Solid state PCB exposure

by SV3ORA

After many years of avoiding to make Homemade PCBs, I have finally decided that a PCB exposure box is very much needed, especially when dealing with high frequency projects. I had many projects which required double sided PCBs, so I have decided to make a double sided PCB exposure box, using two 125W mercury vapour lamps, each 30cm above the PCB, which was sandwitched inside two glasses. This solution worked well but the enclosure was about 1m high. Apart from the lethal voltages, the two 125W lamps were also consumming a lot of power and they were getting very hot. The need for a better solution was born.

Technology has advanced very much the last decade and new solid state devices are out from time to time. With the new UV LEDs availability in cheap prices, a solid state UV PCB exposure box seems a more efficient, cheaper, smaller and safer solution for the labratory.

The key point of making a good solid state UV PCB exposure box is the choise of the lamp. LEDs usually exhibit high light directivity ranging from about 20 to 40 degrees @ 3dB. Whereas directivity is desired in some applications, it is not good for an exposure box. The solution most homebrewers choose is the use of many 100mW LEDs placed close together, so the light is spread almost evenly throughout the whole PCB. Others, use refraction sheets, to refract the light, so that it is spread evenly throughout the PCB. Whereas these solutions work, they require a massive amount of leds and lots of soldering. A better and easier to make solution exists, the straw-hat LED.

This type of LED exhibits a much wider angle reaching up to about 140 degrees. Nevertheless, as the spreading gets bigger, the radiated power per square cm degreases. This requires the use of more powerfull LEDs, to overcome this effective power loss. The LEDs used in my exposure box are rated at 500mW, instead of the common ones which are 100mW. They have 5 LED dies inside the same package, instead of 1. By exhibiting a much wider angle and being more powerful, these LEDs can be placed far appart and less of them are required in total.

There are even more powerful LEDs to choose but all of them require external heatsinks to operate, which is not convenient in this application. The LEDs used, have overgrown internal cathodes, which are used as internal heatsinks. Thus an external heatsink is not required. The picture below, shows such a LED in operation.

As it can be seen, the viewing angle is much greater than that of the common LEDs. An angle of more than 90 degrees can be easily noticed. The technical specifications of the LEDs, are shown in the tables below.
Specifications
  • Source Material:InGaN !
  • Emitting Colour:0.5W 8MM WIDE VIEW ULTRA VIOLET UV 0.5W LED 
  • LENS Type:Water clear
  • Color Temperature: --
  • Luminous Intensity-MCD: Typ: 30,000 mcd
  • Reverse Voltage:5.0 V
  • DC Forward Voltage: Typical:
  • DC Forward Current:100mA
  • Viewing Angle:140 degree
  • Lead Soldering Temp:260°C for 5 seconds

Absolute Maximum Rating (Ta = 250C)

PARAMETER

MAXIMUM RATING

UNITS

DC Forward Current

100

mA

Peak Pulse Forward Current (1)

150

mA

Avg. Forward Current (Pulse Operation)

100

mA

Operating Temperature

-40 to +100

0C

Storage Temperature

-40 to +120

0C

Lead Soldering Temperature

2600C for 6 seconds
(1.0mm or 0.63 inch from Body)

(1)  Pulse conditions of 1/10 duty and 0.1msec width 

  

Electro-optical Characteristics (Ta = 250C)

PARAMETER

SYMBOL

CONDITIONS

MIN.

TYP.

MAX.

UNIT

Forward Voltage

VF

IF = 100mA

3.2

3.4

3.6

V

Reverse Voltage

VR

IR = 100mA

 -

 5

-

V

Dominant Wavelength

lD

IF = 100mA

395

400

405 

- 

Viewing Angle

2q1/2

IF = 100mA

 

140

 

Deg.

Luminous Intensity

IV

IF = 100mA

-

 30,000

32,000

mcd

d


d

 

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