Continued from [iPhone X Teardown] Parts-containing Main Board (3)
Even though the area of the board is 2/3 that of the previous board, parts are mounted on three of the four surfaces. So, the area on which parts can be mounted is almost the same (2/3 x 3 = 2). There are expected to be a relatively small number of pins between the AP and RF boards, indicating that the diameter of the interposer's solder via is not too small and that the aspect ratio is sound.
Probably, the AP and RF boards were separately manufactured and mounted, and parts were assembled through up to three reflows.
"This structure was realized probably because Apple designed the A11 Bionic main processor in house," said an engineer who works for a substrate material maker.
"There has long been the idea of stacking boards by using solder via, and it is classic in a sense," said an engineer who works for a substrate material maker.
For example, solder via that fills a through-hole substrate with solder paste is often used for connecting flexible substrates for hard disk drives on both sides. Nevertheless, the engineer said, "It is rare that such a method is employed for a microfabricated board."
As for the reason why this method was employed for the iPhone X, engineers estimated that Apple is good at solder connection.
"Apple did not use connectors for the connection of the flexible and rigid boards and frequently used 'hot bar,' which applies a soldering iron from the back side of a flexible board," said an engineer who works for a substrate material maker. "Also, solder connection is highly reliable, and that is an advantage."