Electronic – Solder stop mask and tinning sequence

Photos of your degraded flex-PCBs would be helpful in diagnosing the issue.

You need to figure out if your PCB degredation is due to electical or environmental stress.

Environmental stress could be things like high ambient temperature, humidity, UV, corrosive environments (high salt content) and the like. It could also be application-related stress - excessive mechanical stress or flexure.

Electrical stress is less likely - usually solder mask degredation is caused by heat (excessive current) or breakdown due to excessive voltage causing arc-over. Your description of a low-voltage sensor doesn't fit this bill.

If you can't establish a sure environmental or electrical cause, it would be worth your while to get in touch with the flex PCB vendor and ask for some failure analysis. They should be able to establish the cause of the degredation.

It is a good idea to have a solder mask ridge between pads, as you say, to reduce the chance of short-circuits. It is not very important that the thermal pad on the PCB is exactly as large as the thermal pad on the component. With a smaller PCB pad you will most likely still get a good electrical connection and good enough heat transfer. (The exception would be when you are trying to use the component near to its maximum temperature and power.)

So, if you are not using the component near its thermal limits, reduce the size of the PCB thermal pad to allow a solder mask ridge around it.

Although not a direct answer to your question, another alternative is to use a smaller solder mask expansion. Perhaps 0.05mm would be enough depending on your manufacturing process.