Strange/"useless" copper traces on PCB - what are these for? - Page 1
Forum section: electronics / beginners

author: 8008
author_rank: Contributor
timestamp: February 22, 2024, 10:10:58 am
content: I am repairing a vintage computer (Epson HX-20). The batteries of that machine went leaky at some point in time and the leak fluid harmed one of the boards inside.This is the LCD board, containing a small LCD, five chips upd7227 for driving the LCD and few other passive elements.The board seems to be a 2 layer board with many vias. So far no question.But what I found when looking for destroyed/interrupted copper traces, are a mass of "useless" copper traces, going nowhere. They start at some chip pin, but end without any connection, usually at board edge or in some small holes on the PCB (and these are not vias).Are they related to RF / HF shielding? What is the use of these traces?Dennis

author: MrAl
author_rank: Super Contributor
timestamp: February 22, 2024, 10:17:19 am
content: Quote from: 8008 on February 22, 2024, 10:10:58 amHello there,I have a couple guesses.The ones that run 'off' the board at the edge, these may be for a different model where the board is larger. When they cut it down there are traces that are not needed for that model.The ones that go to that small hole, the hole may be drilled for one model and not another. When the hole is drilled, it disconnects three traces so maybe they are not connected for that one model PC board.Maybe someone else has some idea also.

author: Ranayna
author_rank: Super Contributor
timestamp: February 22, 2024, 10:26:55 am
content: Sometimes these are used as contacts for electroplating a whole panel. The actual contacts are on the part of the panel that is broken off.Or for electrical testing of the whole panel, again, the contacts get lost when the boards are de-panelized.The cut traces might be to block programming. Nowadays this is usually done within the chip itself.Or it could be to differentiate between various models. Or maybe different variations of similar chips without requiring a separate board layout.

author: Berni
author_rank: Super Contributor
timestamp: February 22, 2024, 11:48:47 am
content: The lines going off the edge of the board are likely used for testing and are then broken away once no longer needed.This way they might be able to test 10 boards at once in a single PCB panel by just slipping on a card edge connector in one corner of the panel. Could also be used to mass download firmware in there for the whole panel rather than having to do it one by one. These things can save you both time and the cost of designing a bed of nails test jig for the board. In large volume mass production every human manipulation of the board has to be accounted for as it really adds up in volumes.As for the drilled part. That might be just a variation of a product. So instead of using a jumper resistor to short the pins of the chip and tell it to do something different, they take a drill and drill away the track.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 22, 2024, 01:18:53 pm
content: Quote from: Berni on February 22, 2024, 11:48:47 amSome types of gold plating also need electrical connections - you often see this on dsplays using zebra strip LCD connections

author: 8008
author_rank: Contributor
timestamp: February 22, 2024, 02:19:00 pm
content: Thanks for all the possible purposes for these traces. To summarize:1) test traces (with parts of them removed during production process)2) multi purpose PCBs for different configuration, where parts of PCB are cut off3) holes serve as interruption for configuring chips for different models4) support traces for gold plating during board productionI cross checked with schematics. The upd7227 are not programmable, so 3) does not apply in this case.The traces disappering in that hole in my 2nd picure are row selection lines for LCD R0..R8 of chip #4 from schematics.The display is a 120x32 one (4 lines with 20 chars each, yes, very small, it is a machine from the mid 80ies). It looks that it is organized internally as a 240x16 display, needing only 2x8 row lines, so only two upd7227 are used in regards to row lines (R1-R8 and R9-R16), The four other upd7227 Rx outputs are going to these holes and are not used at all.I have marked all the unused Ri pins in attached schematics.So I am still not 100% sure what the traces are used for. Highest probability seems to be really gold plating support or testing.