锡膏剪切稀化：一个鲜为人知的问题

乡亲们

焊膏是一种非常复杂的高粘度 "流体"。当遇到剪切应力时，焊膏的行为是 "非牛顿式 "的，这意味着焊膏的粘度不会随着剪切应力的变化而保持不变。触变性是焊膏的理想特性，因为它能使钢网印刷的焊膏 "砖块 "在印刷后保持形状，而低粘度在受压时又能很好地填充钢网孔径。

许多人可能会认为，这种相对复杂的现象就是故事的结局，然而，焊膏在印刷过程中至少还有一个众所周知的重要特性：对停顿的反应。停顿反应差的焊膏，在闲置 15 分钟后就会变硬。

Another important solder paste property has only recently become well known: shear thinning.My Indium Corporation colleague Tim Jensen, was one of the first people to point this out.Shear thinning is a property of some solder pastes in which the viscosity becomes lower and lower as the paste is repeatedly printed.See the figure above. The x axis is number of prints, the y axis is the viscosity.It is normal for the viscosity to go down during the print, but the viscosity should recover as the "good paste" does, not have a downward trend as the "bad paste."The resulting drop in viscosity, that the bad paste exhibits, will often result in too much paste being printed and potentially lead to defects such as shorts or solder balls.Unfortunately, if not tested for, shear thinning might first be observed after a paste has been implemented on the line.

如果您对测试浆料抗剪切稀化能力的方法感兴趣，请给我留言，我将给您发送测试协议。