High Power LEDs
When I received my high power LEDs from China, I realized that LEDs are SMD components. I have no chance to test or run them without making printed circuit board. Thus, I ordered 10 pieces positive acting presensitized pcb boards from Ebay. I am going to make pcb by myself with some help from instructions.
I decided to make schematics for LED control board using PADS, because we have been using that software in school. I have heard that Eagle is also good and I am going to try that in some day.
Main goal was to fit maximal amount of LEDs to 100 x 150 mm pcb. I did calculus and found out that 88 LEDs fit to that size. Early goal was to have 100 LED, but I think 88 Led is enough. Basic schematic was made.
There are two connectors, one is operating voltage which is calculated more in-depth later, but it is approximately 30 V and other connector is ground. One resistor is limiting current and another one is just for case if I did some early calculus wrong. In this way I have more space when creating final design.
There is final layout design for this LED control board. All LEDs are 1 mm to 2 mm from each other and sides are full of mounting holes for good thermal connection. Nearly whole upper side of the board is ground level. I have really no clue if it good or bad pcb, but lets just try that, because it looks fine to me. LED control board(CopperPour) is the one which is printed to transparent sheet.
I read a few instructables for making your own PCB at home. Here are some good tutorials and I do not go in-depth details about my process.
I printed my layout.pdf to a two transparent sheets. Then I taped those sheets together for getting thicker contrast different for printed and non-printed area. After that I placed printed sheet and presensitized pcb board to an empty photograph frame. In that way sheet and board will stay firmly in place. I used ordinary LED light of our kitchen and I placed that picture frame about 15 cm under the light. The first pcb board was exposured about 20 min and it was not enough, so the second one was exposured for an hour and then it worked.
Transparent box contains sodium hydroxide and red one contains Natriumpersulfate
I buyed 100% NaOH, which is also known as sodium hydroxide. I made about 12 % liquid. Then placed that exposured pcb board to the liquid and after 5 min all the exposured lacquer was gone.
When develop was done it is time to etch all the copper away. So I poured 0,5 l water and 100 g Natriumpersulfate and placed board to the liquid. I heated liquid a bit over the skin temperature for having faster etching. Etching took about 2 hours. Result is shown below and as one can see it right and left corners are not etched corretly. That is because I was too rapid in development.
Etching is done, look corners at bottom!
After etching I removed all the remaining lacquer with nail polisher.
The first completed pcb turned out to be badly designed. LED measures were about 0,1 mm wrong, so I could not fit all the LEDs side by side. In that manner I made another design and I remake all the pcb process for making new pcb board. It turned out fine. Here is the new layout
LED control board R3)POUR)
Now when we have much bigger cap between signal and ground level it is easier to put LEDs on place with soldering paste. I used small plastic bag to make cone where I placed soldering paste. In that way it was really easy to apply proper amount of soldering paste to component pads. You need to be careful with the amount of soldering paste, it is very easy to use too much of it. Fairly good advice would be to use half of the size of the component pad which is going to be soldered, that is very little! Here is picture of my assembly.
Soldering paste is in it place and now it is time to place LEDs
Everything seems to be fine for soldering!
I do not have any special oven for heating soldering paste, so I used normal cooking oven which have recirculated air option. So I heated oven to 250 Celsius degree and placed two baking trays inside. One tray on top shelf and other one on bottom shelf for interrupting heat radiation.
PCB at the middle shelf.
After one minute I notice that flux inside solderin paste became to dissolve. Unfortunately at the same time LEDs transparent plastic bulbs became larger, so it seemed that this plastic compound did not withstand so much heat… Nevertheless I decided to go further with soldering and approximately after 4 minutes tin alloy was properly melt.
Soldering paste will get shiny when it is ready.
I found 1,5 mm drill bit from my home, so I used ordinary drilling machine with it. I know that 1,5 mm is kind a big hole, but I did not want to buy smaller drill bit. Drilling went okay, because I design big pads for those holes.
Throught hole soldering
No need for resistors, because darlington pair TIP120 which I use, limits current in that value which I want. More about this in next posts.