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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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EpEver MT50 Control Panel Teardown

MT50 Control Panel
MT50 Control Panel

Here’s the MT50 controller from EpEver, that interfaces with it’s Tracer MPPT solar charge controllers, and gives access to more programming options on the charge controllers, without the need for a laptop. The display is a large dot-matrix unit, with built in backlight. Above is the display on the default page, showing power information for the entire system.

PCB Rear
PCB Rear

The rear plastic cover is held in place by 4 machine screws, which thread into brass inserts in the plastic frame – nice high quality touch on the design here, no cheap self tapping plastic screws. Both power & data arrive via an Ethernet cable, but the communication here is RS-485, and not compatible with Ethernet! The PCB is pretty sparse, with comms & power on the left, LCD connection in the centre, and the microcontroller on the right.

RS-485 Transceiver
RS-485 Transceiver

On the left of the board is the RS0485 transceiver, and a small voltage regulator. There’s also a spot for a DC barrel jack, which isn’t included in this model for local power supply.

STM32 Microcontroller
STM32 Microcontroller

The other side of the board holds the main microcontroller which communicates with the charge controller. This is a STM32F051K8 from ST Microelectronics. With a 48MHz ARM Cortex M0 core, and up to 64K of flash, this is a pretty powerful MCU that has very little to do in this application.

PCB Front
PCB Front

The front of the PCB has the ENIG contacts of the front panel buttons, and the LCD backlight assembly. There’s nothing else under the plastic backlight spreader either.

LCD Rear
LCD Rear

The front case holds the LCD module in place with glue, and the rubber buttons are placed underneath, which is heat staked in place.

LCD Model
LCD Model

The LCD is a YC1420840CS6 from eCen in China. Couldn’t find much out about this specific LCD.

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nb Tanya Louise – Gas Locker Corrosion Part 1 – Removing The Old Locker & Replacing the Deck Plate

Severe Corrosion
Severe Corrosion

This is a part of the boat that hasn’t really had much TLC since we moved aboard, and finally it’s completely succumbed to corrosion, opening a rusty hole into the engine space below. I’ve already used a grinder to remove the rest of the locker – and even this had corroded to the point of failure all around the bottom just above the welds. The bulkhead forming the rear of the locker has also corroded fairly severely, so this will be getting cut out & replaced with a new piece of steel.
This was originally a 1/8″ plate, but now it’s as thin as foil in some places, with just the paint hiding the holes.

Replacement Steel
Replacement Steel

I’ve cut out as much of the corroded deck plate as possible –  it’s supported underneath by many struts made of angle iron, and got the new 3mm replacement tacked in place with the MIG. I’ve not yet cut out the rotten section on the bulkhead, this will come after we’ve got the steel cut to replace it, as electrical distribution is behind this plate – I’d rather not have weather exposure to the electrical systems for long! Unfortunately more corrosion has showed itself around the edges of the old locker:

Thin Steel
Thin Steel

Around the corner the steel has pretty much totally failed from corrosion coming from underneath – applying welding heat here has simply blown large holes in the steel as there’s nothing more than foil thickness to support anything.

Some more extensive deck replacement is going to happen to fix this issue, more to come when the steel comes in!