Is White PCB Actually White? - Page 1
Forum section: electronics / projects

author: The Doktor
author_rank: Regular Contributor
timestamp: July 17, 2023, 02:55:56 pm
content: I didn't actually know how to phrase this question for a post title, it's not as stupid as it sounds. The basic question is whether or not a white PCB is white all the way through, or is it only the solder mask? I am thinking of using a PCB for an application where I want light to shine through from the back, it doesn't have to be perfectly colourless, but normal PCB green would not be good. I have actually Googled a bit on this but have not had luck finding an answer. Maybe someone here has experience. Probably be a .4mm from JLC.Ed

author: MarkS
author_rank: Unknown
timestamp: July 17, 2023, 02:58:47 pm
content: It's just the solder mask. The actual PCB is the color of the FR4 used, typically a beige color.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 17, 2023, 03:07:52 pm
content: The board material FR4 itself is kinda beige-ish or yellowish, somewhat translucent so passes some light through, and you can see the fiberglass structures in it. Even if just 0.4mm, light transmission is poor so you lose a LOT of light, and can see the structural patterns which might be undesirable. Additionally, the FR4 often has manufacturer markings (often in red) that become obvious when backlit. Any soldermask, even white, will decrease the light transmission even more.What kind of visual look are you expecting, exactly?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 17, 2023, 03:13:20 pm
content: It's translucent yellowish - e.g. if you shine an RGB LED through it, the colours will look OK and brightness will be fineNote even a black soldermask can be moderately translucent, so if you want a shape with a clean edge, mask it with copper.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 17, 2023, 04:27:03 pm
content: is black black lollfor pcb's, same here, kinda translucent with a strong led flashlight

author: strawberry
author_rank: Super Contributor
timestamp: July 17, 2023, 04:44:14 pm
content: thinner PCB will be more transparent. flexible-PCB could be better

author: Siwastaja
author_rank: Super Contributor
timestamp: July 17, 2023, 06:22:17 pm
content: Quote from: strawberry on July 17, 2023, 04:44:14 pmFlexible PCBs would be on orangish-brown Kapton substrate, the color is quite strong but at least it's much more clear, transparent and without a structure.In any case, I think it's best to ask the OP the classic question: what are you actually doing?

author: The Doktor
author_rank: Regular Contributor
timestamp: July 17, 2023, 06:43:08 pm
content: Quote from: SiwastajaI was just thinking I should probably add that information when you made your postI am planning to build a clock using three analog meter movements. these are the meters I am planning to modifyhttps://www.aliexpress.us/item/3256804305521911.html. I'm considering adding a backlight to them, but just adding an LED inside does not quite accomplish the task very well. I pushed a single white LED all the way up to 40 ma, will sufficiently illuminate the front of the dial that it is barely readable, but it creates a mighty bright ring around the outside edge compared to the illumination on the meter face. Probably something like four to six white LEDs running at a reasonable current level what creates sufficient illumination just by traveling around the edges of the face, but I don't really like the bright white area. Having a semi-transparent area for the light to shine directly through the meter Face seems like it may be a better idea.I had been considering how to make replacement meter faces. I have heard of people printing out face and attaching it with adhesive, but I have rather limited use of my hands and that would be an incredibly difficult thing for me to accomplish. I also desire something that looks a little more Factory. Considering how cheaply you can obtain PCBs it seems that a white PCB with black solder mask might is just the ticket. And if you remove all the copper, some light can then get through. I'm now looking at one of my 1.6 mm green PCBs, and I think even at that thickness with solder mask on both sides a sufficient amount of light get through.I also just realized that I probably didn't need to ask this question. I was trying to decide whether or not to leave a layer of copper, but actually the correct answer would be why? I am probably going to use this technique to make the meter face anyway, and if I don't add an LED to the meter, it will make absolutely no difference to the appearance whether or not I leave copper on the back. Considering that many sites say the white solder mask contains a lot of pigment, I will use solder mask only on the front face.

author: abeyer
author_rank: Frequent Contributor
timestamp: July 17, 2023, 08:08:15 pm
content: You might also consider a two-tone acrylic for laser or cnc cutting/engraving. You can find online manufacturers and/or a local shop, and won't be much more than pcb costs, but will give a lot more options for nicer looking translucency imho.(Also, if you aren't absolutely set on backlighting... most illuminated meters were actuallyfrontlit with a bulb between the case and the dial face.)

author: The Doktor
author_rank: Regular Contributor
timestamp: July 17, 2023, 08:28:14 pm
content: Quote from: abeyer on July 17, 2023, 08:08:15 pmIt needs to be quite thin to fit under the needle, thus the .4mm instead of the cheaper 1.6mm. As to the meter, already got them, not much room for front lighting.

