Homebrew_PCBs

Is it possible to use UV resin as a substitute for photoresist?  Copper would be coated with UV resin and an image would be made in the resin with a UV laser.  The uncured resin would be rinsed away and the board etched.  Would the resin adhere to the copper?   Is it worth running an experiment to try it?

Jeff

 

Is it possible to use UV resin as a substitute for photoresist?  Copper would be coated with UV resin and an image would be made in the resin with a UV laser.  The uncured resin would be rinsed away and the board etched.  Would the resin adhere to the copper?   Is it worth running an experiment to try it?

 

Jeff

>

> I've eventually come to the conclusion that the cheap and
> easily-obtainable negative brush-on photo-resist from China ($9
> including shipping to the US for 100g of concentrated resist that
> provides 300g when properly diluted, which is a ton of photoresist!) is
> the way to go. I use an Epson inkjet with cheap and easy refillable

>

> I've eventually come to the conclusion that the cheap and
> easily-obtainable negative brush-on photo-resist from China ($9
> including shipping to the US for 100g of concentrated resist that
> provides 300g when properly diluted, which is a ton of photoresist!) is
> the way to go. I use an Epson inkjet with cheap and easy refillable

 

Hi Jeff,

I've experimented widely in search of materials that could act as photo-imaged resist for PCBs and it has not been terribly successful.  You end up with a lot of problems, such as having the UV resin flow over time while being exposed (Particularly if your PCB is mounted on a X-Y table and undergoes a fair amount of acceleration), potentially obscuring or providing insufficient coverage for  traces.  --And naturally since the resin is liquid you can't use a simple transparency with artwork to cure it; as you indicated, a laser is required, or potentially some sort of fancy lens assembly to focus an image of your artwork with high brightness/contrast from a DLP projector (Similar to what a SLA 3D printer would use), or even a scheme similar to an old-school photo plotter that "draws" the traces with a particular shape focused through a lens onto the board.  It's all a hassle and the complications can consume a lot of time.  

I've eventually come to the conclusion that the cheap and easily-obtainable negative brush-on photo-resist from China ($9 including shipping to the US for 100g of concentrated resist that provides 300g when properly diluted, which is a ton of photoresist!) is the way to go.  I use an Epson inkjet with cheap and easy refillable carts to print onto a transparency, allowing me to create a lot of copies of the board with a single transparency (Unlike the direct-printing process with laser toner that is strictly1:1).  

I will add a side-note that many Epson printers don't seem to be able to use transparencies as-is since they rely on a light sensor to detect the paper, which naturally makes the printer think it is out of paper when using a transparency.  I use a couple pieces of double-sided tape at the top of the page to stick a normal piece of paper to the transparency, which works just fine as long you know where to put the tape (Since it can cause it to feed incorrectly with tape in the wrong positions).

It's amazing what good results you can get from this cheap photo-resist, it can look pretty awful after painting on the resist but after drying it you can get results that are very close or even better in some cases than thin-film laminated resist sheets.  I've gotten the best results from using a little plastic spray bottle and some solvent to dilute the resist, spraying it on like you would with a can of aerosol spray paint (i.e. beginning to spray before you reach the board, avoiding excessive paint application); you want avoid drips.  You prop the board up and go back and forth, moving up/down slowly after each pass to make sure each layer overlaps, without enough to actually drip.  About 20 minutes in the oven at roughly 150C/65C will cure the resist.

I've used the thin-film resist (Doing the laminating myself since it's so much cheaper, especially when you goof things up as much as I do) for many years but it definitely isn't an easy or guaranteed-successful method; part of the problem is that it ages and from what I've heard from the manufacturer it only has a shelf life of around 9 months to a year, though I think that can be extended by keeping it cold in the refrigerator.  This produces an effect where the contrast gets worse and worse and you end up struggling to get a good result since it gets much more difficult to expose it properly and still be able to rinse off unexposed resist without affecting the exposed sections.  Extremely frustrating!  It also doesn't tend to stick to the PCB as well, becoming somewhat brittle and inclined to flake off on thinner traces.

