What are these black strips on the PCB? - Page 1
Forum section: electronics / projects

author: Kryptychon
author_rank: Contributor
timestamp: April 30, 2023, 11:17:29 pm
content: Hello,I've been fiddeling around with our washing machine because the motor died (mechanicl issue).I took the chance and cleaned everything from the dust of the brushes including the control PCB.That's when I noticed these black strips on the main control board. It's a single layer board which contains the MCU, some relais and the triac for the motor speed control.What are those black strips and what are they good for?The closest answer I could find so far is this stack exchange thread:https://electronics.stackexchange.com/questions/198245/what-are-these-black-strips-on-pcbAny other thoughts?Thanks!RegardsKryp

author: wraper
author_rank: Unknown
timestamp: April 30, 2023, 11:26:00 pm
content: Cheap-ass carbon jumpers.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: April 30, 2023, 11:28:05 pm
content: Wow. Is that a single sided PCB? If so it can be a conducting bridge wire. Strange to see this kind of cost cutting when multi layer PCBs are such a commodity. They way the copper traces running into the black overlay then continue in copper again suggests these are conductive bridges.

author: wraper
author_rank: Unknown
timestamp: April 30, 2023, 11:29:58 pm
content: Quote from: Kryptychon on April 30, 2023, 11:17:29 pmThey are not the same. On that photo it's just a silkscreen ink for additional insulation against terminals overhanging over the traces.

author: Kryptychon
author_rank: Contributor
timestamp: April 30, 2023, 11:32:53 pm
content: Wow! Thanks for the responses.That makes sense. I did not pay attention but you're right, there are traces "going in" and "goind out" of these black strips.The board is indeed single sided. Everything looks quite cheap and cut down to minimize costs. The carbon ink strips fit the overall impression of a very cheap board.

author: wraper
author_rank: Unknown
timestamp: April 30, 2023, 11:32:59 pm
content: Quote from: dobsonr741 on April 30, 2023, 11:28:05 pmCost cutting. I've seen similar garbage in PC DVD writers. Also they use 2 layer phenolic PCB and use this stuff for vias as normal copper vias are not suitable for phenolic boards AFAIK.

author: coppercone2
author_rank: Super Contributor
timestamp: April 30, 2023, 11:56:57 pm
content: when there is asphalt on a pcb you are on a high way to hell

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 01, 2023, 03:40:35 am
content: I guess they wanted longer 0jumpers? There are a ton of those too...With all those extra parts I wonder how much they are actually saving, but I guess the bean counters have done their job here.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 01, 2023, 10:35:26 am
content: all this to save a jumper wire lollmust really cost less ??

author: jpanhalt
author_rank: Super Contributor
timestamp: May 01, 2023, 10:42:12 am
content: I agree, their placement looks like jumpers, but suspect that what you are seeing is not a carbon jumper itself. In one photo, it looks like something is buried underneath. My suspicion is that insulation is below the jumper wire (e.g., solder mask), the wire is soldered in place, and that "tape" (either printed or cut) is applied on top to protect the wire.

author: mzzj
author_rank: Super Contributor
timestamp: May 01, 2023, 11:15:27 am
content: Quote from: dobsonr741 on April 30, 2023, 11:28:05 pmLarge scale production is sometimes weird. I have seen more than few televison pcb’s where phenolic board is full of rivets in component holes. Probably to strenghten the poor-ass phenolic board.

author: tszaboo
author_rank: Super Contributor
timestamp: May 01, 2023, 02:58:40 pm
content: Quote from: Kim Christensen on May 01, 2023, 03:40:35 amI really wonder how much savings are we talking about when these techniques are used. I've seen quite big 4 layer boards quoted at less than an EUR for thousands of boards. So I guess the savings must be in the order of dozens of cents.But then again, I never designed something that was made in the millions. Someone with more experience could maybe chime in?

author: Haenk
author_rank: Super Contributor
timestamp: May 01, 2023, 04:34:14 pm
content: With all those zero Ohm jumpers and bridges (which require additional steps during production), I just can't imagine how this could be cheaper than just using a two sided PCB. Which might even reduce PCB size by quite a bit.Or it's an old design from the time PCBs were more expensive?

author: wraper
author_rank: Unknown
timestamp: May 01, 2023, 04:43:28 pm
content: Quote from: Haenk on May 01, 2023, 04:34:14 pmFor two side you need to go for more expensive FR-4. Isn't via plating an additional process too?

author: ajb
author_rank: Super Contributor
timestamp: May 01, 2023, 09:04:23 pm
content: Quote from: Haenk on May 01, 2023, 04:34:14 pmThe economics of high volume consumer manufacturing are much different from what most of us who only make things by the tens to thousands deal with. I repaired a dishwasher where some of the buttons had failed, and found that the manufacturer had this crazy complex injection molded plastic flexure assembly to link the physical buttons to the switches on the PCB, which were, apparently, designed to fit a front panel with the buttons laid out differently. The complexity of that part easily added tens of thousands of dollars of NRE versus if the buttons were lined up, but divide that over a few production runs of ~100k and the marginal cost is nothing compared to the costs in designing, qualifying, manufacturing, and stocking an additional PCB assembly with the switches in the right place.Some years ago I visited a factory that made domestic appliances, and just that one facility was producing 15,000 units per day at the time--which was well below their full capacity. One whole side of the factory was a line of house-sized forming presses chewing through 500 tons of coil steel a day, and in the middle of each one was a massive multi-stage forming die, probably 2-3m long, and 1m high/wide, full of intricate cutting and forming surfaces precisely machined in tool steel, with lots of mechanical linkages for ejectors and slides to form side features. The design, manufacture, and assembly cost on those things must be huge--not to mention the 1000 ton press that operates it--but the whole point is that a fully formed sheet metal panel pops out about once a second all day every day.Here, the marginal cost of screen printing conductive tracks onto the PCB is probably pennies even if you account for the equipment investment and setup, and in return, they save on every unit by using a dirt-cheap substrate, only having to etch/mask one side of the board (which means not having to worry about layer registration), and not plating through holes. It doesn't surprise me at all that this method makes sense at their volumes.

author: james_s
author_rank: Super Contributor
timestamp: May 01, 2023, 10:18:32 pm
content: It's amazing to me the sort of volume at which that stuff is produced. My fridge is more than 20 years old, my dishwasher more than 15, washer and dryer must be close to 20, they were used when I got them, my stove is at least 10 years old, microwave oven close to 15, I just don't replace major appliances often but I guess people must do it.

author: Georgy.Moshkin
author_rank: Frequent Contributor
timestamp: June 09, 2023, 01:02:42 am
content: I think that it isimpossibleunlikely that all products on the market are made by highly talented engineers and thoroughly cost optimized. What you are looking at is often just work get done. Often it is first or second iteration of the design. Decisions are based on engineer experience and manufacturing line capabilities. Even if engineer was told to cost optimize, there are always other considerations, such as time to market, etc. Sometimes you can notice "cost optimization" in products manufactured for decades, and many consumers don't like if reliability is reduced.