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.
粘度是助熔剂的流动阻力或助熔剂的内部摩擦力。粘性材料的例子有蜂蜜、花生酱和酸奶。它的单位是（力 x 时间）/面积或国际单位[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 fluidgenerally 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:
- 含水超能胶（氰基丙烯酸酯）具有粘性，但不粘稠
- 番茄酱和酸奶有粘性，但不粘稠
- 以猫王为主题的拉斯维加斯婚礼俗不可耐
