Hi,
This is easy. If your just doing prototypes, then you could get by with a syringe of solder paste and a hear gun. You can put down a little solder paste on each page and then place the passives and heat gun the board.
If you have other active parts, you can use of those wave soldering iron bits and solder these. However if your doing BGA, then this is a little more tricky, but just flux the pads and heat gun them down gently. However you would need to do the BGA's before the passives as the BGA needs more heat and should not be done at the same time as the passives.
In the early days we assembled dozens of prototypes this way, and they all worked.
However if you want something more robust, then I suggest a small convection batch oven, and a manual stencil printer. You can load the stencil and then print the board, and the batch oven will solder. This leaves what machine you select to load the board. I once owned a MYDATA TP-9 machine. It was slow, but it worked well, and you can get them very low cost second hand. The feeders on this machine are the same as the later machines use, so you can get them cheap as well.
This should be a low cost and solid setup. We used double sided tape to hold the boards in place on the stencil printer. The batch convection oven was a little tricky to setup, and the biggest issue was it took a long time to cool down after a board. However it had a window in the lid so we could count the seconds in reflow, and then the board sat on a wire mesh matt, so we very carefully lifted the mesh matt out of the oven, and getting the board out of the oven let it cool down nice and fast, and stopped it getting burnt.
We made hundreds of boards that way, and it worked well. MYDATA's are easy to setup and you can use the vision to program the position of each component. This setup would be very easy to use and quite low cost.
Good luck!
Grant
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