Introduction: Fantastic Double Sided PCBs at Home Using Photoresist Method

By blog inside
About: Currently studying at Yildiz Technical University as ,Mechanical Engineering student in Istanbul,Turkey. Like other instructablers creating new things and imagining gives me great plesaure. I also participated…

   As a hobbyist I always want to have my pcb in a professional way with 10thou trace to trace distance, double side and error free traces. Of course that was "nearly " impossible to make this kind of pcbs using toner transfer methods at home.

I have tried toner transfer method ,Press N peel paper and photopaper. Result ?
 None of these gave me nice results.   Because these methods are only good if you are doing a single-sided boards.

When you are making the double sided boards, ironing causes toner to melt and this makes the papers slip a bit. BUT if you are working with 10th traces ther shouldn't be any slips.

With UV photoresist method (photolithographic) which is also used in production of silicon chips http://pcplus.techradar.com/node/3059  the results are near perfect.

Step 1: Equipments and Stuff

For Photoresist plating a bare Pcb:
 ---1 can of POSITIV-20 sprayhttp://uk.farnell.com/kontakt-chemie/positiv-20-200ml/positive-resist-positiv-20-200ml/dp/801010   --nearly 20TL(Turkish liras in Turkey)

---or if you have access to sensitized PCB  dont' bother to buy a Photoresist spray, pre made pcbs are way better but In Turkey there isn't any so i used spray.

--- Hair dryer to dry your positiv 20 sprayed pcbs
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For Exposing PCB's
 ---1X Pair of glass at any size you like --Free
---2x 8watts of UV fluorescent lamps( I don't know the exact type uva-A or uva-b but these blacklight tubes are fine) ---3TL each
---2x Fluorescent modules which you can use it directly with  fluorescent lamps --6TL each
---Heavy books wtih more than 400 pages and hard cover, dostoyevski is a good choice for this job :D
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For Developing and Etching:
--- 1X Sink Cleaner in granulated form, infact it is just NaOH (Sodium Hydroxite) 0.5TL
--- 1X 1 liter Hydrogen Peroxide 7TL
--- 1X 1 liter Bleacher (hydrochloric acid) 1TL

Total cost: 46.5 TL equals to 31$


Step 2: Positiv 20 Spray Application

If you have pre sensitized pcbs you can skip this step.

Before Using the spray on board, first lay down some paper towels under the pcb , This step is important because if there is excess lacquer on the first side of pcb, towel  sucks the excess lacquer and that makes a nice homogeneous layer. Also try to spray it in a near-dust-free enviroment

While spraying onto pcb ,start from top and then plate the pcb with zigzag wrist movements.

When you finished the first layer start to spray second layer immediately bottom to top ,if you wait the first layer start to cure and this will cause second layer to be non-uniform.

After finishing the fist side, wait for 2 minutes then set hair dryer to middle speed and dry the board for 5 minutes or less depends on layer thickness.

Then apply the same steps for other side.

P.S:   You don't need a dark room for photoresist applications. Just avoid the direct sunlight that's all.


Step 3: Setting Up UV Exposure Glass

This is the most basic uv setup I think. I will upgrade it to a dead flatbed scanner later.

As you can see in the photo distances are tagged.

You just need few books, a clean glass  about 3mm thickness and 2X 8 watt UV tubes and electronic fluorescent modules. 

This setup gives me 12 minutes of exposure time. Remember exposure time depends on distance and photoresist layer thickness

Step 4: Aligning Top and Bottom Side Translucents

Print your pcb design onto translucent paper with laser printer or ink jet.

For top side, mirror your design and print it with at least 600dpi resolution , I used inkjet printer and I think it is better then laser printers about blocking light.

For bottom side print your design normally (not mirrored).

After printing the transparencies wait them to dry then cut it and align it on the UV Glass by referencing vias and holes. After the alinging use same tape to secure them.


Step 5: Exposing Pcb

Secure the transparencies onto pcb with tape,

The main trick in exposing is using a lot of pressure onto the board, this can be achieved by hard covered books and heavy books :D of course you can use a second glass on top and then use clamps to pressurize pcb.

Step 6: Developing

You need 1 Liters of water and 7grams of caustic soda or sink cleaner or NaOH (they are all same)

While preparing the developing solution the tricks is not to exceed 7gr of NaOH otherwise the solution will be concentrated and it will wash out all the photoresist suddenly.

When you put the pcb into the solution the tracks will be visible in 30 seconds, after the developing process finished wash the pcb with water.

Step 7: Etching

I use hydrogen peroxide and HCl For developing. Its a matter of choice to use FeCl3.
I don't like waiting so long for etching, so I use this method.

CAUTION: This process generates chlorine gas which is poisonous for your health. Use it at your own risk.

Step 8: Results

Not bad eh ? :D

I found the results pretty satsifying, After making some pcb's you will gain experience about exposure times

The picture below is my first pcb using photoresist method, and it has a track clearence about 10mils.

Hope you enjoyed my first instructable, my next instructable will be about tin plating at home with cheap chemicals. 

I also have some step by step guide in turkish at    www.ceyhunderinbogaz.com

Bye!