Scraping off soldermask to expose ground plane - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: February 28, 2021, 04:14:54 pm
content: I have a circuit board that I'm trying something with. In a nutshell, I have a resistor in a very simple, passive circuit that is normally connected in series with a node, but I would like to see how it performs when it is in parallel with a node. Now, to accomplish this, I plan to carefully scrape the soldermask off and expose a portion of my ground plane to connect one end of my resistor to. (Normally, I would create a new board for this, but since this is a prototype of an existing board, this seems to be the fastest, cheapest way) Considering how soldermask doesn't really do much in terms of protection against noise, crosstalk, etc., would it do any harm if I expose part of the ground plane like this to form an impromptu pad? It doesn't seem all that different than leaving an unpopulated component free (like a capacitor connected to ground on one end).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2021, 04:33:54 pm
content: This is 100% a standardhackingprototyping technique, and has no consequence beyond the ugliness of appearance, and relative difficulty of soldering.If by "ground plane" you mean a buried (inner layer) plane, that's fine too, it just takes more digging. Have had to do that before, too.Tim

author: Ian.M
author_rank: Super Contributor
timestamp: February 28, 2021, 04:36:56 pm
content: Except at *extremely* high frequencies, soldermask has no electrical effect whatsoever except insulation and it cant be trusted for insulation as there is a significant risk it will have pinholes in it. Its only there to provide physical protection (e.g. against corrosion) and to aid assembly by controlling the area over which solder will flow.Therefore there is absolutely no reason not to scrape back soldermask where required to expose copper to connect to. I suggest using an Xacto knife or similar with a sharp convex curved blade to remove the bulk of the soldermask and get down to copper, trying to be reasonably neat and staying within the area you want to expose, followed by a fiberglass abrasive pencil brush to clean the remaining solder mask off to leave bright bare copper ready to tin.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: February 28, 2021, 05:36:49 pm
content: Thank you for your reply. Luckily, for this board, I'm only dealing with the top copper layers and not any inner layers.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: February 28, 2021, 05:45:42 pm
content: Thank you for your reply. I don't plan to run at very high frequencies, and this is just a proof-of-concept, so it's not much to worry about. Luckily, my ground plane is on the top layer of my components, so I can probably scrape around the edges since it's close enough.

author: zzattack
author_rank: Regular Contributor
timestamp: February 28, 2021, 06:02:31 pm
content: Quote from: Ian.M on February 28, 2021, 04:36:56 pmI follow exactly the same procedure, and always felt that the fiberglass pen helps create a (significantly) more flat surface, which helps tremendously with solder flow.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 28, 2021, 06:43:02 pm
content: I like to scratch with a sharp, pristine, small flat heat screwdriver. Fiberglass pen can be used to clean the remaining soldermask speckles. Then apply flux from a flux pen and solderability is just fine.This is something you need to do very regularly in prototyping. I always forget some pulldown resistor or similar from the first prototype PCB revision.

author: rfclown
author_rank: Frequent Contributor
timestamp: February 28, 2021, 07:08:18 pm
content: I do this all the time. I'll even create pads by cutting around a square in a ground plane area. I do it all with an exacto knife. Fine for RF circuits.I'll make quick boards with just copper clad board and an exacto knife (or dremel with fine diamond tip). Create pads, cut microstrip lines. With simple circuits and SMT components, it isn't any different than making a board, except you have no ground vias. For that I'll drill and solder a wire top to bottom.

author: Ian.M
author_rank: Super Contributor
timestamp: February 28, 2021, 07:38:32 pm
content: My qualifier "Except at *extremely* high frequencies ..." above refers to the slight difference in characteristic impedance, due to the dielectric constant of the soldermask, between a bare microstrip and one covered by soldermask. Its unlikely to be significant unless you are working at gigahertz frequencies or higher, and soldermask is often NOT used on microstrip, because its lossier than a bright plated finish directly exposed to the air.Modding high UHF and SHF boards successfully*, with repeatable and intentional results needs a RF expert and $expen$ive$ RF test equipment to verify the result as even a bump of solder in the wrong place on a track can significantly affect a circuit.* Unless the designer has already done all the hard work for you, e.g. an integral antenna WiFi module with pads for an external antennae connector, where all you have to do to convert it to external antenna is to fit the right connector and move a coupling cap to connect it in place oif the on-board one.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 02, 2021, 01:16:29 pm
content: also seehttps://www.eevblog.com/forum/beginners/what-common-chemicals-can-remove-soldermask/