What is Solder Wicking?

Solder Wicking (also know as Solder Drainage) occurs when the solder flows away from the pad on a PCB, accumulates on the surface of the board and enters nearby via holes and tracks.

Usually, the component terminal has a lower melting point than the pad to which it is being soldered. As a result, the component terminal reaches its melting point before the pad and the solder flows away from the pad and accumulates in unwanted places.

See the image below, Solder is placed on the pads on to which a component is placed. Once the component is soldered on to the pad, the solder melts and flows towards the via hole and associated area.

Apart from the low melting point of the component terminal, the varying reflow profile of the solder material is also responsible for the solder wicking. This varying reflow profile depends upon the amount of Tin and Lead (Pb) used, while preparing the solder alloy.

Vias and copper tracks are obviously not protected through the solder mask layer that is why they are much susceptible to the solder wicking problem. In extreme situations, the solder can reach the backside of the board through the via, forming ‘bumps’ on top of it.

The solder wicking problem can be controlled by paying attention to the listed parameters:

• The temperature difference between pad and terminal
• Wettability of pad i.e. its joining capability
• Gap dimension between pad/lead.
• Melting time of solder at the rivet

One solution to this problem is the use of a Kapton tape to cover the unmasked vias on the board to block the molten solder from leaking through the vias. Once the components are soldered and the PCB is cooled the tape can be peeled off.