SMD components and moisture

[MacAttak]

I've never really thought much about it before, just accepted it "as is". But why are SMD components so sensitive to moisture as compared to through-hole parts? And why the comparatively short shelf life?

And why does this "moisture sensitivity" and "expiration date" only apply up until the moment of PCB assembly, and not really matter once the board has been assembled and is in service?

Surely there must be good technical reasons for this. Part vendors go through a lot of trouble to pack SMD parts with desiccant packs and moisture-sensitive blotter strips, and to document the sample's production date.

[marshallh]

This is because SMD assembly processes are almost all reflow nowadays. If moisture gets into the (porous) plastic packaging, it will heat up and expand possibly blowing pinholes in the packaging and worse around the die itself.

Through hole soldering process: generally wave soldering. It's like the donut glazing method but with molten solder. Only the joint itself is actually heated and the package doesn't care if it has trapped moisture. In the early 90s some mfgs stuck on SMD parts on the bottoms of boards and ran them through the wave solder line also.

[MacAttak]

Ah yes that makes sense. I totally wasn't thinking about the difference in industrial assembly where SMD boards get baked for a few minutes during reflow, while through-hole boards just get the "quick splash" from wave solder. Entirely different thermal profiles going on there.

[rf-loop]

I have seen one (very different)  special thing related to water and SMD components.

I live in country where sometimes may be even -50 degree celsius. (very usual in winter time 0 to -25 degree).

I have see expensive equipments what have stored in totally wrong place. There have been lot of temperature variations (under and over zero) and humidity in air have condensed on the PCB and also frozen frequently.  Under SMD component  condensed water, then frozen... smelt... frozen...smelt...frozen... of well I can see lot of SMD componets just as pulled out (soldering broken and component just as bounced off the circuit board.)

(volume increases by 10%, and the force is large and many cycles may really move componet out from PCB).

Very pity I have lost these pictures.

[Falcon69]

wow, amazing what a search turns up.

I recently got some 0402 LED's from mouser and they sent them in sealed packaging with that moisture soak up packets inside.  There was a moisture id card inside as well.  AN expiration date and such on the outside.

Now my question has been answered as to why they sent it that way.  Good thing I sealed the package back up right after I opened it up. Was just being cautious.

[Alana]

Should i take those moisture related expiry dates into consideration if i am hand soldering?

[ddavidebor]

No

[codeboy2k]

The other issue with expiry is oxidation of the metal endcaps used in SMD parts, like resistors , caps, diodes.  So a reel of surface mount resistors that are past 1 year from the date on the reel may not solder well anymore in automated assembly.

If you are hand assembling with that part, you may be able to get it to work, and flux helps. But the automated assembly relies on the flux in the paste and good clean parts to work well.

So, if you are buying parts on reels for automated assembly, don't buy anymore than you think you can build up in about 6 months or so, no matter how much of a deal it is to buy 100,000 parts at one time.  Just buy more when you need it and keep on top of the lead times. This is better than having a bunch of old parts that don't wet anymore and can't be soldered on the line. Then you need to throw those away and start over, and there's now more wastage when you thought you would be saving money.

[nanofrog]

For heavily oxidized end caps & leads, they can usually be cleaned with a pencil eraser (i.e. lightly abrasive ones), or with a fiberglass scratch brush prior to soldering (be careful of the dust with these, as it can itch like crazy if it gets embedded in your skin).

[ddavidebor]

or a strong flux, but that must be cleaned (usually)