Phil
Welcome to Board Talk. We are the Assembly Brothers, Jim Hall and Phil Zarrow of ITM Consulting, here to attempt to solve your various SMT assembly process problems. Let's see, what do we have today?

Jim
Well, Phil, today from SA we have a concern about high humidity. The question is, "Will higher than normal humidity in our facility affect reflow soldering? What impact might it have?"

The second question, "Are there special concerns regarding our solder paste when we are exposed to high humidity environment?"

Phil
The answer to that is yes, yes and yeah, most likely. Of course, in terms of solder paste, all solder paste, whether they're no clean or OA, water soluble are all hygroscopic which means they're absorbing moisture from the air as they sit out there on your stencil, on your board, you know, basically during the work life of the solder paste. And as they absorb various degrees of moisture, that of course is going to affect the chemistry of the flux itself or of the solder paste. So, it is going to have an effect on things and generally you'll find solder paste has a specification, an upper and lower limit in terms of the comfort index

What do we usually see on the specs about an upper limit of, what, about 65%, 70% RH? 

Jim
Some pastes are formulated specifically for higher humidity environments. If that's a concern, you can look for it. And on the low end, typically it's 30% to 40%. Which can be a problem up here in New England when we turn the heat on and it tends to dry things out.

Phil
The solder paste threshold comfort, as far as humidity, is first of all slump or bleeding after printing. You know, there is of course our good friend solder ball – it's just another – yet another cause of solder balls. And because basically the flux is being somewhat diluted, if you will, you may see an effect on wetting. The ability of the flux to wet properly, because of the chemistry being altered as we mentioned before.

Jim
As Phil said, both no clean and water soluble pastes are hydroscopic. These humidity effects tend to be more pronounced in water soluble paste because they have alcohol or water-based solvent systems. Although the paste manufacturers have been working hard to minimize those effects.

In regards to solder paste in general, you also need to think back into your stencil printing. If you get too much humidity, moisture absorbed from humidity while it's on the stencil, it can affect the printing properties as well as what happens when you try to reflow it. This is particularly true in low humidity when the evaporation of solvents raise the viscosity, cause drying of the paste, plugged apertures and all those unpleasant occurrences in our stencil printing operation.

Phil
So, pay very close attention to the specification of the solder paste you're using, do your due diligence in terms of trying to control your relative humidity and, of course, your temperature in your work facility. If you are in a situation beyond control, consult your solder company to see if they have a formulation for those extreme humidity situations. Beyond that, I would say thank you for tuning in.

Phil
So, thanks again. Until next time, this is Phil Zarrow and Jim Hall of Board Talk and ITM Consulting saying whatever you do –

Jim
Don't solder like my brother.

Phil
And don't solder like my brother.