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I had a selection of 0805 LEDs in my last Mouser order and was a bit surprised to discover they arrived in heat-sealed (rather than folded and taped with sticker) bags containing a desiccant bag and some sort of litmus-esque paper indicator saying something about baking them if the dots on it went pink (instead of blue).

head-sealed bag with caution stickers

A quantity of through-hole LEDs I had in the same order just came in the regular ESD-protective bag, folded over with a sticker.

What's up with SMD LEDs that they work so hard to keep them dry? Surely their dryness can't be assured once they're installed. Does it have to do with their solderability or something? Why would that be different than other SMD parts?

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    \$\begingroup\$ It is NOT different than other SMD parts. \$\endgroup\$ – Mark Apr 25 '16 at 23:31
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    \$\begingroup\$ FYI this only really applies to reflow soldering. If you are hand soldering them, you don't really need to worry about it. \$\endgroup\$ – Passerby Apr 25 '16 at 23:53
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Through-hole components are generally hand or wave soldered. These apply on local heating to the pads and not to the component itself.

SMD parts on the other hand are generally reflow soldered. This involves putting the entire part in a hot oven for an extended time. The components are generally made from porous materials which by their nature absorb moisture from the atmosphere. If they get water inside them, putting the parts in an oven can cause it to rapidly turn to steam which in turn expands and can fracture or destroy the part. As such the devices are packed for shipping in such a way as to reduce the risk of exposure to moisture.

Parts that have been out of a sealed bag or which have been sitting for extended periods are generally baked at a lower temperature first to cook off any water before being reflowed in order to stop fracturing.

In the case that the parts are being hand soldered, baking is probably not necessary, especially since it will likely not be a production run. But the distributer doesn't know this, so they do their duty and ensure the parts are properly packaged.


The same applies to ICs as well as LEDs. In fact there are also different classes of sensitivity - some parts are more moisture sensitive than others, and as a result require different levels of packaging and/or have different shelf lives.

In your case it is a Level 2 device which is one of the lesser sensitive parts - pre-baking isn't strictly required unless they are exposed to >60% humidity measured at room temp and they can be stored in the packed bag for many months.

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The LED package will absorb moisture. When you raise the package to soldering temperature, the moisture turns to steam and the increased internal pressure will damage or destroy the component.

We deal with this every time we work with moisture-sensitive SMT devices such as ceramic capacitors. They need to be baked before they are soldered - either by hand or via reflow.

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