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 17, 2023, 10:28:16 pm
content: A picture tells more than a thousand wordsI think it'll do pretty well, you can see how easily the green led from the computer mouse goes through the substrate, but you'll need to remove copper, resist & silk screen from the area.

author: The Doktor
author_rank: Regular Contributor
timestamp: July 17, 2023, 11:12:46 pm
content: DavidAlfa, thanks, that's quite informative. Also depressing. Looks like the white mask transmits light about as well as the copperMaybe I'll try bare board, no mask, and printing for scale & numbers in copper. Cheap enough to try.

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 18, 2023, 12:22:28 am
content: Nope, the mask completely blocks the light.The light you see between the pins is because there's no mask of any kind there, just the substrate.

author: macboy
author_rank: Super Contributor
timestamp: July 19, 2023, 01:46:31 am
content: Quote from: DavidAlfa on July 18, 2023, 12:22:28 amSo... It literally does transmit light as well as the copper (as in none at all).

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 19, 2023, 02:35:42 am
content: How many times do I have to repeat you need to remove all the copper/resist/silk layers in the area you need to pass light through?Yes, any of these layers will block the light!

author: donlisms
author_rank: Frequent Contributor
timestamp: July 19, 2023, 06:51:37 am
content: I think you can now order the transparent aluminum boards, yes?

author: gnuarm
author_rank: Super Contributor
timestamp: July 19, 2023, 07:13:36 am
content: The backlight seems to work better than I expected. Have you thought of a side light? You can get very tiny SMD LEDs that are like an 0603 resistor, maybe smaller. You can solder the LED upside down so it shines into the PCB. Mount it on the back with solder mask in front of it and the light might travel around the board coming out at the open areas.As others have indicated, either the solder mask or the copper layers will block the light. There's also a silkscreen layer which will block the light as well. You could have lit areas, with the silkscreen layer containing text, blocking the light. Or, you might just leave the solder mask off entirely and do it all with the silkscreen layer. That depends on which works best with your tools.Some experimentation is in order to get a good result. But small boards are inexpensive, so not a lot of cost.Oh yeah, you may need to know the current to drive your meter. I don't see that on the aliexpress page, just voltages. If it uses a lot of current, you may need to tailor the drive circuit. Is this coming from a DAC?

author: The Doktor
author_rank: Regular Contributor
timestamp: July 20, 2023, 01:37:51 am
content: Quote from: donlisms on July 19, 2023, 06:51:37 amYes, but it's a very expensive board. Also, the mask still blocks the light anywayQuote from: gnuarm on July 19, 2023, 07:13:36 amThe meter is ~1mA full scale. 3 of them will be controlled by an ESP32, 2 pins per meter most likely. LEDs, if used, will be separately controlled, also by ESP32.

author: gnuarm
author_rank: Super Contributor
timestamp: July 20, 2023, 01:50:00 am
content: Quote from: The Doktor on July 20, 2023, 01:37:51 amTwo pins per meter value? Don't you need an analog output? Or are you going to use PWM? That would only be one pin.What is the full scale voltage of the meter. Or, in other words, what is the resistance?

author: The Doktor
author_rank: Regular Contributor
timestamp: July 21, 2023, 01:42:08 am
content: Quote from: gnuarm on July 20, 2023, 01:50:00 amA resistor will be chosen so the meter can be driven just slightly past full scale, to allow calibration in software. The meter will be connected between 2 pins so polarity can be reversed to get back to zero very quickly.

