Viscosity and tack are two important material properties that define a flux. People often “tack” them together as one, thinking they go hand-in-hand. Typically, this is the case, but not always.
Viscosity is the resistance of a flux to flow or the internal friction of a flux. Examples of viscous materials are honey, peanut butter, and yogurt. It has units of (force x time)/area or SI units of [N•s/m2].
Tack is the stickiness of a flux or its ability to hold components. Examples of tacky materials are tape, glue, gum that has been chewed, and also honey. It has units of force or SI units of [N].
The difference between these two properties is apparent when you leave a jar of solder paste sitting out. Over a period of days or weeks, the solder paste hardens and becomes extremely viscous. The solder paste also dries out and becomes less tacky. Viscosity is going up, but tack is going down.
Consider the metal load of a solder paste, or the weight percent of alloy in the flux vehicle. A higher metal load equates to a higher viscosity. More metal in the paste means it's harder to stir.
What does this higher metal load do to the tack?
The tack decreases as metal load increases because there is less flux present. Remember, flux is the source of that sticky tack property.
A high viscosity fluid generally equates to a high tack (i.e. honey). So how can we remember that this isn’t always true, and that they are totally different properties? Here are some examples that will help the lesson stick:
- Watery superglues (cyanoacrylates) are tacky, but not viscous
- Ketchup and yogurt are viscous, but not tacky
- An Elvis themed Las Vegas wedding is just tacky