Nowadays very few people use single-side reflow, because double-sided reflow can save board space, allow product volume to be small, most of boards we see in the market belong to double-sided reflow process.
Because "double-side reflow circuit board" needs twice reflow soldering, there will be some restrictions in its process, the most common problem is when the board pass reflow oven second time, the parts already soldered on the down side may fell because of the gravity, especially when board entering high temperature zone, this article describes parts placement precautions of double side reflow process.
What SMD parts should be placed in the first side in reflow soldering?
Firstly, relatively small parts placed in the first side in reflow soldering, since deformation of PCB in first reflow is relatively small, the accuracy of solder paste printing will be relatively high, so fine parts is more suitable for first reflow.
Secondly, fine parts have lower risk of falling down in the second reflow process. Since the first assembled side is facing down side during second reflow, when the board enters the high temperature reflow zone, less likely fall down from the board because the weight is light.
Thirdly, part of the first assembly side must go through reflow oven twice, so must be able to withstand reflow temperature twice, normally resistor and capacitor are required withstand at least three times reflow temperature, which is to meet requirements of the board may be need reflow more than twice because board maintenance and repair sometime.