Original Post

The preliminary tests for 0.4 mm pitch #multilayerpcb #DIY #BGA #solderless process finally exceeded all my expectations (less than 1 ohm via resistance, and more than 100 megaohm inter-via), after introducing a silly step (thanks to the reballing video makers, showing how meditative that process is!)

Shortly:
1) Laminate pink (for laser!) Avery stickers (Neon 5975).
2) Laser-drill BGA holes in laminated Avery (engraver 3W 0.050mm/pixel).
3) Stick the Avery to your BGA.
4) Laser-drill "conductive ink mask" (the same BGA holes) in another, NON-laminated Avery.
5) (Carefully, precisely aligning the holes) Stick the non-laminated Avery on top of the laminated.
6) Apply CircuitScribe conductive ink on each hole (paying attention to the bubbles of air under it, and to the whole CAPILLARY PHYSICS of the process).
7) (The most important step!) To facilitate the removal of the air bubbles (and to improve drying of the conductive ink!), STAB each ink-filled hole using a SMALL COPPER WIRE. Make sure your chip lays on a firm surface, otherwise the chip can pull the ink between two holes on the adhesive side, short circuiting the vias.
8) (Don't wait too long before this step) Remove the non-laminated Avery (it has short-circuited conductive ink on top of it anyway).

PM me for details.