Finally, with some luck on my side, I am left alone for the afternoon. This also means I have access to the important large table. You see, I have not done anything to reduce the size of the solder paste stencil but rather, left it as it is.
THE SOLDER PASTE
When you hand solder components, the solder you use would be in the form of a thin malleable wire of various thickness. But for re-flow soldering, the solder comes in the form of a grey paste. If you look at the paste with a microscope, you would see thousands of small little solder balls suspended in flux.
The Solder paste I am using is Type 4 from RELIFE. It claims to have good wicking and melts at 183ºC. I am not sure why I used Type 4 but I do remember it is great for SMD components.
HOW TO APPLY THE SOLDER PASTE
Just like silkscreening, you use a squeegee and drag the ink/ paste across the stencil. The process is very easy and you can use any object as long as it has a straight edge. For me, I bought some yellow plastic ones which is meant for car window tinting.
Very quickly, I learnt that there is a technique to apply the paste as it was not a simple as I thought.
To prevent thermal shock to the PCB, I placed it on the hot plate as soon as it starts to warm up. It took quite some time for the hot plate to reach required temperature. There were quite a lot of fumes coming off and once the solder paste starts to melt, it means the peak temperature has been reached. The PCB is then let to ‘cook’ for about 18 seconds before the hot plate starts the cooling down process.
If I used hand soldering, this board would have taken me at least a few hours. With the re-flow, it would be significantly faster. This method gives me a much higher reliability although it has some issues:
Too much/ little solder
I cannot solely blame on the stencil’s 0.13mm thickness because there are other factors too. One possibility is the way I dragged the squeegee and the speed which leads to the paste not settling down. Plus, maybe the stencil was not sitting firmly on top of the PCB which allowed for leaks.
During the testing, there were two LEDs which did not light up reliably. The testing led back to the network resistors which, although has sufficient solder, did connect. There is another possibility that I allowed the liquidous state period to be too brief. This is the peak temperature where the solder has started to melt and (through capillary action) aligns the SMD components. Gravity also made sure the SMD components are sitting on their respective solder pads correctly. All these needs time. Perhaps on the next board, I should extend the period to at least 40 seconds or more.
Here is the video of the re-flow process on the PCB. It was very exciting for me because this is now an actual re-flow process and not some PCB testing anymore. Thankfully, the first time results was better than I anticipated. Enjoy!
With this issue more or less settled, it’s time to make sure the whole system works. Then I would have to start on the BOM aka Bill of Materials to see how much I would need to get the whole system manufactured. Plus there is the Assembly Manual as well.
What started off ans a simple three PCB board design has now evolved to eight…