TL;DR A visible void beneath an exposed-pad QFN does not, by itself, decide whether a PCBA should be rejected. Buyers need an acceptance rule for the exact component, PCB layout, assembly process and operating conditions before production release.
Why a single void limit is unreliable
Texas Instruments’ QFN Layout Guidelines, dated July 2006, advises designers using a 0.127 mm stencil to X-ray reflowed boards and verify that at least 50% of the thermal-pad area is soldered. That guidance addresses a specified process condition; it is not a universal pass threshold for every QFN or application.
Void size and position also matter. Analog Devices’ undated AN-772, Revision 0, reports that small, distributed voids covering up to 50% of the paddle had only a marginal modeled thermal effect in its 6 mm × 6 mm LFCSP example. It separately cautions against large voids that could affect electrical or mechanical performance. That result cannot be transferred to a different component without validation.
What buyers should decide before release
Identify the exact orderable part and check its current datasheet and assembly guidance. Agree on how X-ray images will measure total void area and significant individual voids, then review the result against the pad, thermal-via layout and the product’s heat-dissipation requirements.
Set the inspection method and acceptance criteria with the assembler before the build. JETFGO’s English PCB Assembly page says X-ray inspection can form part of a quality plan according to the board design and project requirements; it does not publish a QFN void acceptance limit.
A measured void is evidence for an engineering decision. Approve or reject the assembly against documented, part-specific criteria and the intended operating conditions.
References Texas Instruments, QFN Layout Guidelines, SLOA122, July 2006: https://www.ti.com/lit/an/sloa122/sloa122.pdf Analog Devices, AN-772: A Design and Manufacturing Guide for the Lead Frame Chip Scale Package, Revision 0, undated: https://www.analog.com/media/en/technical-documentation/app-notes/an-772.pdf
Related JETFGO pages PCB Assembly & PCBA Manufacturing Services: https://jetfgo.online/en/products/pcb-assembly