Ipc-9704 Pdf !free! -

: Engineers glue microscopic sensors called strain gages onto specific "high-risk" areas of the board, usually near large BGAs or mounting holes.

You may find previews or outdated 1999 drafts. The industry currently uses IPC-9704A (confirmed 2022). Any link to an old version is useless for certification audits. ipc-9704 pdf

Complete Guide to PCB Strain Gage Testing for BGA Reliability - PCBSync : Engineers glue microscopic sensors called strain gages