Debugging solder paste

I am currently progressing with my first complete “product”, that is, electronics design including professionally made PCBs and plastics. Assembly will be done by us, which caused me to use solder paste for the first time.

The first run was bad: the solder points where brittle and the components came off easily. Could there be a better moment to play with our new USB microscope? (100 Bucks on eBay, great investment I think!)

Bad solder joint

Bad solder joint. The joint was brittle and the compontens came off easily. You can still see the little balls that make up the solder paste. Diagnosis: reflow temperature was too low!

Turns out that I used accidentally the wrong setting on the reflow oven. I though we still had some leftover lead-based paste in the applicator, but apparently it was lead-free paste after all. D’oh!

Good solder joint

Good solder joint. Here you can see that the little balls melted, resulting in a smooth surface. Also, the solder paste was sucked close to the component.

Now I just have to find out what the symptoms of old solder paste are. I heard that you can keep it only for a couple of weeks, but the paste I used here is about a year old. This had me wondering what solder joints would look like when you’ve used paste that was too old.

NB: Sorry for the colors by the way. VLC seems to switch the red and the blue channels on the snapshot (but doesn’t on the video itself), and I haven’t found an easy way around it yet.

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