How essential is gold plating for PCIe edge connectors - Page 1
Forum section: electronics / projects

author: Nogtail
author_rank: Regular Contributor
timestamp: December 08, 2022, 07:13:50 am
content: I have a product with a PCIe card form factor. The product is intended to be relatively low cost, so a cheap PCB + BOM is essential. With an ENIG surface finish the cost is acceptable, even for small quantities. The quotes I've got for hard gold plating have more than doubled the price of a fully assembled board. It also limits where I can get the board made, as a lot of the lower cost shops don't offer it as an option.How bad is it to use an ENIG finish for PCIe edge contacts? I know it's a lot thinner than hard gold, but for a product that is only likely to be plugged in/unplugged a small handful of times will this matter? I can't seem to find any good information on the durability of an ENIG finish for edge connectors. Does anyone know of products that use an ENIG finish for edge connectors?Thanks!

author: ataradov
author_rank: Super Contributor
timestamp: December 08, 2022, 07:47:38 am
content: I have no scientific information on this, but in this posthttps://www.eevblog.com/forum/kicad/card-edge-connectors/msg2003468/#msg2003468I have pictures of ENIG PCIe connector after more than 100 insertions. There is a visible tarnish, but the connector remains operational.I don't have any data on whether it would be a problem for a long term operation. This tarnish may be enough to cause oxidation or something like that.

author: Kenneth Rosenstroem
author_rank: Newbie
timestamp: December 08, 2022, 09:15:18 am
content: I do not understand why people use gold plating. Silver is a lot more conductive than gold. It is also harder than gold, so the wear and tear should last longer with silver. Of course, there is oxidation, but if the connection is made (connector), it should not oxidise where there is contact. Maybe I am wrong, if so, please enlighten me. I know silver is used for speaker cables, and cable connectors in high-end HiFi systems, so why not in electronics? As a bonus, it is also cheaper than gold

author: tszaboo
author_rank: Super Contributor
timestamp: December 08, 2022, 10:05:55 am
content: ENIG is probably fine. Bare copper is terrible, you touch it once and your fingerprint will be visible etched into it forever.Though I don't think hard gold plating is very expensive step if you order larger quantities. Unless you want to order the chinese special online pricing. You absolutely can get better pricing if you ask quotes. Talk to your fab.

author: Hawaka
author_rank: Regular Contributor
timestamp: December 08, 2022, 11:06:15 am
content: Quote from: Kenneth Rosenstroem on December 08, 2022, 09:15:18 amSilver is indeed more conductive, but the main goal of gold plating is corrosion resistance. I do believe that it could corrode even if it is inserted, just like a screw in a nut. A contact does not stop corrosion in-between.Silver might be cheaper as raw material, but all PCB supplier already have gold plating process. I do not know any with silver process. Therefore cost will be surely higher if you really want silver.From my understanding hard gold is used for longevity. If only ENIG is used, then the plating would be slowly pushed away from the contact point as it is soft. This could result in bad contact after a long insertion period. At least that’s what my colleague once.

author: strawberry
author_rank: Super Contributor
timestamp: December 08, 2022, 11:10:13 am
content: Quote from: Kenneth Rosenstroem on December 08, 2022, 09:15:18 amsilver is in good quality switches and it is x100 better wear resistance than solder plated contacts (solder turn black after some connect/disconnect operations(solder shavings and dust paste))silver oxidize after some ~10y. cleaning and applying fresh grease/oil will do another ~10y

author: dmills
author_rank: Super Contributor
timestamp: December 08, 2022, 11:16:34 am
content: Silver oxide is conductive, it just looks ugly. The problem with silver is the sulphate which is an insulator and forms due to sulphur in diesel exhaust.The trap with gold is that it only really works well when the mating contact is also gold (In particular, gold on tin is a nightmare, an effect called 'Tin fretting', it is real and fucked me once.

author: E-Design
author_rank: Regular Contributor
timestamp: December 08, 2022, 01:03:02 pm
content: Quote from: Nogtail on December 08, 2022, 07:13:50 amSince you state that it will only cycle a small number of time (say 10 or less) in the life of the product, ENIG only (no hard gold) should be fine. Ive done it before and has proven reliable. It really is all about the cycles the connector will see in its life.

author: TimFox
author_rank: Super Contributor
timestamp: December 08, 2022, 02:49:47 pm
content: Silver does not oxidize under normal conditions.The black tarnish that happens in normal indoors operation is sulfide.In good switches with silver contacts, the surface is cleaned by the wiping operation of well-designed sliding contacts.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: December 08, 2022, 03:20:40 pm
content: I think I would evaluate my client's wants and needs and whether a better plating adds enough value to them that they are willing to pay for it.

