Enable nRF52840 regulator DC/DC mode with short-circuit in DCC and DEC4

[lawisme]

Hi,

I am searching for some methods to blocking the abnormal PCBAs which got short-circuit in DCC and DEC4 after SMT process. These abnormal PCBAs will cause BT pairing process fails in business firmware stage, however it is not efficient for manufacturing to use this method to block them out. I try to have these PCBAs to go through the DTM test with enabling the regulator DC/DC mode, but sometimes pass and sometimes fail. Is there any suggestion for me? Thank you!

[ketiljo]

The DEC4/DEC6 net is the power supply to the radio. So any short to ground will render the radio inoperable. The DTM test should fail if you have a real short here. If you measure the current drawn by the device with the radio in RX mode for instance, you should see a different if DEC4 is shorted.

[lawisme]

Not short to ground, but short between DCC and DEC4 (as the X-ray picture below). I suppose the short will affect the power supply and the RF performance. But it is hard to measure.

[ketiljo]

If DC4 and the DCC pin are connected, it should have the same effect as if the DCDC inductor is shorted. The DCDC will not operate properly. It might be able to supply the radio somehow but the current draw when the radio is operating will be very different. So you should be able to detect it by measuring the current when the radio is in RX mode.

[lawisme]

I think it is a good way by measuring current to check this issue when debugging. But in my situation, it is hard to measure the exact current value after SMT process since MCU shares the power supply with other components. And it is not able to remove any 0 Ohm resistances to make MCU circuit separated from the the other components in the production process. So any other suggestion? Thanks a lot~

[ketiljo]

Not really. But if you compare the total current drawn by a known good unit and a unit with shorted DCDC inductors, there should be a difference. You don't have to measure the current for the nRF device by itself. 

[lawisme]

I will have a try on it. Anyway, thank you very much!