Reflow soldering - Part (6) First circuit board soldered

"Done... Switch off and open door"

The first circuit board has had the surface-mount components soldered onto it using my new reflow-soldering system. On close inspection afterwards, there was no sign of any heat damage and no repairs or rework were necessary. The tiny parts all seemed to be perfectly soldered.  I later soldered the through-hole-terminated connectors and switches with a soldering iron.
As soon as the board was back to room temperature, I powered it up and successfully programmed the PIC microcontroller. Some initial electrical tests were also positive.
Earlier I had spent some time practicing picking up, holding steadily and placing the parts in a controlled manner onto the circuit board. After the solder paste has been applied there is limited time. A mistake would probably smear the paste, requiring cleaning and start-over. I found tweezers, curved at the ends, to be the best. In the interests of speed, I first took all the components I would be needing from my stock, and separated them by type in a compartmented tray. I decided to place the 44 pin PIC on the board first as a mistake was most likely to occur with the placement of that component.    

Reflow soldering - Part (5) Using the stencil

Preparing to apply the solder paste

I now have the stencils through which solder paste is applied to the printed circuit board ( PCB ). The material is 0.1mm thick stainless steel. Manufactured using a laser cutting process, the minimum cut-out dimension and spacing is also 0.1mm. However the steel was far more suitable for laser cutting than the transparencies, mylar and plastic materials which were also evaluated.

The PCB is held securely between spare PCBs, or off-cuts of other material having exactly the same thickness as the circuit board; checked with vernier calipers. The stencil is then aligned on top so that the solder pads, ( where the SMT parts will be placed ), are visible, and then one edge is taped down to form a hinge.

I use two spreaders; a small spatula and a plastic card. The spatula is for getting the solder paste out of the pot and, after transferring the paste, the card is used to spread the paste over the stencil. As the paste is not cheap, any excess is returned to the pot.