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Solder wicking occurs when either surface--lead or pad--has poor solderability, causing the solder to flow to one surface in preference to the other. It may also occur if there is a significant difference in the reflow temperature of the pad or pin. Correct temperature profiling will eliminate this possibility.

The J lead termination shown in Figure 1 is a classic example of the wicking defect.

Figure 1: Classic wicking defect on a J-lead termination. Photo courtesy Texas Instruments.

The example of solder wicking in Figure 2 is clearly caused by the printed board surface finish. The solderability of the gold is poor due to a plating problem, and the paste has not wetted the pads during reflow. Close examination of the leads shows a slightly bulbous appearance on them. The solder has tended to wet the lead rather than the pad. Often in this situation the printing process is blamed for not printing paste successfully on to the pad.

The solderability of the printed board should be tested using a wetting balance.

Figure 2: PCB finish has caused a solder wicking problem here.

Figure 3 is another classic example of solder wicking where the reflowed solder haswicked up the component leads. In this case it is clearly the fault of the printed board solderability.

Don't be too quick to blame the PCB supplier; it may be an assembly problem. Copper surface coating can be degraded by washing the board, by long storage times in excess of six months, and by high cure or reflow temperature. Make sure you confirm the true cause of the problem.

Figure 3: Solder beading from an LCCC.

Via hole wicking was a common problem in the early days of surface mount reflow. Poorly trained design engineers placed via hole either in or too close to pads. During reflow the solder was drawn down the hole, starving the joint area. Via or through holes should be a minimum of 0.010" away from solder pads and connections should be either 0.006-0.008" in width.

It is possible to place vias in pads provided the size is small enough to block before all the solder volume is lost. This technique generally increases the cost of the printed board.

Figure 3: Via placement in pads or too close to solder lands is an old problem but still happening in the industry due to poor education of engineers.

Chip Leaching	Component Cracking	Cracked Joint	Crystallized Joints	Failure to Reflow
Joint Failure	Joint Voiding	Lifted Component	Open Joints	PCB Measelling
Plastic Coplanarity	Plastic Softening	Poor Pad Wetting	Poor Solderability & Wetting	Solder Beading
Solder Short	Solder Wicking