Questions about standby current defects during production

Question

hi 
 
 We have developed it to the specifications below and are preparing for mass production. 
 We would like to inquire about a 10% standby current test defect occurring in pre-production (100EA). 
 
 1. Specifications 
 - nRF52832 
 - SDK 17.1.0

2. Self-review results 
 - PCBs with standby current defects continue to defect (100%) 
 - Normal PCB with no standby current defects continues to be normal (100%) 
 
 - If you retouch the nRF52832 IC with a heat gun, the standby current will return to normal for a while and then increase again. 
 
 - Changing peripheral devices such as crystal, LDO, etc. does not solve the problem. 
 
 - Normal current: 100uA -> defective PBA 230uA

=> Is there anything you can guess as the cause?

hmolesworth · Accepted Answer

This issue has been raised dozens of times over the years in one format or another, and relates to a floating Rx pin (sometimes with an external device with separate power supply back-driving the VDD via Rx). The fundamental issue was identified back in 1980 where a floating CMOS or MOSFET input pin can cause feedthrough from internal VDD to GND of typically 130uA when the voltage on the pin floats to the FET thresholds. Why would the input pin float to the threshold voltage? The input impedance on the pin is very high, and any electrical fields or miniscule stains on the PCB from the assembly and cleaning process can cause induced voltage or leakage sufficient to move the input voltage to the FET threshold. The nRF52 series input pin has effectively a bus-hold feature, details unknown, but this does not stop the pin from floating due to such external effects. The solution is trivial in many cases, as you note: simply enable the internal pull-up to remove effects of stained board leakage (or high-humidity leakage) and remove the power from an external device or use a level-shifter in the case of a connected driver. In my view the problem historically arose from all the nRF52 examples using the default uart settings with the pull-up not enabled, both in the SDK52 and nRFConnect. Why is the Rx pin more susceptible than other unused pins? Typically there is a trace on Rx to a connector or other device, so it is more susceptible to stain leakage, whereas usually there are no traces on unused pins. 
 See my notes here: potential-risks-if-voltage-is-applied-to-a-gpio-pin-when-the-chip-is-not-powered-vdd-0-0v 
 TI App Note on Feedthrough: App Note scba004e 
 Other ways to handle unused input pins - set them as active low outputs and drive low. Does this increase sleep current? No, and I quote (my measurements): 
 // Here are my measurement results, which prove the pullup/pull-down resistor is connected to the pin regardless
// of the input setting. nRF52832, 32MHz crystal, no 32kHz crystal, no Reset pin, SoftDevice loaded but not
// enabled, no peripherals enabled, idle. Power CR2032 coin cell, no ground other than PPK-2, no J-Link.
//
// This first table is default port settings after a reset, no changes:
//
// PPK-2 Meter Conditions
// ====== ====== ==========
// 1.66uA 2.971V with errata workarounds, no i/o
// 1.51uA 2.985V no errata workarounds, no i/o
//
// This Next table is port settings made after a reset, all 32 port pins identical, nothing connected to any of
// the 32 port pins, no errata applied:
//
// PPK-2 Meter Direction Input Pullup Drive Level Sense Level Output
// ====== ====== ========== ============== ============ ============== ============== ===========
//
// 1.49uA 2.989V (PIN_OUTPUT | PIN_DISCONNECT | PIN_PULLNONE | PIN_DRIVE_S0S1 | PIN_SENSE_OFF) Driven Low
// 1.49uA 2.999V (PIN_OUTPUT | PIN_DISCONNECT | PIN_PULLDOWN | PIN_DRIVE_S0S1 | PIN_SENSE_OFF) Driven Low
// 6.47mA 2.894V (PIN_OUTPUT | PIN_DISCONNECT | PIN_PULLUP | PIN_DRIVE_S0S1 | PIN_SENSE_OFF) Driven Low
// 6.47mA 2.889V (PIN_OUTPUT | PIN_CONNECT | PIN_PULLUP | PIN_DRIVE_S0S1 | PIN_SENSE_OFF) Driven Low
//
// 1.51uA 2.959V (PIN_OUTPUT | PIN_DISCONNECT | PIN_PULLNONE | PIN_DRIVE_S0S1 | PIN_SENSE_OFF) Driven High
// 6.42mA 2.877V (PIN_OUTPUT | PIN_DISCONNECT | PIN_PULLDOWN | PIN_DRIVE_S0S1 | PIN_SENSE_OFF) Driven High
// 1.53uA 2.938V (PIN_OUTPUT | PIN_DISCONNECT | PIN_PULLUP | PIN_DRIVE_S0S1 | PIN_SENSE_OFF) Driven High
//
//
// This shows setting a port pin into active low output driven low is as good as leaving a pin floating
// when unused, with the added benefit that there is no possibility of port feedthrough if a floating pin
// drifts through the threshold.

Questions about standby current defects during production

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