I am using Tin-Lead solderpaste to make my prototype circuits. This is the exact solder paste:
ALPHA OM-5100 (https://alphaassembly.com/Products/Solder-Paste/OM-5100)
Alloy: 62Sn/36Pb/2Ag, 63Sn/37Pb and 62.8Sn/36.8Pb/0.4Ag (NT4S, Anti
Powder size: Type 3, (25-45 µm per IPC J-STD-005) or Type 4 (20-38µm).
I use a stencil to apply the solderpaste. Next, I place all the components one-by-one.
The solder paste contains flux. However, I'm worried the flux evaporates while I'm placing the components. It generally takes several hours - up to 8 hours for my latest PCB - to get all components in place.
I've got the impression that the solderpaste doesn't "flow" sufficiently. For example, I would expect slightly misaligned resistors and capacitors to straighten up when the solder melts. But that doesn't happen.
Another example: Imagine some solder paste that isn't applied very good:
During the reflow process, I would expect it to flow like this - as if it is attracted to the copper pads and repulsed by the naked FR4-material or coatings(where no copper is present):
But that doesn't seem to happen either.
Should I spray some flux onto the board before I shift it into the reflow oven? If yes, what spray would you recommend?