I have been doing hardware design and PCBA prototyping for a while and I have issues with quality of reflow soldering. I have a small lab including a pick'n'place machine, stencil printer (eC-stencil-mate) and reflow machines (Zb5040hl and ec-reflow-mate). I thought the issues that I am facing is caused by the Chinese reflow machine so recently I purchased a second hand reflow machine, manufactured by EuroCircuits. But still I have same issues:
- solder balls around the SMT component
- stains on the pcb due to fluid of the paste
- solder balls on the solder joint
It looks like the paste explodes during preheat/soak or it just floats away from the pad of the components...
The soldering procedure is done like this:
- Get out the paste from fridge (4°C). Paste was opened ~4 month ago and it is stored in fridge for that time.
- Mix the paste with hand tool for ~2 min
- apply paste to a clean stencil
- print the paste with 1 swipe on the stencil
- remove PCB from stencil printer and place components to the board (manually or with machine)
- put it into the reflow machine and reflow
I selected the SN42/BI58 paste because of its low temperature specifications. Sometimes I work with temperature sensitive elements and it is nice if we don't apply more than 200°C. The reflow temperature is ~139-141°C with peak soldering temperatures of 175°C. I tried to follow the reflow profile as much as possible with the EC machine but still I see the same issue.
Datasheet states a 2.5 °C / sec rate for heating and it is not violated. The highest rate is ~1.2 - 1.5°C / sec at preheating stage.
Soldering with chinese machine (peak 200°C is reached in 280 sec):
Soldering with EC-Reflow-Mate (peak 180°C reached in 210 sec):
Soldering with EC-Reflow-Mate (Peak 180°C reached in 330 sec):
Soldering profile for the last (330 sec):
What can be the issue? Can the paste be scrap? Am I introducing bubbles into the paste during mixing?
Update: Testing the suggestions, I made major improvement. Using brand new paste, not mixing, same profile as 3. test (330 sec) I do not see much solder balls, just small flux. I don't know if the flux is normal or still I need to tune the profile.
Pastes (1. new not opened, 2. Freshly opened, 3. old paste. I do not see too much differences between old and new):
Paste was opened ~4 month ago and it is stored in fridge for that time.you didn't mean "Opened and left in the fridge with its lid's taken off", did you? Don't get offended please, it's my English... \$\endgroup\$