Need help with homemade PCBs - Page 1
Forum section: electronics / general

author: electrotwelve
author_rank: Contributor
timestamp: October 20, 2020, 08:18:14 am
content: I've been trying to make PCBs at home albeit in a better way using better UV exposure units and quality chemicals. I bought a fewpre-sensitized PCBsfrom RS but I have not been having luck exposing them. The attached image here is after 5 minutes of exposure under a 30W UV lamp at a distance of about 50mm. The result was marginally worse when exposing for about 2.5 minutes. Usingthisdeveloping liquid. What am I missing? This is what they look like,

author: Yansi
author_rank: Super Contributor
timestamp: October 20, 2020, 08:30:45 am
content: No image is attached.You can't combine random photoresist with random developer solution. There are quite a few types of them. Two main differences being if the photoresist is POSITIVE (exposed areas will be gone and etched), or NEGATIVE (exposed areas will not be etched).POSITIVE photoemulsions use mainly low concentration (<2%) NaOH (lye) as a developer.NEGATIVE photoemulsions are based on different chemistry.Make sure first you know you combine compatible chemicals. Also read the manufacturer recommendations on UV light wavelength and get an idea about the exposure time required in your setup: Use a strip of the PCB and expose it in say 15 second steps (by uncovering more area each 15 seconds).There are a LOT OF variables to control when making PCBs at home, on the other hand the processes are quite tolerant and robust. You need to keep trying and perfecting your processes.

author: electrotwelve
author_rank: Contributor
timestamp: October 20, 2020, 10:58:12 am
content: Quote from: blueskull on October 20, 2020, 08:25:35 amThe photo mask istransparent sticker sheet. I believe its about 0.15mm thick. I printed the mask on that and then stuck it on the board. I wonder if the sticker adhesive interferes with the photoresist. I placed the board such that its right under that lamp. This is my exposure unit for now. The side panels have reflective Aluminium tape on the inside. Also shown is thelamp.

author: electrotwelve
author_rank: Contributor
timestamp: October 20, 2020, 11:03:19 am
content: Quote from: Yansi on October 20, 2020, 08:30:45 amIts not random. From what I could find about the part numbers of the boards, it is a positive resist laminated board. The manufacturer's sheet states that with their brand mask films the adequate exposure time is 2.5 mins. With others it can be upto 5 minutes.the UV light wavelength stated by the lamp manf is 380-395nm. I'll try exposing a strip to calibrate the lamp. I've had mixed success with other setups and nothing was consistent and repeatable. However, as you say, there are a lot of variable. I'll keep trying!

author: ace1903
author_rank: Regular Contributor
timestamp: October 20, 2020, 11:37:55 am
content: 30w in UV range is huge power. Either big percentage is in visible spectrum or number is simply wrong.Glass acts as filter on UV spectrum. Different types attenuate differently.To test the process you can put masking tape (isolating type that electrician use) over some part of ground plane.It will show you if developer solution is good enough and help you to see if you need to shorten exposure time.Maybe later I will upload pictures of the setup that I used at home for years.Recently major issue is to find a printer that produces black areas with good density.

author: fcb
author_rank: Super Contributor
timestamp: October 20, 2020, 11:47:05 am
content: You might have over-exposed the whole thing? Looks like you are really cooking with that lamp. What did it look like when etched?How good is your image masking - make a test by using something solid (piece of metal) to block UV and prove your masking is not UV transparent.

author: Smallsmt
author_rank: Frequent Contributor
timestamp: October 20, 2020, 01:23:43 pm
content: I would try to keep more distance > 20cm then adjust light time.Expose only a small part of the circuit board and cover the rest with black cardboard.Every minute you move the cardboard a little bit to build an exposure stair.1 - 10 minutes steps for example.After the development you can choose the best time and use it in the future.

author: electrotwelve
author_rank: Contributor
timestamp: October 20, 2020, 01:55:02 pm
content: Quote from: ace1903 on October 20, 2020, 11:37:55 amYes that's the plan now. To see your setup would be very helpful.

author: electrotwelve
author_rank: Contributor
timestamp: October 20, 2020, 01:59:04 pm
content: Quote from: fcb on October 20, 2020, 11:47:05 amHmmm I'm beginning to think that may be the case now. I did not etch it because it looked like all copper. Here is what the mask looks like. Looks good to me TBH. Printed on a transparent sticker sheet. I'll try the 'calibration'. Once exposed does the board have to be developed right away or can it stay there while I expose other areas it in steps?

author: electrotwelve
author_rank: Contributor
timestamp: October 20, 2020, 02:00:14 pm
content: Quote from: Smallsmt on October 20, 2020, 01:23:43 pmWill try this! Doesn't 20cm seem too far?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 20, 2020, 02:04:23 pm
content: Do a test exposure in steps of 30 seconds ( i.e. cover part with foil and pull it back every 30 secs) , up to 5 mins - this will tell you where you are with exposure time.No idea about using LEDs, but insect killer tubes work great - typically 5 mins exposure ( the type with the white phosphor, not the black "blacklight"or clear germicidal types)And if possible, use a silicate based developer instead of sodium hydroxide - this gives a much wider margin between "develop" and "strip"

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 20, 2020, 02:05:54 pm
content: Quote from: electrotwelve on October 20, 2020, 02:00:14 pmYes, though you need a distance that gives a uniform exposure across the board - a high power small LED source as shown probably won't do that at any sensible distance.Another tip - for the artwork, laser print on tracing paper - use the thickest stuff you can find - at least 90gsm