author: langwadt
author_rank: Super Contributor
timestamp: December 08, 2022, 03:52:35 pm
content: Quote from: E-Design on December 08, 2022, 01:03:02 pmafaik a regular PCIe connector is only rated for 50 insertion cycles

author: strawberry
author_rank: Super Contributor
timestamp: December 08, 2022, 04:15:49 pm
content: Quote from: TimFox on December 08, 2022, 02:49:47 pmheed someone to exercise switches regularly otherwise oxide thickens too muchold style switches are better than miniature (thicker silver and more contact pressure)

author: Nogtail
author_rank: Regular Contributor
timestamp: December 09, 2022, 02:32:34 am
content: In my case I'm mostly using the connector for power/low speed data. The power requirements are also very low so I'm not too worried about slightly higher resistance in the connectors unless it gets crazy high. I'd imagine the connector will get 2 mating cycles for 90% of customers - once for production test then again for installation into a system. I'd imagine around 10% of customers might be "enthusiasts" who give it a few more cycles. I might try and do some accelerated life testing when I get the prototypes to see how well ENIG holds up.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 16, 2022, 08:35:42 pm
content: Its just a way of moving gold into landfill never to be revovered.It looks cute but given the very few inserions cycles (usually 1) it makes little difference.Once you gave a gas tight mating any contact finish will be relaible.We managed for most of the past 100 years without gold plating on connectors and the world did not end.

author: tszaboo
author_rank: Super Contributor
timestamp: December 17, 2022, 02:54:41 pm
content: Quote from: Terry Bites on December 16, 2022, 08:35:42 pmHave you tried a vintage headphone recently? Constant contact issues with the SCART connector? Internet disconnecting because of the terrible connectors on the POTS line? CRT TV loosing picture that get suddenly resolved if you smack it from the side?Don't act like this is a non existing issue.

author: tooki
author_rank: Super Contributor
timestamp: December 17, 2022, 03:29:53 pm
content: Quote from: Terry Bites on December 16, 2022, 08:35:42 pmWell that’s true, except for the part (namely, the entirety of it) where it’s actually completely incorrect.https://www.digipas.com/documents/Effects-of-Fretting-Corrosion-on-Au-Sn-Plated-Contacts.pdf

author: tooki
author_rank: Super Contributor
timestamp: December 17, 2022, 03:33:10 pm
content: https://eprints.whiterose.ac.uk/4940/1/repository_liskiewicz_paper09.pdf

author: tooki
author_rank: Super Contributor
timestamp: December 17, 2022, 03:35:45 pm
content: https://suddendocs.samtec.com/productspecs/fretting-corrosion.pdf

author: MisterHeadache
author_rank: Regular Contributor
timestamp: December 19, 2022, 04:34:24 pm
content: Quote from: dmills on December 08, 2022, 11:16:34 amWhat he said. Mixing plating in electrical interconnects is a very bad idea (I know, that's off topic from OP). Silver's sensitivity to sulfur also gets pooh-poohed too often because it depends on the exposure PPM over time. And there's lots of sneaky sources for sulfur exposure (e.g, nearby rubber gaskets, cardboard packaging, and greases, to name a few).

author: niconiconi
author_rank: Frequent Contributor
timestamp: December 19, 2022, 04:45:00 pm
content: Quote from: Kenneth Rosenstroem on December 08, 2022, 09:15:18 amFrom what I've read, immersion silver just looks horrible after oxidation so clients and consumers don't want it.Quote

author: vidarr
author_rank: Frequent Contributor
timestamp: December 19, 2022, 04:47:51 pm
content: I am very serious about my computers and I always do gold and I have never had a problem ----ever. Many of my machines get taken apart and re-arranged dozens of time. Most of my very best machines I even keep on test benches, because of this.Re: Gold on tin:"... an effect called 'Tin fretting',..."I don't think you will run into this problem, if you buy from quality manufacturers. I don't even know if they do this method now, but I am not an expert on this exact thing. I own tinned copper cookware and know how to retin them. That is the extent of my tin knowledge. I don't doubt this other guy had this problem, but I never heard of it before.

author: dmills
author_rank: Super Contributor
timestamp: December 20, 2022, 10:03:21 am
content: I had to have the office recall over 100 units of a fairly expensive broadcast industry box because on a ONE character error on the BOM that made one half of a mating set of connectors tin not gold.These were Samtec parts, so not junk, and were failing in fly packs where the kit was moved by air freight.You could actually see the tin fretting but needed a decent microscope to identify it, lots of red faces over that one.

