As a hardware engineer, you will often need to direct contact with PCB suppliers and circuits assembly factories. So, a well understanding of the process of circuits assembly is a must, then you will not be fooled when you are visiting carefully selected PCBA companies in China.
Technically a PCBA process includes: PCB manufacturing+components sourcing+SMT+fireware flashing+function test.
Soldering the circuits by machines has the same theory of soldering by hand" add solder paste, place components, heating. The only difference is the machines goes way faster and accurate by men's hand job.
Re-flow (under construction)
Wave soldering (under construction)
Printing solder paste
The first step of circuits assembly is to print solder paste on the board, we know that there is a file in Gerber called paste mask, it is for stencil making. What is stencil? It is a thin steel plate (0.1mm). There are lots of pre-cutted areas on the plate by the shapes on the paste mask file, then we put the stencil on the PCB, the cutted areas will be the soldering pads on the PCB.
Next prints a layer of solder paste on the stencil, the cutted area will be covered with solder paste which is on the solder pads of circuits board, the paste thickness is 0.1mm. We call the machine who do this job "solder paste printing machine", first you put the stencil in the machine, second put the board in a tray, it will hold the circuits board just beneath the stencil.
Upper the stencil there is a big brush pushing the solder paste, puching it from east to west and turn back, the groove on the stencil (which is the soldering pads) will be covered with a layer of paste.
After this, take the circuits board off, put them in next machine, for surface mounting .