Easier Way to Trace SMT Connections on PCB? - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: March 08, 2021, 05:39:11 am
content: I have what appears to be a multi-layered board with blind vias. I'm trying to trace where a surface mount resistor connects to after it dives into one of the layers by way of a blind via.Obviously I'm touching each point with a meter, however, this is a very large board with lots of components. I'm wondering if something exists that would touch many points within a small area allowing the ability to narrow down to an area; and then use a meter to find which component.

author: Berni
author_rank: Super Contributor
timestamp: March 08, 2021, 06:44:43 am
content: Get a fine metal wire brush (Or just strip some stranded cables), connect one of the multimeter leads to the brush then sweep across the board until you get the beep. Then use a probe to find the exact pin in that area.

author: tautech
author_rank: Super Contributor
timestamp: March 08, 2021, 06:45:24 am
content: Products that use metallic bristle bushes.You could probably cobble something together using an old DMM lead and desoldering braid.

author: ajb
author_rank: Super Contributor
timestamp: March 08, 2021, 06:50:20 am
content: You could connect one of the meter leads to a flexible conductive pad, or a piece of high strand-count wire stripped and splayed out like a brush to cover a larger number of contacts. The big downside to any method that tries to contact a large area at once is that it won't provide the point contact pressure required to pierce any non-conductive surface material, like flux residues or oxides, so it will only work if the board is fairly clean. You'll also have a harder time getting good contact with things like QFN pads where the exposed conductive material is in the corner of the package and the board, but that can be achieved with a bit of specific attention to those areas.You might be able to do something with RF injected into the trace, but that's going to be sensitive to board construction and the circuitry installed on it.I've occasionally thought it would be an interesting project to build a flying probe machine, essentially two XY positioning systems with movable probes on each side of the board. If you could map out all of the contact points on the board (either manually, which would be tedious, or I guess with machine vision, which would be hard) you could then just click a button and have it exhaustively check connectivity across the whole board. If the board is unpopulated the result would be a pretty unambiguous netlist, but on a populated board the best you could do would be a map of the electrical parameters measurable between pairs of points, which is pretty easy to interpret if you have a lot of direct connections, but could get tricky for boards full of interconnected passive and discrete semiconductor parts.

author: kripton2035
author_rank: Super Contributor
timestamp: March 08, 2021, 06:59:07 am
content: https://www.eevblog.com/forum/testgear/wavetek-sf10-short-finder-a-very-useful-tool-if-you-can-find-it/

author: bostonman
author_rank: Super Contributor
timestamp: March 09, 2021, 02:35:57 am
content: The wire brush trick worked great.I found the connection within five or ten minutes. Yesterday I spent about two-hours, and prior about another hour, without luck.After this, I plan to make a better brush setup, but this was truly a time saver. The only issue I have now: I found a connection, but the other side of the component has low resistance to other components. This is causing the meter (in diode mode) to beep and show a value much higher than 0.Obviously this triggers me to stop and look, but only to discover it's not a direct connection. I think changing the range fixed this, but it makes the meter react slower.Anyway, thanks for your help, you saved me tons of time and aggravation. Wish I thought of the idea on my own.

author: amyk
author_rank: Super Contributor
timestamp: March 09, 2021, 03:31:36 am
content: Quote from: ajb on March 08, 2021, 06:50:20 amThe various PCB reverse-engineering services around, many of them in China, have such equipment.

author: Berni
author_rank: Super Contributor
timestamp: March 09, 2021, 06:07:45 am
content: Good to hear you found it.Multimeters with adjustable continuity beep can be handy around these things. They can often be set between about 1 Ohm to 1K, but it is a fairly rare feature, mostly found on bench DMMs.