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Etching PCBs With Dry Film Photoresist

Since I do my own PCBs on a somewhat regular basis, I decided it was time to move to a more professional method to etch my boards. I have been using the cheap toner transfer method, using special yellow coated paper from China. (I think it’s coated in wax, or some plastic film).

The toner transfer paper does usually work quite well, but I’ve had many issues with pinholes in the transfer, which cause the etched tracks to look horrid, (not to mention the potential for breaks & reduced current capacity), and the toner not transferring properly at all, to issues with the paper permanently fusing to the copper instead of just transferring the toner.

BigClive has done a couple of fairly comprehensive videos on the dry film photoresist available from AliExpress & eBay. This stuff is used similarly to the toner transfer method, in that the film is fused to the board with heat, but then things diverge. It’s supplied either in cut sheets, or by the roll. I ordered a full roll to avoid the issues I’ve heard of when the stuff is folded in the post – once it’s creased, it’s totally useless. The dry film itself is a gel sandwiched between two protective plastic film sheets, and bonds to the board with the application of heat from a laminator.

The board is first cleaned with scotchbrite pad & soap to remove any tarnish & oil from the copper.

Dry Film
Dry Film

Once the board has been cleaned, one side of the backing film is removed from the gel with adhesive tape, and the dry film is placed on the board while still wet. This stops the film from sticking immediately to the clean copper, one edge is pressed down, and it’s then fed through a modified laminator:

Modified Laminator
Modified Laminator

I’ve cut away most of the plastic covering the hot rollers, as constant jamming was an issue with this cheapo unit. All the mains power is safely tucked away under some remaining plastic cover at the end. The board with it’s covering of dry film is fed into the laminator – the edge that was pressed down first. This allows the laminator to squeeze out any remaining water & air bubbles from between the two so no creases or blisters form.

After Lamination
After Lamination

Once the board has been run through the laminator about 6 times, (enough to get it very hot to the touch), the film is totally bonded to the copper. The top film is left in place to protect the UV sensitive layer during expsure.

Photomask
Photomask

The exposure mask is laser printed onto OHP transparencies, in this case I’ve found I need to use two copies overlaid to get enough opacity in the black toner sections to block the UV light. Some touching up with a Sharpie is also easy to do if there are any weak spots in the toner coverage. This film is negative type – All the black areas will be unexposed and washed off in the developer tank. I also found I had to be fairly generous with track spacing, using too small lines just causes issues with the UV curing bits of film it isn’t supposed to.

Exposing The PCB
Exposing The PCB

The PCB is placed on a firm surface, the exposure mask lined up on top, and the whole thing covered with a sheet of standard glass to apply even pressure. The UV exposure lamp in this case is a cheap eBay UV nail curing unit, with 15 high power LEDs. (I’ll do a teardown on this when I get some time, it’s got some very odd LEDs in it). Exposing the board for 60 seconds is all the time needed.

After Exposure
After Exposure

After the board is exposed, the areas that got hit with the UV light have turned purple – the resist has hardened in these areas. It’s bloody tough as well, I’ve scrubbed at it with some vigour and it doesn’t come off. Toner transfer was a bit naff in this respect, most of the time the toner came off so easily that the etchant lifted it off. After this step is done, the remaining protective film on the top can be removed.

After Developing
After Developing

The film is developed in a solution of Sodium Carbonate (washing Soda). This is mildly alkaline and it dissolves off the unexposed resist.

After Etching
After Etching

Now it’s into the etching tank for a couple of minutes, I’m still using Ferric Chloride to etch my boards, at about 60°C. Etching at room temperature is much too slow. Once this is done, the board is washed, and then dipped in the strip tank for a couple of minutes. This is a Sodium Hydroxide solution, and is very caustic, so gloves are required for this bit. Getting Ferric Chloride on skin is also a fairly bad idea, it stains everything orange, and it attacks pretty much every metal it comes into contact with, including Stainless Steel.

This method does require some more effort than the toner transfer method, but it’s much more reliable. If something goes wrong with the exposure, it’s very easy to strip the board completely & start again before etching. This saves PCB material and etchant. This is definitely more suited to small-scale production as well, since the photomask can be reused, there’s much less waste at the end. The etched lines are sharper, much better defined & even with some more chemicals involved, it’s a pretty clean process. All apart from the Ferric Chloride can be disposed of down the sink after use, since the developer & stripper are just alkaline solutions.

 

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Nanoptix “Spill-Proof” Thermal Receipt Printer

Nanoptix Spill-Proof Thermal Printer
Nanoptix Spill-Proof Thermal Printer

I have yet another receipt printer, this one appears to be brand new. It’s possibly the smallest thermal 80mm printer I have at the moment, and has both USB & Serial interfaces.

Controller PCB
Controller PCB

There’s not much to these printers at all. Removing a single screw allows the case halves to separate, showing the guts. The controller is based around a Texas Instruments TMS320VC5509AFixed-Point DSP. It’s associated Flash ROM & RAM are to the right.
Power supply is dealt with in the top right of the PCB, with the interface ports further left.

Print Head
Print Head

Here’s the thermal mechanism itself, with the large print head. The stepper motor to drive the paper through the printer is just peeking out at top right. The paper present sensor is just under the left hand side of the print head.

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nb Tanya Louise Deck Welding

The boat being over 50 years old, there are some parts that are suffering from rather bad corrosion. The bow deck plate is about the worst, so this is being replaced in it’s entirety.

However a hole has developed in the stern deck, this has rusted from the inside out due to condensation in the engine bay.

After Grinding
After Grinding

After taking a grinder to the area, this is how it looks. The steel has gone from 1/4″ to paper thin, not surprising after 50 years or so!
It would be a massive job to cut out the entire plate for replacement, so a patch was made from 5mm steel, and welded over the hole:

Patch
Patch

Here’s the patch partially welded. The holes closer to the bottom are another small area of damage, and another patch will have to be cut for this. It’s covering the deck drain channel so it’s frequently under water, so it’s inevitable that this section would corrode.

All that is left to do now is to finish off the welding, grind everything smooth & repaint.

Another small job complete!

73s folks