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?

  • \$\begingroup\$ Whenever the manufacturer says you should. \$\endgroup\$ Jul 28, 2016 at 10:28
  • 1
    \$\begingroup\$ Bigger vendors often pack moisture sensitive items with color indicators so you know by inspection whether they actually need tempering. If you have a deadline, though, it seems silly to let a $30 bag of LEDs hold you up. Buy new ones. Better yet, if you can afford to spend an extra 10% or so on parts, go as close to turnkey with your assembler whenever you can. Let them do the ordering. This really smooths the process out. \$\endgroup\$ Jul 28, 2016 at 16:52
  • \$\begingroup\$ I think that tempering is a wrong term. I've only ever heard it referred to as conditioning. \$\endgroup\$ Jul 28, 2016 at 19:25
  • \$\begingroup\$ @ScottSeidman: It was more like 2000x0,10=200€ but the reel was missing a few hundred. The main problem was, that the responsible person from the purchasing department wasn't aware of a potential "tempering/conditioning" requirement and the lead time for a new reel was 2-4 weeks as well. Otherwise I agree. \$\endgroup\$
    – Rev
    Jul 29, 2016 at 6:26
  • \$\begingroup\$ @KubaOber: If you confirm that "conditioning" is the correct terminology in English, I could change that if "tempering" is misleading. \$\endgroup\$
    – Rev
    Jul 29, 2016 at 6:27

3 Answers 3


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:

Datasheet with MSL information

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.

  • \$\begingroup\$ Thanks, MSL is a good "keyword" I was missing to work with. \$\endgroup\$
    – Rev
    Jul 28, 2016 at 11:33

Tempering is required for components that say in the datasheet they need tempering, or specify some low moisture content.

The general problem is that the material in some components can absorb moisture over time when kept in humid conditions. This can cause problems when these components are suddenly heated to soldering temperature. Worst case, they can even explode due to the greatly increased pressure of the trapped water.

Certain ceramic packages are notorious for this, and LEDs are often on a ceramic substrate. Your assembly house was probably just covering their butt. They saw the open bag and didn't want to take a chance that you let the parts pick up significant moisture. Playing it safe is usually a smart move when dealing with unsophisticated customers.


Some assemblers may only have one dedicated room for moisture reduction, one temperature, and one time period. In other words it could be that your specific parts could have been moisture reduced much more quickly if they had an oven and process just for them, but if they don't want to spend the time and resources to specialize, they may have taken a "one size fits all" approach.

In these cases you'll need to weigh the cost of doing it yourself (including shipping time back and forth) or ordering new sealed parts against the time their process takes.

Two weeks seems excessive, but if the company generally doesn't want to deal with this, then it makes sense for them to essentially penalize you for providing unsealed parts by slotting your work in later. You might want to choose a different partner if you regularly need them to deal with unsealed parts, or you may want to adjust your process to allow for the time increase, or to avoid sending them parts which might end up slowing production down.


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