author: Psi
author_rank: Super Contributor
timestamp: December 20, 2022, 10:10:39 am
content: Silver plating is a bit of a nightmare for PCBs, sometimes it will tarnish super fast and other times it wont.It can be quite unpredictable.

author: Haenk
author_rank: Super Contributor
timestamp: December 20, 2022, 03:07:31 pm
content: I think there is a reason, why memory modules and computer "cards" switched to gold plating many years ago - and never went back. Even though those are hardly ever moved around.And honestly, there is almost no gold involved at all.

author: vidarr
author_rank: Frequent Contributor
timestamp: December 20, 2022, 07:02:02 pm
content: Quote from: Haenk on December 20, 2022, 03:07:31 pmThis.

author: tooki
author_rank: Super Contributor
timestamp: December 21, 2022, 09:58:20 am
content: Quote from: Haenk on December 20, 2022, 03:07:31 pmIf only the gold recovery lunatics would understand this…

author: langwadt
author_rank: Super Contributor
timestamp: December 21, 2022, 10:15:46 am
content: Quote from: tooki on December 21, 2022, 09:58:20 amstill alot more gold in e-waste than ore

author: tooki
author_rank: Super Contributor
timestamp: December 21, 2022, 04:49:58 pm
content: Quote from: langwadt on December 21, 2022, 10:15:46 amBe that as it may, the point is that DIY gold recovery at home is a money pit and likely environmental disaster.

author: thm_w
author_rank: Super Contributor
timestamp: December 21, 2022, 11:45:00 pm
content: Quote from: dmills on December 20, 2022, 10:03:21 amGood example.It must have meant the parts that were connected were able to move enough to cause the fretting. If they were shipped unconnected or rigidly connected, and the end product was not in a high-vibration environment, then presumably there would have been no issue.In some cases pins can be lubed too:https://www.nyelubricants.com/what-is-fretting-corrosion

author: free_electron
author_rank: Super Contributor
timestamp: December 22, 2022, 12:32:15 am
content: THe process is different.Gold doe snot work well directly on copper. it will diffuse itself in the copper over time. so you need a nickel barrier.ENIG : electroless nickel , immersion gold. There is no electricity involved in this process. This is a chemical reaction. deposit nickel , deposit gold. once the nickel is covered it is hard to increase the thickness of the gold further.Hard gold is an electroplating process. you need a shorting bar on the contacts and the gold only grows where current flows. Once plating is completed the shorting bar is milled off the side of the board.So hard gold is an additional process step and requires additional steps ( adding shorting bar to the design , milling it off post hard gold plating )So yeah it is a big cost adder

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: December 23, 2022, 05:20:59 am
content: The thing that frustrates me most about this thread is the plethora of people saying "I don't think it's an issue, therefore it's definitely not an issue". Just because you can get away with it sometimes doesn't mean it's a non-issue.If we're going to Dunning-Kruger our way through things then what's the point of having a technical forum?I suppose the same characters will point to the above video as proof that bypass capacitors are worthless, because that circuit still happened to work without them

author: tooki
author_rank: Super Contributor
timestamp: December 23, 2022, 12:05:16 pm
content: Quote from: Geoff-AU on December 23, 2022, 05:20:59 amAnd many forget that just because it worksnowdoesn’t mean it’ll still work years later.Same with ESD damage, where there are always people who say “I never use an ESD mat or band and I’ve never had ESD damage”, because when an ESD-compromised component dies a year later, they don’t attribute the failure to an ESD event (since most ESD events are not detectable by human senses), and instead think it’s random failure.

author: free_electron
author_rank: Super Contributor
timestamp: December 24, 2022, 08:07:55 pm
content: Quote from: Psi on December 20, 2022, 10:10:39 amit depends on the atmosphere. any with sulphur in it is a disaster for tin plating. even rubber bands ...

author: free_electron
author_rank: Super Contributor
timestamp: December 24, 2022, 08:09:55 pm
content: Quote from: Geoff-AU on December 23, 2022, 05:20:59 amor still sprinkle 100nf caps everywhere when running clocks at higher than 100MHz ...

author: srb1954
author_rank: Super Contributor
timestamp: December 24, 2022, 08:29:15 pm
content: Quote from: free_electron on December 24, 2022, 08:07:55 pmSome locations are particularly bad for sulphur compounds, especially volcanic and geothermal areas such as Rotorua, New Zealand.I used to work for a company that supplied equipment for installation in Rotorua and the sulphur compounds in the air there completely corroded away all the copper and tin from PCBs. In many cases all that was left of the PCB tracking was the thin gold plating where an edge connector used to be. To solve the problem the equipment had to be sealed in an air tight enclosure with filtered air pumped in to maintain a positive internal pressure to keep out the contaminated atmosphere.