Wicking
Wicking turns up in 3 fields: adhesives, PCBs, nonwovens. Here is what it means to the people in each one.
Wicking in adhesives
Describing very thin adhesives that run into already assembled joints by capillary action.
Wicking grades of cyanoacrylate, anaerobic and UV adhesives have water-thin viscosity and are applied at the edge of an assembled joint, threads or crack, where capillary action pulls them in. They lock pre-assembled fasteners, seal porosity and tack wires and coils. They also run where they are not wanted, so dispensing is controlled with fine needles and small drops.
"Hit the set screws with the wicking grade after torque, a drop on the edge and it pulls right in."
Wicking in PCBs
Solder or plating chemistry drawn by capillary action where it is not wanted, up a lead, into a via or along glass.
Wicking describes unwanted capillary flow in two places. In assembly, solder wicks up component leads or down open vias in pads, robbing the joint and leaving it starved or voided. In fabrication, wicking is copper or plating chemistry penetrating along glass fibers from a drilled hole wall; it is measured in microsections and limited by IPC class because it shortens the spacing to neighbouring conductors and can seed CAF.
"Solder is wicking down the open vias in the thermal pad, plug them or tent them from the bottom."
Wicking in nonwovens
Capillary movement of liquid along or through a fabric, measured as how far liquid climbs a strip in a set time.
A strip of fabric hangs with its end in liquid, and the height reached after a fixed time is recorded in each direction. Wicking depends on fiber wettability, pore size and fiber orientation, so carded webs usually wick faster in MD. Absorbent and acquisition layers need good wicking to spread liquid, while barrier layers are designed to resist it.
"MD wicking is 40 millimeters but CD is only 15, so the fluid runs straight to the ends of the pad."