UV Exposure unit for PCB Production

Sunday, January 06, 2013 11:00 AM

Introduction

In this project i'll introduce my homemade UV exposure unit for PCBs Production.
This UV exposure unit can produce a double side pcb, and the circuit quality is good. this UV box is able to produce easily a 0.3mm width traces and can go down to 0.1mm all depends on the PCB quality. The PCB final result depend also on the etching process.






Extractor fans to remove hot air from the box:



All you need to make this UV box is :
- Wood box ( 540x280x150 mm for the bottom and 375x280x150 mm for the top)
- 8 UV lamps (8 Watts each one).
- 4 Electronic Ballasts.
- Miscellaneous electronics Stuff :
       * 6 segment displays, pushbutton, 2 switchs, keyboard, 5 LEDs, 2 relays
       * Linear power supply (Capacitors, regulators ...) + Step Down Transformer (220VAC - 12 VAC) + Graetz Bridge
- Control circuit board (PCB, Microchip Microcontroller PIC16F877A, transistors, some circuits....)
- White paint for the interior.
- Black spray paint for the exterior.
- Wires, Wire duct.
Here is what the box looks like at the beginning (after putting some wood putty and smothing the surface) :






UV lamps

The lamps are BLACKLIGHT 350 from SYLVIANIA and consumes 8Watts.
I use 4 little UV lamps in each side instead of 2 big UV lamps to obtaint an uniform light thereby to get a good PCB final result.
Their peak wavelength is 350nm, which is inside the range recommended by the PCBs boards manufacturer.







To power the 8 UV lamps i use 4 electronics ballasts. 2 lamps for each ballast.

The benefit of using electronics ballast instead of a classic ballast are :
- The classic one are so heavy compared to electronics ballast.
- Classic ballast becomes too hot after a short period of working time, and it is very dangerous especially when the box is made with wood and this may damage the box over the time even it could put the fires in the box.
- doesn't need a starter.
- Ignition is instantaneous.

I put 2 UV lamps in series and power it through a single 18 Watts electronic ballast.
Using this method i drop the number of ballast from 8 to 4 so i save space and money.

Here's the wiring diagram :






Lamps spacing

The tubes are separated at the same equal distance of 55 mm to allow a perfect exposure of the PCB.
The distance between the UV lamps to the PCB are 70 mm.

The interior is paint with white color to obtaint more UV light by the Reflection phenomena.


I use a fan as you can see in the top of the picture to evacuate the hot air to the exterior.



Main circuit

Above the electronics ballst i add a piece of plexiglass that containt the electronics stuff necessary to control, supervisse and power the UV exposure box.









This piece of plexiglass containt a transofrmer, linear power supply, relays and the main PCB.



The main PCB consist of two superimpose PCBs. These PCBs are homemade.






The top PCB, the red one, which is the main circuit containt a microchip microcontroller PIC16F877A. The programmation of this microcontroller is made with Assembler .
This PCB containt also :
- 3 x 74LS47 BCD to 7-Segment Decoder for the segment display.
- 1 x 74LS08 QUAD 2-INPUT AND GATE used with the keybord.

Layout of the top PCB (Red color) :

The different connector of this PCB are:
- Reset Push-Button terminal.
- Led_selec_sup terminal (Red LED) indicating the top UV lamps are selected.
- Led_selec_inf terminal (Red LED) indicating the bottom UV lamps are selected.
- Keyboard connector (Connecteur_Clavier).
- Switch terminal dedicate to select between the top or bottom side (Switch_Aff).
- 30 pins connector for connection data with the other green PCB.

The second PCB is the green one. This PCB provide the power (12V, 5V), through a terminal, to all the circuit (bottom and top PCB).
this PCB containt :
- 6 x 74AC373 Octal Transparent Latch
- Output power stage to drive the relays (transistor, resistor, Flyback diode)

Layout of the bottom PCB (Green color) :

The different connector of the green PCB are:
- Led_inf terminal (blue LED) indicate that bottom UV lamps are lit.
- Led_sup terminal (blue LED) indicate that top UV lamps are lit.
- Relais.inf / Relais.sup terminals are connected directly to the relays.
- Alim terminal input provide 12 and 5 Volts from the linear power supply.
- Two 22 pins connectors are connected to the segment displays.
- Led_on terminal (Green LED) indicating the presence of voltage in circuit.

Linear Power Supply

To provide power to the UV box i use a step down transformer (from 220VAC to 12 VAC) and a bridge rectifier. then i stabilized the voltage through capacitors and regulators to obtaint 12V and 5V.



 

It's mandatory to know the maximum current delivred by the power supply in order to limit the voltage variations Δu, and thus ensure that voltage at the input of regulators remains in the interval allowed by the manufacturer.
Here is the formula :
Δu = I / (C × f).

We note that if the capacity value is important the voltage variation decreases.

The maximum current indicated by the multimeter is 650mA when all the peripherals are ON (2 fans, 2 relays, 6 segments displays and microcontrollers ... etc)
I use three 2200uF capacitors in parallel so the maximum varaition is : Δu = 0,396 V



For a linear power supply, delivring 0.65 is important thus the power dissipation in the regulators are important, the reason why i add a heatsink that i found it in an old computer power supply.
The power dissipated in the 12 volts regulator (LM7812) is 4V * 0.65A = 2.6 Watts.

Control Board

To built the conrol board i cut a piece of plywood (305x170 mm) .
Here are the steps of how i built it :






Using a saber saw i cut the space provided for the segment displays.

The holes are reserved for the LED diode.



With wood putty i tried to repair the segment display holes's locations.






Segment displays's PCB:

Layout of the PCB segment display :

Here's how it look like after painting and installing the components: ( we begin from the top we have )
- ON/OFF switch & Power Green LED.
- 4 indicator LEDs. the red LEDs indicate the selected side and the blue LEDs are on when the UV lamps are lit.
- 2 segment displays for each side (top and bottom). both displays show the number of seconds.
- Reset Push-Button
- A switch to select the side
- A keyboard to enter the number of seconds required for each side.

The number of seconds required is different from one manufacturer to another.


The reason why i use two separate timer for each side is because the UV light received from lamps is not equal for the both sides. This is due to the glasses thickness. As you can see in this photo bellow.


The glasses thickness of the top is bigger than the bottom because we need to put some heavy glasses above the PCB, to remove the air and plays between the layout and PCB to obtaint a good result during the exposure process .



Etching Process

For etching i bought a small custom aquarium (200 mm x 230 mm x 30 mm).
I use two Air-pumps in such a way that all the ferric chloride circulate equally in all the aquariaum. (see photo below)


We notice that the pumps creat a whirlpool inside the aquarium. I use a water instead the ferric chloride to see clearly how the water stream flows inside the aquarium.



I add a heating resistor to heat the ferric chloride :






PCB Results

The results are quite good.
I remove the lacquer with aceton, and then treat both side with a spray varnish (I use green Spray from KF).

The minimum wire width is 0.3 mm in this PCB.

After Soldering components onto a circuit board:



Video

Here's the video of the UV exposure unit: