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]

You seen have too little solder paste applied. The center pad has some air bobbles but that's probably ok. The pads however are more concerning, some om them are circled here: 

Try to use a thicker stencil to apply more paste. 

[Do Van Hiep]

Hi Mr Ketiljito. Thank you for your recommendation. It's helpful for us.

[Do Van Hiep]

ketiljo In case we need to verify air bubbles inside might be affected to our components. What are your recommendation?

[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.