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How does a conventional board house deal with the issue of reflow during PCB component assembly with BGA's on both sides of the board?

Are two different solders used (like regular lead free SAC305 and a low temp?)

If reflow soldering a BGA with a low temp solder, does a BGA that already has solder balls need to be reballed for lower temperature solder?

Or do they just use surface tension to hold the parts on after flipping the board and use the same solder?

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  • \$\begingroup\$ My guess would be adhesive, (strong) conformal, and/or a (very custom) jig. \$\endgroup\$
    – rdtsc
    Commented Sep 6, 2022 at 21:00
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    \$\begingroup\$ If you’re considering such a process, then I’d suggest you ask the assembly house how they do it and how much it costs. Foxconn should be able to assist. \$\endgroup\$
    – Kartman
    Commented Sep 6, 2022 at 23:04
  • \$\begingroup\$ Surface tension will hold small parts. For larger parts, put all the BGAs on the same side and reflow that side last, or glue the BGAs before reflowing the other side. \$\endgroup\$
    – Mattman944
    Commented Sep 7, 2022 at 0:18
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    \$\begingroup\$ @Kartman to talk to foxconn you need qtys of 100k \$\endgroup\$
    – Voltage Spike ♦
    Commented Sep 7, 2022 at 4:25
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    \$\begingroup\$ It's actually not that much more for NREs for the quotes I got double sided will only add ~200$ to assembly costs. I was able to put power on both front and back in the same area and share vias (put in some extra also) \$\endgroup\$
    – Voltage Spike ♦
    Commented Sep 8, 2022 at 14:08

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The soldering process for each side of the PCB is the same.

Usually the surface tension will be enough to hold a BGA in place, given the number of balls compared to the weight of the chip. If it is not the components will be glued, but it is quite rare to need to do it.

By design one should avoid putting heavy components on each side of the board. On the bottom there should only be small components.

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