Not sure if this is the best place for this question but couldn't seem to find anywhere else that it fit, and
seeing as I'm new to doing the surface mount components myself I figured I'd ask here.
I don't do alot of surface mount, so generally I leave it up to a third party to do the assembly. After seeing a video
on doing it with a hot air rework station I decided to give a simple board a go. I got a cheap plastic stencil done
and applied the paste to each of the 5 boards I was doing then proceeded to populate the boards with the components.
After populating all 5 boards I one by one started using the hot air on the components.
My problem was that every few components (specifically the resistors - all were 1206 - starting big to get the feel) after
moving the nozzle around and around (with the air flow very low) the component would appear to *pop* off the board.
Now I've heard of tombstoning and such but this was literally 'jumping' across the board.
I figured it could be one of the following:
a] heating paste too quickly and not allowing for a soak (correct term?)
b] paste was out on board for a little over an hour (as I did all boards cause I was slow going) and dried leaving little flux?
c] paste I have is a couple months past exp date (kept refrigerated and not used often) I did leave out to get to room temp.
d] unbalanced paste or heat to each 2 pads on component - paste appeared to coat really well using plastic stencil.
My end results were good but, I would like to improve and I'm wondering if anyone can identify what I'm
doing wrong that causes the resistor and occasional capacitor popping/jumping?
I tried several temps on the hot air station tried 200 to 285 in 15 degree increments to see if it helped, but found little
difference other than the low temps took too long to get the paste to reflow, got better results >250 degrees.
I will attempt a toaster over (with home made controller with my solder paste reflow profile) in upcoming weeks... but if
anyone can help point out my errors it would be greatly appreciated.
Chris