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My PCB will roughly have the following stackup:

35um copper foil (tracks)
100um FR4 prepreg
200um FR4 core
100um FR4 prepreg
35um copper foil (tracks)

This gives a total board thickness of around .400mm
Since I've only ever seen 1.6mm boards, I wonder how a .400mm will be. Too flexible? Will the core keep the board relatively rigid, or will I be able to bend it like a ribbon cable? Will there be problems with drilling, assembly? Can I still have a BGA soldered onto it?

Edit: board dimensions are 80x50mm

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    \$\begingroup\$ Not like a ribbon but still it will be pretty flexible. Now that's a relative term and it's difficult to convey properly. But I'd suggest getting a 0.4mm pcb from somewhere and see it for yourself. Also, it depends on PCB size so try getting something similar to what your original design is. \$\endgroup\$
    – Whiskeyjack
    Commented Feb 6, 2017 at 6:47
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    \$\begingroup\$ BGAs are prone to solder contact issues (fatigue, cracks) due to bending and warpage in PCBs so I would certainly not put it on a 0.4mm pcb. Normal smd IC's have legs that can bend a little bit relieving the stress on the solder joint when bent. \$\endgroup\$
    – Douwe66
    Commented Feb 6, 2017 at 6:51
  • \$\begingroup\$ The smallest PCB I've worked with was 0.032" which is about 0.8mm.... that gets flimsy enough that a panel begins to sag holding it from the edges. I can only imagine that a 0.4mm would need special considerations for handling and mounting. \$\endgroup\$
    – Daniel
    Commented Feb 6, 2017 at 6:54
  • \$\begingroup\$ Even getting it bound up when putting the screws in may put an undesirable warp in it. Of course, you didn't say how big it is, which is a huge factor there. \$\endgroup\$
    – Daniel
    Commented Feb 6, 2017 at 6:55
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    \$\begingroup\$ "35um copper foil (tracks), 100um FR4 prepreg, 200um FR4 core, 100um FR4 prepreg, 35um copper foil (tracks). This gives a total board thickness of around .400mm" - Sounds to me more like a total of .470mm, almost 20% more than what you quoted ;) \$\endgroup\$
    – marcelm
    Commented Feb 6, 2017 at 12:34

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I've done a 1 mm thick board with dimensions about 3 x 15 cm. This design was noticeably more flexible than a similar 1.6 mm board, and flexible enough that I worried about the possibility of board flex causing damage to mounted components or the solder joints holding them to the board.

Given that the flexibility of a board depends on the cube of the board thickness (because it depends on the area moment of inertia of the cross-section of the board), and your proposed stackup is less than half the thickness of mine, I expect you will need to take special care to mount this board at multiple points, or provide some other stiffening mechanism, to avoid damaging the board or the parts on it.

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