I have a product design that includes 2 PCBs soldered together at 90 degrees. Currently the boards are jigged and then inductively soldered together at 6 points, with solder wire being fed through 6 separate mandrels. They’ve been doing it that way for 10 years and have been living with about 87% first time yield, and about 500 PPM of latent defects caused by poor soldering that pass testing but fail in the field. 
This is an ideal case for a flex circuit, except that board rigidity is required in some areas, hybrid flex/rigid costs too much. 
Is it possible to twist a narrow strip of FR4 pcb through 90 degrees? Say 20mm long and 6mm wide? Has anyone done this in a real product?