Sometimes when buying components, you’ll receive two different batches of the same component from the same manufacturer, but which are each marked with a different MSL level. One batch might be marked MSL 1, while a different batch of the same device is marked MSL 3. How can this happen? When it does, which MSL is correct?
The answer is that both MSL levels are correct for their specific batches. MSL is not determined by the engineer who designs a component, but rather determined by the specific bill of materials for the device’s packaging used by a given assembly site. One assembly site’s package BoM might warrant MSL 3, while a different assembly site using a different package BoM might be able to get away with MSL 1 instead. This is not a fault or an error with either batch of products, and it has no effect on the performance or functionality of the components in question.