Abnormal inspection when checking by X-Ray machine

[Do Van Hiep]

Hello Experts,

When I use a X-Ray machine to check appearance of NRF52840 . But I found out that there were some abnormal phenomen as same as a photo below:

Any body can explain what they are and is it normal or defect?

Thank you.

[ketiljo]

Air in the pads should be avoided as it might affect the connection of the pad. But some air bubbles under the center pad is hard to avoid.

[Do Van Hiep]

1. What will we do to reduce air bubbles in the pads?

2. Do you have any recommendation about reliability testing methods to verify this bad issue?

Thank you.

[ketiljo]

If you have open vias in the pads, the hole in the via will suck some of the solder. In this case, you need to plate the vias. Else, more solder by using a thicker stencil will normally fix this problem,