 18, 2019, 06:20:23 pm
content: Watching louis rossman repair macbooks, I noticed the gold layer isn't really dissolved even after reflow, and wicking the pads! You can see it here at 8:05I know flux might be contributing to the gold color but it's not that thick, there has to be some layer of gold still left under there. How fast do you reckon gold dissolves in tin?

author: ajb
author_rank: Super Contributor
timestamp: February 18, 2019, 06:39:01 pm
content: Quote from: T3sl4co1l on February 18, 2019, 04:55:17 amAccording to onefab, disadvantages of tin plating include:QuoteI'm not sure if it's enough tin that tin whiskers are a real concern or if it's more of a psychological factor, but it sounds like it just doesn't stack up well against the other options.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2019, 11:31:48 am
content: Quote from: OM222O on February 18, 2019, 06:20:23 pmThat's the flux.Consider, even if it were gold under there, why would we be able to see it? Wick can't possibly removethatmuch plating, such that the tin layer is see-through (< 100nm). Consider a similar situation: why don't we see pink through a well-wicked surface of tinned copper?If it were flaking off rather than wicking away, it could be, but that would be one hell of a dry joint, and it looked like it tinned very thoroughly. And anyway, a poorly-tinning gold layer would be, not necessarily a direct contradiction -- never underestimate the complexity of surface chemistry! -- but it would be kind of horrible to behold.Alternate possibility, the nickel underplate shows up, and, through the solder, somehow manages to get oxidized, and the joint just turns dry as hell. That would be weird (solder isn't permeable to air), and I think could only be indicative of a bad plating to begin with (the nickel oxidized before the gold was applied, and incompletely cleaned?).And probably some more, further out, possibilities. Maybe an info overload here, but I like to show how much depth there can be in every subject, and how to reason through it.Quote from: ajb on February 18, 2019, 06:39:01 pmCould be. I wonder if "tin flash" may be too thin to form whiskers, or at least of substantial size? It can tarnish through, or I sometimes see finishes that look awfully like that has happened, anyway. Dipped tin/solder is thick enough that the oxide washes away from liquid metal underneath, but too-thin layers can just stay crusted.Tim