Solderability Problems at Low Humidity

Phil 
Welcome to Board Talk. We are the Assembly Brothers, Jim Hall and Phil Zarrow, of ITM Consulting, and we're here to attempt to answer your questions on SMT process, equipment, materials, procedures, or anything else you might have in your mind, and, also, maybe throw in a rant or two when it's appropriate.

Let's see. Today's question is from a Mr. Scott D., but we don't know what town he's from. Scott has a humidity question. Scott writes that some experts say that a circuit assembly production facility's temperature should ideally be set between 70 and 78 degrees Fahrenheit and the humidity of 40 to 70 percent when working with electronic assemblies.

The facility I work in has a current temperature of 76 degrees and the humidity level of only 16 percent. Will that humidity level affect the solderability of our assemblies?

Jim
Where do you get humidity down to 16 percent? That is incredibly dry. I think we can assume that Scott lives in one of the desert states.

Phil
Yeah, the desert states or the snow belt states. If he's writing this in the middle of winter and they've got the heat cranked up and it's hot enough, it's dried up, you can strike a match on anything. So that's probably the situation. We would have to think that if Scott is in that aforementioned desert that his humidity changes erratically when summer comes as it typically does in the snow belt. But in any event, to answer Scott's question ...

Jim
No, Scott, you will have no problems with solderability at that low a moisture level and that moderate a temperature. You should have no problems. Also, your MSD components, you can have significantly improved increased out-of-the-bag exposure times. Looking at the tables from moisture sensitive devices in IPC Jedex Standard O33, if you have a Level 2A, your exposure time goes from 28 days up to 124 days and a Level 3 goes from 7 days up to 10 days.

So those are nice advantages. But we have some other concerns that we think are very significant. One is ESD.

Phil
Yeah, absolutely. As we say, that dried air, oh, boy, sparks must be flying.

Jim
Don't you see sparks flying all over the place whenever you're touching non-conductive materials? I would think this would be painfully obvious in a highly hazardous situation for all of your sensitive ICs.

Another issue is your solder paste, its work life on the stencil and on the board in terms of tack and workability, particularly if you're using an OA water soluble paste, which tend to dry out.

At that low a humidity level, you could be seeing problems. Perhaps Scott is observing soldering problems that he's attributing to solderability and they're really resulting from the fact that his solder paste is drying out.

Certainly, the solderability of materials is not affected by low humidity, but the working life of your solder paste could be a significant issue. That could give you some of the same kind of problems.

I think if you go to your solder paste spec, most of them are going to tell you that you should not be operating at 16 percent relative humidity.

Phil
By the way, some of the newer things we've seen the solder paste companies develop in the last few years, although this may still be off the scale, are solder pastes that have enhanced capabilities in humidity extremes.

For example, for high humidity situations, such as we find typically in Southeast Asia. But as Jim mentioned 16 percent is incredibly low humidity.

That's it for this session. This has been Jim Hall and Phil Zarrow, the Assembly Brothers, saying remember ...

Jim
Don't solder like my brother ...

Phil
and don't solder like my brother.

Jim
And keep those kids away from the solder pot.