Editorial Team - PCB Directory
May 20, 2020
The Wetting Balance Test is conducted to measure the wetting forces between molten solder and the PCB as a function of time. The wetting balance test measures the time it takes for the solder to wet the termination pads of the component on a PCB.
During the test, the PCB is inserted into and removed from a molten solder pot at high speeds (1 to 5 mm/ sec). The solder pot is held steady for 5 to 10 seconds while the PCB is inserted and removed in the vertical dimension. Forces like surface tension and buoyancy act on the board in the vertical dimension.
The net active forces are measured using a linear variable differential transformer (LVDT) and used to draw a wetting balance curve to show the total force (mN) or normalized force (mN/mm) as a function of time. It provides information about the extent of wetting.
Apart from wetting forces, there are rejecting (non-wetting) forces too, as plotted in the figure on the negative axis. The rejecting forces act in the upward direction and the wetting forces act in the downward direction.
The rate of insertion of specimen is high enough that it reaches the depth of the solder pot before the wetting forces came into action. Initially, the rejecting forces are high, that’s why the curve rises from the negative axis. After some time, the wetting forces overcome the rejecting forces, which is depicted by the curve on the positive axis. The point where the magnitude of both the forces match each other, they cross the zero-axis called Time-to-Zero. When solder sticks to the PCB specimen, it compels it to move downward, resulting curve movement in the positive direction. The slope of the rising curve gives the indication of the wetting rate.
The Wetting Balance test is fast, automated, and an entirely qualitative method, which only specifies whether the specimen under test has passed/ failed. We need a statistically-proven test method to get clear-cut information on solderability validation. For solderability evaluation, the wetting curve is divided into three pass/fail valuations based on IPC-J-STD-003 criteria.
Click here to learn more about this test and curve.
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