I think the main advantage the cheap, Chinese liquid, brush-on photo-resist has is related to how "fresh" it is, since I've never seen much of a difference from it ageing. It always provides extremely sharp results (Even when the brush strokes on the board from applying the resist are not great).  Another major advantage is that it will adhere very strongly to virtually any PCB surface, even if it's oxidized or scratched, etc.  This is in terrible contrast (ha ha) to dry-film resist which is very temperamental about sticking to less-than-perfect PCB surfaces, and this gets worse and worse as it ages.  Applying dry-film resist in a hobbyist environment is somewhat of a dark art since it's designed to be applied in large, commercial laminators that are very good about applying a precise amount of heat and uniform pressure to a virtually perfect PCB surface, something that can be very difficult to do at home.  Search around for DIY dry-film photoresist lamination and you'll find a large number of opinions on how to do it, ranging from modified budget laminators (Or expensive, semi-pro models) to "wet process" methods that involve spraying the board down and even forgoing heat.

Anyway, this has gotten a lot longer than I intended but the short answer is don't waste your money on UV resins.  Using a laser to expose dry-film or the cheap brush-on resist is probably the best way to go if you want to avoid printing out transparencies to image your boards.  The brush-on resist is also going to be far cheaper than any UV resins, as well as being easy to apply and develop/etch.  Search 301857576850 on eBay for an example of the brush-on resist, I'm not affiliate with any of these products and this is just my opinion.

HTH-

Larry

Hi Larry,

What do you mix with the photoresist to dilute it?  Do you dilute the full 100g at once or dilute small amounts when you pull it out to use it?  Do you have any thoughts on a spin coater to apply it?

Jeff

KPR is currently available. It is sold by Transene Company : http://transene.com/pkp/

This place also has materials and supplies: http://www.camconchemical.com/

It is not clear that KPR can be purchased in hobbyist quantities. The material has a shelf life, so it is impractical to to buy much more than you can use over a few months.

On the subject of the "Blue Paint" from e-bay: I too am confused by the presentation on the website. It says to dilute the material with an unidentified diluent. They don't seem to offer that material for sale, not do they tell us what it is. They do mention that a possible substitute is banana oil. I don't know about any of the other members of this forum, but I don't keep banana oil in stock because I have never encountered  a squeaky banana. Has anyone figured out what the solvent is?

With regard a spinner for applying photoresist, this website has some info on building and using your own spinner. Much of the other material on the site, although interesting, is not new, and some is out of date: http://www.tayloredge.com/reference/Electronics/PWB/pcb.html

Harvey

 

Yes and no, with regards to using a UV resin for a photoresist.  The original popular photoresist was KPR (Kodak PhotoResist):

https://en.wikipedia.org/wiki/KPR

That photoresist is UV sensitive, and will harden when exposed to UV light.  Then, it is developed with KPR-developer, a rather nasty mix of volatile and toxic materials (Xylene?) (although I think I remember that safer, alternative developers were available at one point).

The largest problem with KPR is that it is a "negative type" photoresist, in that you need to UV expose the areas which are going to leave Copper.  The world has been searching for an effective "positive type" photoresist for decades.

Now, I haven't tried to obtain any KPR for about 35 years.  I know that, at least at one time, it became quite difficult to obtain.   I'm not sure of the availability now, though.

Dave

Banana oil is slang for amyl acetate or isoamyl acetate because they smell like bananas. Amyl acetate is used in lacquer thinners. Lacquer thinner, acetone and alcohols will thin this resist. Alcohol is probably not a good idea due to the amount of water in most available alcohols.

I have spin coated a few small (2" x 3") boards so it can be done. However it is not quite as simple to achieve a satisfactory result as some of the videos of spin coating you may see on the internet.

Craig