Caustic Soda concentration

[kl1k]

I'm busy experimenting with using photosensative spray, uv light and caustic soda and not having much luck. The spray and uv exposure seems to go ok, but when I put my board in the soda, it either removes all the purple stuff from my board or does nothing - some of the time I go see the tracks on the board and they look quite good - but the rest of the board is wiped.

I've fiddeled around with the caustic concentrate - 1 teaspoon per 1L, or 1 tsp per 1.5L - I just can't seem to get it right.

Do you think its my uv exposure, caustic soda?

 

[flatulent]

You need to do a two dimensional set of data points. A range of UV exposures and a range of soda concentrations.

The trouble may be the photosensitive coating. Is this put on by the manufacturer or by you. If by you it could be the copper cleanliness or your spray thickness or the age of your spray chemicals.

 

[kl1k]

I'm spraying the photo resist on.

 

[flatulent]

You can do some separate experiments.

1. Soda concentration experiment. Cut several strips of PC material. For several trial runs of different soda concentration put one strip in the solution and time how long it takes to etch the copper off. This should give you an idea of which concentration to use.

2. Resist experiments. For all of these first clean the copper with rubbing alcohol to remove any oils then use vinegar (mild acid) to remove any oxide.

a. spray thickness subtest. Spray the surface n times for n = 1 to some large number. Expose to the UV or not depending on which makes the resist be the state that shields the copper from etching. Put the strip with resist coating in the soda solution for the time found in 1 above and see if the resist protects the copper from etching.

b. If the resist needs to be exposed to UV to make it shield the copper you need to experiment with the exposure. I suspect that too much will not hurt and only cause "undercutting" of the resist and make the traces slightly different widths.

 

[kl1k]

Thanks for the tips - I'll make a few trial runs and let you know how it turns out.

 

[platonas]

I agree with Faltulent,
I will comment further and express some personal experiences.

1. First, avoid hot days while applying photoresist spray. It does not spread evently in hot environment (higher than 30C). If it is hot, cool the spray can but to not to a very low temperature. Personally I put it in the fridge for about 10 minutes. I also cool the PCB but this is not a good idea since it easyly condensates atmospheric humitidy.

2. Better spray some more matterial and compensate with more exposure time than leave spray gaps.

3. It is important to clean your board very well. Use methodes as described by Flatulent. Personally I use lemon juice (soft acid) with soap to remove oxides and dirts and then clean the board with thinner or acetone or pure alcochol.

4. Caustic Soda has a suggested mixure of 7grams in 1 litre of water. Start with this quantity and if it does not work, add small quantities of CS untill you can observe results on your PCB. Do not add too much because it can remove even unexposed photoresist spray.
When you find the correct dosage, take a note to avoid future time waste.

For the expposure I use normal fluorescent lamps. Not the UV ones but the normal light lamps. I use 2 lamps of 1,5 feed lenght each. The exposure time is between 9 to 13 minutes. The advantage of this long exposure is that, few more min utes of overexposure result in a small overall persentage and hence there is not so critical. I sometimes forget the board for 15 minutes and it rarely destroys the photoresist spray.
Certainly exposure time depends on the exposure distance from the lamps and the thickness of the separating material. I use glass of about 3-4 mm at a distance of 15 cm.

After exposing your board, normally you can distinguish the unexopsed from the unexopse part of the spray before you put it in the caustic soda solution. There is a slide colour variation. If it isn't so then eigther you over or under exopse the board.

it either removes all the purple stuff from my board or does nothing

That means your board is either over or underexposed. You have to play with exposure time. First try to expose less and observe any results.

some of the time I go see the tracks on the board and they look quite good - but the rest of the board is wiped.

This maybe from an uneven light source. In some areas it is strong and in some other is weak. Do not use a spot source of light.

regards
P.

 

[kl1k]

Thanks for the replies - a few more quetions.

I seem to be getting better but its still not 100%

1) How thick should the photo resist be sprayed on. After I've sprayed it on and its dried you can clearly see the purple film - there's no gaps.

2) After you put the board in CS - how do you know when to take it out? I can see the tracks clearly but the pcb is still have a layer of the purple stuff on it. Should the pruple film vanish completly (except for on the tracks)

3) Which is a better etchent to use - Ferric Chloride (mine works really slowly even when heated up) or Hydrochloric Acid+Hydrogen Peroxide

I've done another board (etched) and the top half of it is perfect - nice clear tracks. But on the bottom half none of it is etched. If I leave it in too long it starts to eat away the tracks, but still doesnt touch the bottom section? Does this sound like too thick spray?

 

[nford]

There is an art to putting the board into caustic soda - long enough so that all the purple etch resist that you dont need is gone but not so long that your tracks start to dissolve . It just takes a lot of experimentation to find the right concentration and time for developing.

If you're getting one side good and the other unetched - it either means that your spraying was uneven or that your exposing was not even. The best solution to this is to get more practise at making PCBs. Try different methods of doing things eg. raise the board so its not so close to the exposure lamps, make sure the board is level when you spray it, develop the board in cs solution for a little bit beyond where the tracks are clear.

With the etchant, ferric chloride is known for being really slow and messy. However, the slow part can be to your advantage - if the etch resist on your your boards is not the greatest, the slowness of the ferric chloride gives you time to fix small problems like broken tracks partway through the etching.

 

[kl1k]

THanks - I'll try a few more test board tonight.
As least I now know that there shouldn't be purple etch all over tha board. No wonder its now etching properly

 

[kl1k]

I'm now getting the hang of it - almost 100% now. I had 2 problems for those interested:

1) Before I thought the spray etch was dry after 30 mins - I've now left it overnight to dry and it made a huge improvement!

2) The board was being overexposed by the UV light - 4 mins does the job now.

 

[kl1k]

Got one problem I cant seem to sort out - sometimes the image left behind after the exposure to CS is blurred. Anyone know what could be causing this?

 

[sda]

Chances are that your transparency isnt making full contact with your pcb during developing.
I use a peice of glass (from an old picture frame) on top of the transparency to hold it on. Just make shure its fairly thin, and not of the UV protection type .

cheers
sda

 

[sinatra]

Hello Kl1k.
Maybe your problem is just due to thickness of the transparency. Paralax.

1-Print the negative of your drawing into the transparence.
2-When making the PCB you put the printed faceof the transparency in contact with the PCB UV sensitive film.

In this way you get better definition and get rid of the thickness of the transparence.....paralax, etc......

3-You can still improve the definition putting the UV lamp far from the PCB+transparence. Paralax is then over......

Take care! The UV light is dangerous for your eyes!!! If possible, NEVER look at this light.

Greetings.
S.

 

[kl1k]

Thanks for the reply - I'm currently using tracing paper and am using the printing against the copper. My uv lamps are about 15cm from the board - i'll try moving them further away.