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 20, 2020, 02:24:57 pm
content: Here I use an Ed17 100 Watt coated mercury vapor lamp powered by a 150 V DC constant current inverter, pwm with no filter.It is mounted in a metal hood 300mm above the platen on photo enlarger where the vacuum holder is placed.The white light is intense, brighter than sunlight and I suppose with HG lines. Measured EV 15 reflected on exposure spot meter.I print transparent film on a Epson ET-2720, not pure black, the color has all 4 cartridges spraying to get better coverage.Hex 0F 16 16Coverage is not perfect with small pin holes occasionally, but good enough for hobby.I have started doing smd now.Ink side down against the MG positive copper.Exposure time is critical, I used a method like smallsmt mentioned. and keep notes of the exposures.Exposure time 98 second with not much leeway about that.Problem now is cutting the boards before exposure. I modified a scroll saw but it is still not very good.Also cutting the board can damage the photo resist for about 1mm away from the cut.Edit: Photo added. The pinholes are only visible through the microscope.The pin holes shown in the photo are my laziness in filling.

author: asmi
author_rank: Super Contributor
timestamp: October 20, 2020, 03:07:53 pm
content: I wonder why people still bother with this homemade nonsense when you can have it made professionally for less than what raw materials would cost over here...

author: electrotwelve
author_rank: Contributor
timestamp: October 20, 2020, 03:34:25 pm
content: Quote from: asmi on October 20, 2020, 03:07:53 pmTrue! However, making your own boards to prototype a design in a jiffy is invaluable. Having to wait for more than a week to get boards made and again spending more if the design changes is way too time-consuming and expensive. Not to mention that all Chinese board manufacturers are inaccessible to me right now due to the ongoing tensions on the border...

author: phil from seattle
author_rank: Super Contributor
timestamp: October 20, 2020, 03:48:31 pm
content: Quote from: electrotwelve on October 20, 2020, 03:34:25 pmHope your border situation calms down. Peace always seems elusive.I also find chemical etching and all it entails to be unsatisfying with cost and mess to get an inferior product. I used to do etching but no more. For quick, simple PCBs for testing, I use a CNC mill but that is only if I have no other option. There simply is no substitute for a real PCB with solder mask and silk screened legends.Is there is a potential market in India for an OSHPark like service?

author: electrotwelve
author_rank: Contributor
timestamp: October 20, 2020, 03:52:39 pm
content: Quote from: mikeselectricstuff on October 20, 2020, 02:04:23 pmThis is useful info. Thanks! Arepaper developersthe same stuff?

author: electrotwelve
author_rank: Contributor
timestamp: October 20, 2020, 04:02:04 pm
content: Quote from: phil from seattle on October 20, 2020, 03:48:31 pmI hope so too! All the strife and conflict seems more pronounced in 2020...I see what you mean about professionally made PCBs, but...There definitely is a market for OSHPark-like service here. We have a huge student community who need such a service. Not sure what the economic model entails though. We have a institutions like the IITs who could setup PCB labs that others can use. We have similar setups for mechanical fabrication services.

author: electrotwelve
author_rank: Contributor
timestamp: October 20, 2020, 04:04:23 pm
content: Quote from: mag_therm on October 20, 2020, 02:24:57 pm98 seconds? Wow, that is precise... Awesome setup though!

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 20, 2020, 04:20:20 pm
content: I think +/- 10 seconds ( +/- 10%) would be over/under exposed so I have been using that as a sweet spot.This pcb processing setup came out of my photography and film processing hobby, which I got a bit tired of after many years.Thanks

author: electrotwelve
author_rank: Contributor
timestamp: October 20, 2020, 05:25:31 pm
content: On closer inspection of the lamp, I realized it has a glass panel that covers the UV source which may be blocking any UV coming through at all...

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 20, 2020, 05:45:43 pm
content: Yes The safety glass is not desirable for exposing the photo resist.If you can get an intense light, the exposure time comes down, which helps preserve the resist and the edges on the wanted copper(At least, that seemed to be the experience here.)

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 20, 2020, 05:56:53 pm
content: Quote from: electrotwelve on October 20, 2020, 03:52:39 pmNo.See my page for some info on silicate developerswww.electricstuff.co.uk/pcbs.html

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 20, 2020, 05:58:17 pm
content: Quote from: electrotwelve on October 20, 2020, 05:25:31 pmThe wavelengths for exposing PCBs pass through normal glass just fine.

author: julian1
author_rank: Frequent Contributor
timestamp: October 20, 2020, 06:36:09 pm
content: Did you try etching it? Is the etchant just washing everything away? My memory of using the pre-sensitized boards, is that there is no strong visual difference between the cured/non-cured parts. It is only after dumping the board in etchant that the non-UV cured bits will get eaten up (along with copper) and you will know if UV exposure level was right.I hope I remember that correctly. I found just leaving the boards outside for twenty minutes on a partially overcast day would also work. Maybe useful as a test to rule out issues with the uv lamp.

author: electrotwelve
author_rank: Contributor
timestamp: October 20, 2020, 07:11:35 pm
content: Quote from: mikeselectricstuff on October 20, 2020, 05:56:53 pmThat’s a really useful and detailed guide. Thank you for sharing it. I couldn’t really find sodium metasilicate but i did find a sodium silicate concentrate. I hope that would work.

author: electrotwelve
author_rank: Contributor
timestamp: October 20, 2020, 07:14:08 pm
content: Quote from: julian1 on October 20, 2020, 06:36:09 pmDidn’t try etching it since it looks like all copper with faint hints of the layout. I did read about leaving it in the sun and though I have not tried it it seems limiting since I’d have to work only in daytime.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 20, 2020, 07:18:09 pm
content: Some protoresists need a little help removing the exposed resist after developing to leave a crisp image - a wipe with a wet paper towel usually does it.If the exposed areas don't turn pink instantly when dipped into ferric chloride etchant, it's not developed properly.