Realistic minimum pitches for diy PCB etching - Page 1
Forum section: electronics / projects

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: September 21, 2017, 07:59:58 am
content: Hey guys,I was looking at making my own pcb's for a future project, and wondered what I should be considering in terms of realistic minimum pitches and spacing. For instance, is it realistic to route traces between pins of a DIP or similar package when etching your own PCB? Is there a strong variation between techniques?

author: tautech
author_rank: Super Contributor
timestamp: September 21, 2017, 08:20:26 am
content: Quote from: TheUnnamedNewbie on September 21, 2017, 07:59:58 amI didn't on purpose try to push traces between DIP pins to start with, but rather honed layout and etching skills.20 mil is a realistic target to start with but once you have the process sortedandrepeatable 10 mil (0.25mm) is quite achievable. I tend to stick with 15 mil as I know (for me) it will etch OK.Technique is pick your poison, beit TT, photolithography, Ferric, Cupric or Ammonium Persulphate, most have their way to get good results. What you have on hand can help you decide which process you want to run with along with the cost of acquiring the gear you don't have and need.By far I spend the longest time on layout, TT, etching and drilling take no time at all by comparison.To keep things easy and simple I've stuck with Phenolic PCB for easy cutting and drilling.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 21, 2017, 08:41:31 am
content: When I used to do a lot of homebrew PCBs :Laser print at 600dpi on tracing paper with Microtrack laminate and silicate developer would reliably do 12.5mil track/space, with the occasional 10mil where necessary. Upping the print to 1200dpi helped on finer boards.It helps if you choose line widths/spacing that are an exact number of dots at the print resolution.The limit was the artwork quality I occasionally get typesetter films done for finer stuff

author: tautech
author_rank: Super Contributor
timestamp: September 21, 2017, 09:00:36 am
content: Have a hunt through this threads for some tricks and etchant comments:https://www.eevblog.com/forum/projects/pcb-milling-failure/Member KL27x has a fair bit of experience on DIY etching.

author: hlavac
author_rank: Frequent Contributor
timestamp: September 21, 2017, 09:13:37 am
content: I was able to make 0.2mm spacing / traces work, but barely.With photosensitive PCB and laser printed mask on tracing paper.Switched to inkjet since to get better coverage of larger areas.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 21, 2017, 09:20:44 am
content: Initial tests / uncontrolled process with good margin:20 mil tracks, 15 mil spacingsPhotoresist exposed from a high quality (good 600 dpi print, dense blacks) original using point source light and/or vacuum, on 35µm (1oz) copper clad, etchant well agitated by compressed air, or sprayed, with margin for error10 mil tracks, 10 mil spacingsSame, but with 18 µm copper clad: 8/8 milsSame, but with no margin for error: 6/6 mils

author: sleemanj
author_rank: Super Contributor
timestamp: September 21, 2017, 09:30:57 am
content: I find 12/12 a good default, I also use oblong pads rather than round so you can have them thinner but longer to open up space between pins but still have a reasonable solderable area. I'll drop down to 10mil traces if necessary, I seldom go less, and very seldom go for less separation than 12, preferring to drop trace width first. I use dry film resist and tracing paper artwork, led exposure , no collimation, nor vacuum, just a heavy weight.

author: ElektroQuark
author_rank: Unknown
timestamp: September 21, 2017, 09:34:53 am
content: One of my first home made PCBs some years ago:8 mill traces / 10 mill spaces.1:1 size:About 10x:You can read about my proceduresHERE(in Spanish).

author: asmi
author_rank: Super Contributor
timestamp: September 21, 2017, 05:33:39 pm
content: I've done down to 0.15/0.15 mm at home, but now there is no point in doing so as these boards are incredibly cheap to order in China to have them in your hands in about 5 business days or so. And just about any Chinese fab will make 0.15/0.15 mm with no problems. And that wait time perfectly fits into my "hobby schedule" - design board on Saturday -> send it to the fab in the evening -> get it approved for fabrication by Sunday morning -> order parts for it on Wednesday with 1 day delivery -> receive boards on Thursday or Friday -> assemble it in the morning of Saturday -> find out that I messed up something in layout -> fix issues and design rev. 2 -> goto step 2

author: Fire Doger
author_rank: Regular Contributor
timestamp: September 21, 2017, 07:02:25 pm
content: I have done 6mil but it need some practice. 10mil is very realistic without failed pcbs if you make a repeatable process.

author: Cerebus
author_rank: Super Contributor
timestamp: September 21, 2017, 07:43:20 pm
content: I use the negative UV imageable film (DuPont Riston), the sort you laminate to the PCB and then expose. I use a persulphate etchant. That combination is easily good enough for 4mil/4mil at home and the film is used in industrial production to 1.2mil/1.2mil (30 microns).However, the difficulty is in getting a good enough image and exposure. Using a diffuse (as opposed to collimated) UV light source knocks off around 2mil from what is achievable. Using a good laser printer or inkjet on acetate (image side down) and a fluorescent UV tube the best I've been able to do consistently is 8mil/8mil and I go for 10mil/10mil unless I absolutelymusthave some smaller features.If you can get proper imagesetter film negatives and a single point/collimated light source you can aim higher, but I wouldn't try finer than 4mil/4mil.

author: Wimberleytech
author_rank: Super Contributor
timestamp: September 21, 2017, 08:09:47 pm
content: 10mil with good printer and using TRF. Do a copper fill to minimize the amount of copper to etch. Also fresh etch and constant monitoring of the etch.

author: KL27x
author_rank: Super Contributor
timestamp: September 21, 2017, 08:52:19 pm
content: When I first started toner transfer, 15/15 mil was pretty much fine. Any larger didn't really help that much.I can comfortably do 8/8, now, pretty much all day. And the main limitation is the resolution of my printer. But if you want to get that fine, you have to pay attention to more details in the process. There are several ways to improve toner transfer, but you have not specified, so I will not dwell on toner transfer in particular.In general:1.Area of the board matters. If you want to run traces through DIP pads, this should be no problem. Just make your DIP pin pads rectangular, so you can make them thinner. If they break on the side of the hole, no problem. Also, you can make the traces thinner locally, them fatten them up again. The larger area you try to cover in smaller pitch/spacing, the more likely you will have a failure. Esp if your board is very small, feel free to push the boundaries. Just make several more than you need to account for any failures.2. Thickness of the copper layer: The thinner the copper, the more accurate your result. I would advise you get copper clad in 1/2 oz, 1 oz, and perhaps some single sided 2 oz boards for DIP PSU/power stuff. For tight pitch, use the thinnest copper you can.3. Etchant: etchant matters, particularly when the copper is thicker than 1oz. HCl and peroxide is perfectly capable of making good boards in 1 oz pour, but it becomes difficult at 2 oz. Different etchants have various degrees of undercutting, and HCl + peroxide is far and away at the bottom of the barrel. It is the most effective at removing the resist before the traces are done etching.4. "Shading" When you get to traces smaller than 12, you may want to start paying attention to routing. Esp in an etch tank with any turbulence/agitation/flow, a lone trace with nothing around it will tend to etch more than traces that run together in a pack. When doing fine pitches, I will preferentially use a ground pour to fill in blank space to reduce this sort of etching problem. Keeping consistent space between all areas of copper increases the probability that all etching finishes closer to the same time.