nRF52840 Lead Finish of the BGA Balls

[John]

We are having a very difficult time soldering the nRF52840 to the PCB. Our Operations Department is asking what the lead finish of the BGA balls is. Would be great to get on the phone with you guys to discuss all the things we have tried. You might have some ideas. Thank you!

[Kaja]

Hi John, 

we have this application note: QFN, but the the lead-frame is different for the nRF52840:


Maybe that will solve the issue? 

Best regards,
Kaja

[Kaja]

And maybe this will help, some guidelines for aQFN: 

Best regards,
Kaja

[John]

Thank you, Kaja. I will pass this information on to our Process Engineers and I'll let you know if they have further questions.

[Kaja]

Great, let me know! 

Best regards,
Kaja

[John]

Hello Kaja,

We think we are still having issues soldering the nRF52840 to our PCB. We have been temperature cycling our PCBs from +70C (2 hour soak) to -30C (2 hour soak), and after a few of these cycles we are seeing nRF52840 resets on about 1/2 of our PCBs. These resets always occur near or shortly after the temperature gets to -30C. We tried a destructive test to evaluate our soldedring. We actually pryed the nRF52840 off the PCB with a couple of small flat-blade screwdrivers. When we look at the PCB and IC afterwards, we note either the PCB pad stood with the IC, or the IC pad stood with the PCB. We think this means the solder joints were really good! The one exception is the VSS pad. When we look at the VSS pad, we notice the pad on the PCB is still there and the pad on the IC is still there. We think this means this joint isn't being soldered very well. If the VSS IC pad became disconnected from the PCB, would this casue the 1.3V core voltage or any other internal nRF52840 voltage to go away and cause a reset? The caps needed for the 1.3V buck circuit are still connected to VSS through a via, but does the circuit stop working if the VSS pad separate from the PCB? Thank you very much!