Are there any tricks to making a card edge (gold fingers)? - Page 1
Forum section: electronics / general

author: alank2
author_rank: Super Contributor
timestamp: July 15, 2017, 12:51:33 am
content: I've done a few pcb's before, but never one with this feature. The guy at Elecrow said it was a $16 charge to do it. Do you have to do anything special in the pcb application besides not having a copper pour there and having the pads there?

author: Yellofriend
author_rank: Regular Contributor
timestamp: July 15, 2017, 01:09:14 am
content: If the PCB is for your own use and not used commercially you could do the cheap trick and simply order the PCB with flashgold surface instead of HAL. Flashgold solders well and often is a good protection against oxidation.Otherwise, gold fingers are usually hardgold, I think about 1µm thick. It's a special process that will cost more.

author: ConKbot
author_rank: Super Contributor
timestamp: July 15, 2017, 03:46:47 am
content: http://www.eurocircuits.com/blog/gold-plating-for-edge-connectors/this covers it pretty well. As said above though, for an at home project, feel free to use ENIG instead of a hard gold plating, and skip on the bevel if you want. It'll be harder to mate the connector and the gold will be worn through pretty quickly by repeated use, but getting the hard plating and bevel done both can add to the cost by a decent bit on a short run, depending on the manufacturer.

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 15, 2017, 05:19:31 am
content: good to know, I've been daydreaming about making a custom plugin...an old Tek frame with USB ports should raise a few eyebrows

author: floobydust
author_rank: Super Contributor
timestamp: July 15, 2017, 05:39:29 am
content: But wait... old edge-cards on a PCB were not a hard edge but have a chamfer. They are sanded/router'd so the copper at the edge is ground back. Look at one from say a PC graphics card.Also the corners are rounded (in the other axis) to tolerate some slop and wiggle when putting the cards in.

author: Mattylad
author_rank: Regular Contributor
timestamp: July 15, 2017, 10:21:25 pm
content: Try and keep any other components etc above and away from the edge connector, the board will be masked and run through a gold plating line so its best to keep everything else so it will not be subjected to the same gold plating.Your gold fingers can (in your PCB program) extend beyond the board edge so that there is no gaps or rounded ends at the board edge, as long as you supply the board outline the FAB will cut them back.Ensure that you provide adequate information to the fab to do your gold fingers, the amount of gold, where it is etc.A manufacturing drawing is the best way of achieving this.

author: alank2
author_rank: Super Contributor
timestamp: July 28, 2017, 03:20:56 am
content: That was going to be my next question, should the fingers go right to the board edge or not.Do I need to do a keepout for solder mask on the fingers area?

author: floobydust
author_rank: Super Contributor
timestamp: July 28, 2017, 08:46:34 pm
content: Yes, no solder mask near the fingers, and back off the pads from the finger edge. I think you get burrs otherwise.

author: alank2
author_rank: Super Contributor
timestamp: July 29, 2017, 02:39:19 pm
content: Is there disagreement as to whether the fingers should go to the edge or not?

author: krho
author_rank: Regular Contributor
timestamp: July 29, 2017, 03:28:59 pm
content: If you have any card available you can see that they don't. The edge of the PCB is not straight but it has material removed on both sides my guess would be under 30° angle or so.

author: zeqing
author_rank: Regular Contributor
timestamp: August 04, 2017, 10:18:44 am
content: As the factory. it is indeed that some extra fee needed for the golen fingers and the bevel edge

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2017, 10:45:47 am
content: Note that the fab needs to have a shorting bar across the traces, just past the card edge. That is, they'll fab it slightly too long, with shorting copper across the traces. That's why you see thin protrusions on the above pictured fingers. When plating is done, the shorting area is cut off and the edge is chamfered for ready insertion.They should add this themselves -- but just understand, you won't get precisely the shape you had on the drawing, that little protruding tip is part of the process. And they need the area there to work on, so you can't have any other board area beyond the plane of the connector.Hard gold is usually 30uin, BTW. What thickness you choose depends on how much wear resistance you need -- for one or two insertions, yeah, even regular ENIG is passable. But beware, once the gold wipes off, exposing nickel, it's quickly downhill from there, as nickel has higher resistance, and has a surface oxide which is a semiconductor (leading to small rectification effects -- "passive IMD" in RF).Tim