After 5 boards made by air gun I switched to reflow oven for boards 6,7,8. And finally got the perfect board after 7 previous ones. The board is 100 TQFP 0.5mm, 128 TQFP 0.4mm, and 40pin PFC 0.5 mm.
All boards work, but only the last one did not require anything to do after it got out of oven.
1.Screw in the mounting screws (to have smallish gap between bottom of board and oven pod) before reflow to have minimal contact to the oven pod. The air will be hotter than oven pod, so the temperature from sensor will match the temperature of board with better accuracy. Also there will be no cold joints
2.When printing it is OK to have a single side of stencil fixed with wide mask tape. No need to have 2 points
3.Make sure there is no sticky tape at the bottom of stencil So when stencil is lifted the lifting speed can be controlled
4.The excess solder can be put back into jar, but with condition that it spent no more than few minutes in open air. <5 min
5.The board before printing must be wiped with isopropyl alcohol on coffee filter soaked in diluted remains of older solder paste. That was most successful reflow. The blank board is HASL Pb-free and solder is Pb-Free as well, but different mixture. Having the diluted (few nanometers ) of no-clean flux all over the HASL helps to merge the solders with different formulas.
6.There is no need to clean the board after reflow
7.Watch out for silkscreen hole: one spot got over the fine pitch pins. Scratch it off with steel needle before alcohol treatment
8.Move FPC connector 1 mm down to reduce area of contact for pins. This will let more solder get wicked into between pins and pads and lessen the cold joins.
9.Reflow one board at a time, since oven is so small and weak and uneven
10.Place the board on pad with high pitch part being in the center of oven pad, not the actual board center
11.Lock the capton FPC cable while the connector is still warm right after reflow. Use mini hand tool.
Thing I did not try yet.
12.Use hand vacuum pen with remote control to pick and place.
13. Run the profile without any boards to preheat the oven itself. It seems that first board out of 3 had cold joints, and next 2 had no much rework, except few bridges.
The board is on the picture:
The coffee filters are best wiping material, because it sheds nearly no fibers.