- Not sure about exposure time (for my lamp).
- Not sure about UV light position and height required.
With these you just have to experiment, I'd suggest putting the board fairly close to the lamp (20-50mm or so).
I suggest you take one longish strip of photoresist PCB and put something on top of it that blocks UV light, then expose the strip while moving the UV blocker away at predefined intervals (say, every 10 seconds). What you will end up with is a PCB exposed in steps for different exposure times (10s, 20s, 30s, 40s, 50s etc.), then just pick the exposure time that gives off the best result.
- Not sure if my mask pattern is opaque enough (to UV light).
If it's a normal transparency sheet it should be fine. Printing to these with a laser printer works but this depends on your printer. Large areas of black might get overexposed but this depends on your printer. If your quality of black is bad you can print two sheets and align them on top of eachother.
- Not sure about correct proportions of caustic soda to water
I have been using a 3% solution of NaOH (KOH works too) which seems to work well.
- Not sure about time to leave the board in the caustic soda - whether I'm going too far developing.
You just basically move it around in the solution until it seems developed (black stuff comes off the board), usually it develops quite quickly (under 30s) but this depends on your solution. If your solution is too strong or you develop it too long it will strip off all the photoresist if you keep it in the solution too long. A solution not concentrated enough won't successfully develop the board (again, 3% solution seems to work well). Washing the board under a faucet afterwards works well. You can continue developing after taking it away from the solution. You can actually even continue developing after etching the board a bit, just remember to wash the board. Putting the board in the etching solution shows you pretty well where the board has photoresist and where it doesn't, the copper exposed to the etching solution goes to this "dull" color/texture in under a minute in the etching solution.
- I also dont know if a developed pre-sensitized board stays so, and wont fade as its left in daylight over time. How long before normal daylight affects a pre-sensitized developed board?
A board with the plastic shield SHOULDNT be affected by daylight but to be sure I'd keep them in a place shielded from light (like a drawer).
Btw. I'd suggest you try to expose your boards with a 11W fluorescent table lamp (the ones with a "U"-shaped lamp, they should be pretty common and cost like 10e from Ikea). Put the lamp quite close to the board (like 50mm close, and have a thin plate of glass over the board to keep the mask close to the board). You can get suitable plate of glass from picture frames (again, Ikea is a good place to get these), just make sure its real glass and not plastic. Expose for 12-15min (I've used 13,5min for my boards and setup). I have been using this method succesfully for a long time. Won't work for large boards due to the lamp being so narrow but for small boards it works well.
In answer to your questions,
Remove the third lead, this will let you clear/prepare the pads for re-soldering.
It's probably best to flick the old solder off the leads; do this by heating and bending the lead with the tip of the iron and letting it spring back, DO NOT DO THIS TOWARDS YOUR FACE!
You can buy solder with a flux core. Also, if you can get it, buy lead solder, it is MUCH nicer to work with.
As regards the vid:
Yes the guy tins his iron to remove the leads, you should always tin (add solder to your iron tip) before you do anything, it stops the tip oxidising. It also helps when melting old solder.
To clean the pads generally I would put a bit of new solder on, then wick it off to give a nice clean and shiny surface.
He is applying flux, this will stop the solder "balling up" and sticking to stuff it shouldn't. You can do this, but if you are careful, and have flux core solder, you wont need to.
As regards the rest of your questions, this is just about technique. The guy seems to tack solder (a connection made to hold it in place) the leads at first. When the whole lead is tack soldered he goes around and tidies up the job.
So what YOU need to do.
Remove the lead - Tin and clean the pads and flick the old solder off the end of the lead (NOT TOWARDS YOUR FACE!)
Add a little solder to each of the pads
Place each lead above the pad and push down gently whilst touching the tip of the iron to the solder you placed on the pad. Make sure you remove the iron before removing the pressure.
When all connections are made, get some tweezers and push down on each lead in turn, melting the solder on the pad with the tip of the iron so the lead is properly flat on the pad.
Remember! Before you touch the iron to anything, make sure you have applied some solder to it and wiped it off on a sponge.
Also, if you are buying new equipment, when you first turn the iron on, keep adding solder to the tip and wiping it off. You need to thoroughly tin/protect the tip before you start using it. You can get little pots of hard flux, this is useful and it will help remove the crud on the tip due to oxidation. Also, tin it and DON'T wipe it off when you are done, the tip will still be hot for a while after you switch it off.
Best Answer
In Italy we have a transponder-based payment system called Telepass, which sticks to the windshield using Velcro. One side attachs to the glass with tape, and the other is attached to the device.
It is pretty tough, and usually lasts for years.
It seems it uses Dual Lock by 3M.