microcontroller STM32F030K6T6: VSS pin 16 (internally) shorted to VDD? - Page 1
Forum section: electronics / repair

author: Sorama
author_rank: Frequent Contributor
timestamp: August 12, 2025, 08:05:00 am
content: So I am repairing a pcb with a STM32F which has a VDD (3.3V) short to ground when its VSS pin 16 (VSS from digital VDD?) is soldered to the VSS pad on the pcb.When I lift this VSS pin 16 from its pad on the pcb, short is gone.I measure 2.4 Ohm between VDD(Analog) and its VSS pin 16 (which its the short).However, there is another VSS (pin 32, VSS of analog VDD ?) which is not shorted to VDD and seems to be not internally connected to the other VSS (16) as the impedance between both is indefinite.So how can we explain this?could the VSS of the digital VDD be internally shorted to its VSS (2.4 ohm) and carrying 3,3V while VSS pin is not connected to its pcb pad?

author: ataradov
author_rank: Super Contributor
timestamp: August 12, 2025, 02:21:54 pm
content: A lot of things are possible if device sustained ESD or electrical damage, for example. Generally all related supplies are connected inside the device, but high energy event may blow that link and resulting sputter may short something else.

author: Sorama
author_rank: Frequent Contributor
timestamp: August 13, 2025, 05:52:07 am
content: The pcb is from a hot water boiler like a quooker (although different brand) and some water drops got onto the pcb.What I would like to know from someone here is wether it is normal that both Vss are isolated from each other in a way that one of the Vss pins now carries 2.3V and when soldered to its pad on the pcb, creates a short to ground.In other words: is this pcb irreparable now, as there is firmware on this faulty microcontroller that I cannot retrieve?

author: Kean
author_rank: Unknown
timestamp: August 13, 2025, 06:08:37 am
content: That is definitely not normal, they are not isolated.The datasheet specifies in the Absolute Maximum Ratings section that the absolute difference between VSSand VSSAmust be no more than 50mV.In addition VDDmust be no more than 0.4V above VDDA.

author: Sorama
author_rank: Frequent Contributor
timestamp: August 13, 2025, 08:27:40 am
content: Quote from: Kean on August 13, 2025, 06:08:37 amTnx.Data sheet doesn’t mention which pin is Vss and which Vssa. They are both Vss marked.But fact is that when lifting 1 Vss pin from its pcb pad and leaving the other Vss connected to ground pad, the pcb does start and there is no longer a short between Vdd and Vss.But as the problematic Vss is lifted, it does has 2.3V on it.I will return it to customer and see if it will work with that one Vss not connected.Those TPS62175DQCR step down converters are a real pita to solder…

author: Kean
author_rank: Unknown
timestamp: August 13, 2025, 09:04:54 am
content: Quote from: Sorama on August 13, 2025, 08:27:40 amAh, that is because there is no separate VSSAon the LQFP32 parts, only higher pin-count parts. You would have to assume VSSAis bonded internally to VSS.Quote from: Sorama on August 13, 2025, 08:27:40 amClearly something is damaged internally on pin 16, and possibly on other nearby pins if water dropped there which may cause problems if for example PB1 is used.What you see sounds rather surprising for a power pin, but I have no knowledge of the silicon die layout on these parts.