I know that components should kept in storage either in a sealed bag or in a dry environment.
We had one case where the assembly house got an open bag of LEDs from us and we were somewhat surprised, when they told us that they had to temper those LEDs for 2 weeks (!) to make sure absorbed humidity has been removed before starting assembly. This really hurts if you have a deadline.
I think that if a company gets back remaining parts from one assembly run, its not uncommon that those parts are stored under sub-optimal conditions.
So I was looking for some insights about what kind of components may be prone to require tempering and thus should either be kept in close bags / under optimal storage conditions. At least it would be good to have an idea where tempering may be a requirement.
Can anyone provide some general information regarding this topic?
Best Answer
This is determined by the Moisture Sensitivity level of the component. If a component has been in an uncontrolled (as defined by the MSL requirement) longer than it is permitted prior to reflow, it needs to be baked to ensure all the moisture (well, the vast majority of it) has been removed to prevent popcorning.
Even passive components have this stated in the datasheet, such as the one below:
Note that MSL varies depending on process; I have seen the MSL for a component be 4 for a leaded process (peak reflow temperature 220C typically) and 3 for a lead free process (peak temperature 245C to 250C).
I must admit 2 weeks seems excessive, although I have seen a programme using a low temperature bake for many days due to other issues with the parts.