brake dust sticks to certain PCB traces and not others? - Page 1
Forum section: electronics / beginners

author: dentaku
author_rank: Frequent Contributor
timestamp: February 16, 2016, 05:04:09 pm
content: Last night I opened up an old Yamaha A-550 amplifier salvaged from a truck garage and noticed that the bottom panel had a nice pattern of what looks like brake dust that formed around the vent holes.Then I noticed that some of the traces on the bottom of the PCB have this dust on them and others don't.In fact, the two ~~ inputs to the rectifier are clean but the + and - traces are dusty.I'm wondering if it sticks to certain traces and not others because of heat, amount of current, AC vs. DC?

author: German_EE
author_rank: Super Contributor
timestamp: February 16, 2016, 05:10:24 pm
content: My guess is electrostatic charge, any others?

author: dentaku
author_rank: Frequent Contributor
timestamp: February 16, 2016, 05:18:08 pm
content: I forgot to post the photos. The bottom panel looks like it's just due to airflow but the PCB obviously isn't.

author: AF6LJ
author_rank: Unknown
timestamp: February 16, 2016, 05:21:26 pm
content: I don't think that is break dust maybe welding residue that gets suspended in the air.Wash some of it off into a beaker and let the solvent dry and see if it is...Conductivemagnetic....or has other properties.

author: Seekonk
author_rank: Super Contributor
timestamp: February 16, 2016, 05:25:48 pm
content: Brakes have a lot of metal fillers and certainly what they rubbed against was ferrous. I'd say it was magnetic forces. DC magnetic fields will be constant. Every ferrous material has some magnetic polarity. AC magnetic fields will cause spinning as well as a directional force. That means more dust will go to the DC traces. Certainly all three are a factor inside a cabinet as to where dust lands. And a reason why slots are required for some traces for isolation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 16, 2016, 06:01:30 pm
content: I don't think it's magnetic. If it were, it would be more than happy to settle on the AC traces. But anyway, magnetic fields aren't constant and intense in an amplifier. Current draw (which is approximately equal for both AC and DC traces) varies significantly with output power and program content (dynamic versus heavily compressed music). Most notably, you'd expect about the same accumulation on the output terminals, which carry the load current.Whereas the electric field is ever-present, and differs by an order of magnitude or two between groups of traces. That said, I'd guess it's not using more than maybe 100V total supply voltage, which still seems impressive that it gathers anything at all!Example: office use is much less dusty than a garage, but still, it took >1kV on the one trace to accumulate what little dust it has:http://seventransistorlabs.com/Monitor/Images/Defl5_a.jpg(Bottom left corner, following the bottom edge, up into the middle with some components and PCB cutouts.)I'd call it a cool phenomenon, document it for future reference, and wipe it off with a damp rag. Check the top side components, probably blow it off with compressed air. Should be good for another bunch of years, or until the electrolytics go tits-up.Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 16, 2016, 06:19:10 pm
content: https://en.wikipedia.org/wiki/Static_cling

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: February 17, 2016, 02:28:32 am
content: This phenomenon is quite commonly seen on EHT tracks/traces such as in CRT TVs. Aerosols in the atmosphere such as tar and nicotine from cigarettes combined with dust make quite a sticky residue. I have spent happy hours cleaning the stuff off in a previous life.BT