Continued from [iPhone X Teardown] Parts-containing Main Board (2)
There was no use in continuing to look at the PCB. So, we started to split the two-story PCB by using a rework machine for repairing BGAs, etc.
When the upper board was removed, we found an interposer that functioned as a spacer and connected PCBs with solder. So, they were connected by using the solder via of the through-hole substrate that connected the two (upper and lower) circuit layers without using cables, connectors, etc. The interposer played three roles: spacer, physical connection (fixing) and electric connection. Compared with connection using a flexible substrate, it eliminates the need for space for wiring. So, it contributes to reducing size.
Because the interposer had fine notches, it seemed that Ajinomoto Co Inc's interlayer insulation material ABF film for packages, which can be easily processed, was employed and processed by counter boring.
Hybrid chip or parts-containing PCB rather than main board
Engineers were so excited about the main board having the two-story structure.
"It is a modularized main board or a hybrid chip," said Kiyoshi Abe, who is an adviser for DMM.make and had long been engaged in the development of mobile devices at a major Japanese electronics manufacturer.
"If we consider that the interposer is used for storing parts to be mounted, it is a parts-containing board," said an engineer who works for a substrate material maker.
"What's more, it is well designed," Abe said. "The size of the AP substrate having a large number of layers was reduced as mush as possible."