After learning how to make PCB Boards at a class at #MakerPlace I started making a lot of little inventions primarily for my electric cars I recently got a reflow oven for $250 (awesome) and have been using it for my prototyping. Many of my boards are double sided and I came up with the idea of using two different temperature solder pastes to reflow a double sided board.
Some examples of solder paste formula with melting temperature are
Sn63Pb37 - 182c
Sn42Bi58 - 137c
Sn96.5Ag3.0Cu0.5 - 217c
For this example I used a syringe of Sn96.5Ag3.0Cu0.5 and a pot of Sn63Pb37. I got one of them at Fry's and one at Digikey at different times so I used what I already had. An important consideration is some parts (NPN transistors in my circuit) are rated with a gate temperature of 175c so in an earlier build I found out that my transistors melted themselves off the board (I was using a 137c solder and the cooling circuit wasn't working and neither was the thermal shutoff).
Note : the plastics in many connectors melt / deform at higher temperature reflow. Choices are hand solder them, choose lower temperature solder paste, or choose higher temperature connectors.
Step 1: Align Stencil on First Side.
Choose the side of the board that will go through the reflow oven twice. This one will have the higher temperature reflow cycle so I generally choose the side of the board that has heat sinks flowed to the PCB or if there are none of those I then choose the side with the least heat sensitive parts (e.g. without processor).
Step 2: Apply High Temperature Solder Paste
Apply the high temperature solder paste and scrape off the excess
Step 3: Carefully Place the Parts
Remove stencil and place your parts. You can see that I didn't do a great job with the solder paste, in fact I did an intentionally mediocre job as I was testing the tolerances of making PCB to determine how practical it would be to do small runs in my house. The answer is they are very tolerant.
Place parts with tweezers making sure that LED's, chips, etc. are properly aligned. I like having a second board with no paste on it to see the alignment dots. After the first board is built and tested I use it for the future boards. There are good instructables and videos on how to do this, the main thing is a steady hand. There is a little bit of adhesion that will keep the part in place if it is not bumped or tipped too far.
Step 4: Carefully Place the Board Into the Oven and Reflow
Carefully move the board into the oven
Choose or create a reflow profile for the higher temperature solder and reflow the board.
Step 5: Align Stencil for Second Side of the Board
Step 6: Apply Low Temperature Solder Paste and Place Parts
This is a repeat step X with a lower temperature solder paste.
Step 7: Place Board in Over on Spacers and Reflow
Put some spacers (PCB leftovers work great) so that the board will sit level (un-flowed parts side up) without sitting on any of the parts that you previously place.
Reflow board on lower temperature profile for lower temperature solder.
Step 8: Clean Up and Test Board
I usually test the board and then only clean it if I'm giving it to a customer / friend, but it is a good practice to clean the boards with flux remover at this point. I don't use flux remover in my lab room, but I can easily test the board there ... I think you can see where this is going.
It is also a very good idea to spray the board with conformal coating if you are planning on the board lasting.
That's it. I hope this helps a few of you.
T-Rex - Yes my middle name is Tyrannosaurus and yes I did have the best Hippy Mom EVER.