Homebrew PCBs

THANK YOU for the verbose explanation.
I bought the cheap photo-resist but had not done any experimentation 
with it yet.


On 03/02/2017 01:28 AM, Larry Battraw lbattraw@... [Homebrew_PCBs] 
wrote:
> 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
>