PCB spark gap with an air gap? - Page 1
Forum section: electronics / beginners

author: Khaveer
author_rank: Contributor
timestamp: January 05, 2015, 03:28:53 pm
content: Hello. I took apart a Samsung lcd screen to replace a few bad caps. After a quick inspection of the board I noticed a few spark gaps. At first I thought it was nice of them to add some extra protection, but then I noticed that there were two spark gaps that had air gaps between their "contacts". I never encountered that kind of a spark gap before. Is that a normal thing to do? Doesn't that increase the breakdown voltage?

author: tszaboo
author_rank: Super Contributor
timestamp: January 05, 2015, 04:32:40 pm
content: I think this is what happened:Engineer 0x01:"I'll add this spark gaps, to protect against ESD!"Engineer 0x02:"Oh no what have we done! That will ark over! We should respin the boad, but there is not enought time. Lets make a milling there (BTW, everything that 0x01 does is bad)"

author: colotron
author_rank: Contributor
timestamp: January 05, 2015, 04:42:47 pm
content: Unrelated to the topic but related to the picture.What is the function of the horizontal track/lines covered with solder on the down side?, I use to see those in power supply boards, but never know why they are there.

author: wraper
author_rank: Unknown
timestamp: January 05, 2015, 04:52:04 pm
content: Quote from: NANDBlog on January 05, 2015, 04:32:40 pmAre you joking? This is normal practice to avoid PCB carbonization -> short between line and GND. When doing spark gaps between ground and line potential they are made in a such a way most of the time.

author: max_torque
author_rank: Super Contributor
timestamp: January 05, 2015, 06:09:36 pm
content: Quote from: colotron on January 05, 2015, 04:42:47 pmAssuming they play no part in the circuit (ie they are not power distribution) then i guess they are just "guard" lines, left uncoated in soldermask to ensure that any arc or tracking will go there first, where there is a nicely grounded track, rather than the casing or any nearby metallic object etc?

author: Refrigerator
author_rank: Super Contributor
timestamp: January 05, 2015, 07:52:34 pm
content: Quote from: colotron on January 05, 2015, 04:42:47 pmI believe those are ground traces ( the ground symbol is right there )

author: ludzinc
author_rank: Unknown
timestamp: January 05, 2015, 08:23:01 pm
content: Those slots are not spark gaps - they are clearance slots.These help maintain 'creepage' distance - a barrier between the mains and low voltage sides of the board.

author: Seekonk
author_rank: Super Contributor
timestamp: January 05, 2015, 08:37:34 pm
content: Agree, those are UL clearance gaps and air increases the spark over voltage, but those are also intended spark gaps with those alligator teeth. A rare occurance where two rules apply.

author: colotron
author_rank: Contributor
timestamp: January 06, 2015, 12:39:10 am
content: Quote from: Refrigerator on January 05, 2015, 07:52:34 pmYes, I know that, but I just wanted to know the specific function of having a track portion covered in solder in specific places.Quote from: max_torque on January 05, 2015, 06:09:36 pmMmmm, maybe?. I attach another picture, this one is a old LCD monitor supply board - sorry for the crappy quality. Lots of solder covering tracks, I guess the objective is to increase (slightly) current capability. But there are portions where the solder lines are perpendicular to the track/pour zones, which will worthless to increase current capability along it, so there could be another reason?.

author: ludzinc
author_rank: Unknown
timestamp: January 06, 2015, 01:14:56 am
content: IMHO that's a wave soldered board (see the bigger pads on the corner pins of the IC's - that's the clue) so the solder mask is slotted to allow the board to work with the solder bath.If all of the track / area were exposed, there would be too much 'theifing' of the solder.... And yes, the extra solder is to increase current capacity of those traces.

author: colotron
author_rank: Contributor
timestamp: January 06, 2015, 01:25:11 am
content: Quote from: ludzinc on January 06, 2015, 01:14:56 amOk, it seems quite reasonable, thanks!!!