I’m new in SMD technology and i would like to share with you my issue.
I bought an oven toaster in order to convert it to a SMD Reflow Oven having in mind to use an arduino board with a K-type thermocouple as the PID controller.
As a first step i modified the oven adding an SSR to control the heating elements. I connected a K-type thermocouple to an MAX31855 in order to read the temperature and i added some extra gold thermal tape in the inside of the oven to increase the heat reflection. The code i used is the rocketscream reflow example modified according to my needs.
Hey, nice project. I’m about to try something similar soon.
The profile looks OK to me. The measured curve shows no overshoot or instability, so the PID_K* values should be usable.
The cool down rate is a bit slow. As soon as the solder is solid, you can open the oven’s door, to cool down a faster. This may reduce oxidation. But I don’t think that that’s the real issue.
@Waltr: this is lowtemp solder paste, melting temp is around 180°C, see the datasheet.
I’ld suspect:
the solder paste did not melt (it should be shiny after soldering, shouldn’t it?). Heat with a solder iron and check if it starts to flow. If it flows, the actual pcb temp was much lower than your thermocouple indicates. Make sure the thermocouple is thermally coupled to the pcb; you may also want to check and calibrate the temp measurement (with ice and boiling water?)
something wrong with the paste: too much paste, paste not well mixed (were these the first drops from a tube?), paste too old, paste wasn’t stored in the fridge, etc.
No it is not shiny after the reflow cycle. It looks as it is dry with mat colouring. If you scratch it with a screw drive it is reveals an under layer of melted shiny solderpaste. The thermocouple is attached at the top of the pcb and it touches the surface of it. I haven’t calibrate the thermocouple but in comparison with a multimeter i have the temperatures readings looks to be similar.
The amount of the paste could be too much at the first samples but then i tried using the stencil so that looks not to be the issue as the problem still appears. I also tried different paste samples so there is no mixture issue. Finally i tried to add some more flux in the paste and mixed it well with the result of a small difference in the melting point.
Here is a sample of the latest curves i have used.
With visual inspection, it looks the solderpaste starts melting at 180C and then for some reason to stop. I have configured the oven to stay at 200 for more seconds in order to see if this is the issue but no result.
Good checks. So the paste did melt and the paste should be OK. With the stencil, did the solder still flow outside the pads? Was the connectivity OK?
The mat colouring could be due to the flux residues. It is a no-clean flux. Maybe it needs more heat to really disappear? What happens if you heat it with a hot soldering iron (at 350 °C or so)? Maybe the people at termopasty.pl can help you out on that.