author: gnuarm
author_rank: Super Contributor
timestamp: July 21, 2023, 02:00:34 am
content: Quote from: The Doktor on July 21, 2023, 01:42:08 amI didn't ask you what resistor you will add. I asked you for the resistance of the meter. Either that, or tell me what the full scale voltage and current are.

author: The Doktor
author_rank: Regular Contributor
timestamp: July 21, 2023, 02:03:03 am
content: Full scale is 600V 1mA.

author: gnuarm
author_rank: Super Contributor
timestamp: July 21, 2023, 03:32:40 am
content: Quote from: The Doktor on July 21, 2023, 02:03:03 amThis would be a 600 kohm meter coil resistance.How are you planning to generate the 600V to drive it full scale?Why did you not pick a meter with a lower full scale voltage?

author: edavid
author_rank: Super Contributor
timestamp: July 21, 2023, 03:37:40 am
content: Quote from: The Doktor on July 17, 2023, 06:43:08 pmWhy is it easier to attach a PCB than a piece of cardstock?

author: The Doktor
author_rank: Regular Contributor
timestamp: July 21, 2023, 04:04:39 am
content: Quote from: gnuarm on July 21, 2023, 03:32:40 amThe meters voltage (as delivered) is not important, and they didn't have any in low voltage ranges, out of stock. The coil is the same in all meters of this series, 1mA full scale and somewhere in the tens to hundreds of ohms resistance. There is a series resistor internally to give full scale reading at appropriate voltage. I'm in there changing the dial anyway, resistor is easy. I think I've got a 4.7K in there now, pegs a bit under 5V. Need a bit lower with ESP32.

author: The Doktor
author_rank: Regular Contributor
timestamp: July 21, 2023, 04:11:13 am
content: Quote from: edavid on July 21, 2023, 03:37:40 amLimited use of hands. Very fiddly cutting out a label and getting it aligned on the old face, at least for me. Never thought of just using a piece of cardstock by itself, wonder how that will look? Still a bitch to cut it out, maybe get a friend to do that. I think I'll order some card stock...

author: gnuarm
author_rank: Super Contributor
timestamp: July 21, 2023, 04:11:34 am
content: Quote from: The Doktor on July 21, 2023, 04:04:39 amThan it is not a 600V meter. That is just what's on the face plate.

author: The Doktor
author_rank: Regular Contributor
timestamp: July 21, 2023, 04:16:06 am
content: Quote from: gnuarm on July 21, 2023, 04:11:34 amI don't think there are any 600V meter movements, correct me if I'm wrong. When you hook the wires up to the 2 terminals on the back, it indicates 600V. And you'll read 600K ohms across those terminals. 150K for the 150V meter, etc.

author: gnuarm
author_rank: Super Contributor
timestamp: July 21, 2023, 06:30:57 am
content: Quote from: The Doktor on July 21, 2023, 04:16:06 amThe point is, I asked you about the full scale meter voltage and you said it was 600V. Now you are saying its very low, maybe less than a volt.Ok, it will do what you want. If it really is 1 mA full scale, just pick a resistor that will give you a full scale reading from the voltage you can drive with.

author: MarkS
author_rank: Unknown
timestamp: July 21, 2023, 05:55:19 pm
content: Have you considered just making a small PCB (1" x 1" or similar) with a do nothing circuit and have it manufactured in white? It would cost just a few dollars and you'd have a real example to review.

author: jt
author_rank: Regular Contributor
timestamp: July 21, 2023, 06:37:15 pm
content: As a point of caution about white soldermask - a company I worked at switched fabs and the "white" soldermask turned out with a definite pink hue. I forget what their excuse was. Wasn't really a problem, just not the aesthetic we were going for, especially given the PCB is visible in the end product

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 21, 2023, 09:20:25 pm
content: Quote from: jt on July 21, 2023, 06:37:15 pmWas it an ENIG finish by any chance ?I've seen this - the plating process did something to the soldermask ink, turning it pink. We ended up getting the mask re-printed after plating - this was with a UK PCB place so we had the flexibility to do this.There are also different types of white soldermask with different opacity and print resolution.

author: jt
author_rank: Regular Contributor
timestamp: July 23, 2023, 11:39:05 pm
content: QuoteAh, yes, that was the root cause now that you mention it. Thanks!