Solder Wicking and Heat Robbing Control

Solder wicking is the alloy travelling away from the joint into a region it should not reach, most often up a lead, along a pad or into a via. The result is a joint that is starved of the material that was intended for it.

Heat robbing is the mechanism behind many of these cases: a connection with high thermal mass pulls energy out of the joint, so the alloy stays liquid longer in one place than another and flows in the direction of the heat.

Where the Alloy Travels

The common paths are up a component lead, along a trace connected to the pad and into an open via. In each case the destination is a region that is hot and wettable.

The travel is driven by surface tension and by the temperature gradient, so a joint that is heated unevenly will move material towards the hotter side. Understanding which region is hot explains the direction.

Heat Robbing by Planes and Traces

A pad connected to a plane has a much greater thermal capacity than one connected to a short trace, so the plane conducts energy away and delays the joint reaching temperature. During soldering, alloy can flow along the connection towards the plane.

The counter measure is a thermal relief, which reduces the connection area so that the pad heats more quickly. Reliefs on a soldered through hole pad are standard practice for exactly this reason.

Thermal Relief Design

A relief replaces a solid connection with spokes, and the number and width of the spokes set the balance between heat loss and current capacity. Too few spokes and the pad heats quickly but the connection is poor, while too many defeat the purpose.

The spokes should be symmetric so that the heat flow is even, and the pattern should be repeated on every layer where the pin connects. Asymmetric reliefs produce a directional flow that can be worse than a solid connection.

Via and Barrel Wick Paths

An open via beside a pad is a wick, because the barrel is hot and the copper is wettable. Alloy drawn into the barrel leaves the joint short and can contaminate the opposite side.

Tenting or plugging the via removes the path, and a filled and capped via removes it completely. Where a via must remain open, the pad should be arranged so that the via is not adjacent to the joint.

Wicking Up a Lead

Alloy climbing a component lead is normal to a degree, and it becomes a defect when it reaches a region where it interferes, such as a seating plane or a contact surface. The cause is usually a lead that is hotter than the joint, which happens when the lead has a large mass or is connected to a heat source.

Slowing the heating or preheating the lead reduces the gradient. Where the lead is a contact surface, a mask or a solder stop ring can define the limit of the acceptable flow.

Paste Volume and the Wick Effect

A joint with more paste than the fillet requires has surplus alloy that will find somewhere to go. Reducing the volume to the required amount removes the material that could wick.

The volume should be calculated from the joint requirement rather than copied from a similar footprint, because the demand varies with the lead geometry and the hole size.

Solder Preforms and Controlled Volume

Where the volume must be exact, a preform gives a defined amount of alloy at a defined place. The technique is used for joints where wicking or starvation is critical and the paste route is too variable.

A preform still needs flux and a thermal cycle, so it removes the volume uncertainty rather than the process requirement.

Detection and Records

Inspection looks for a shortened fillet, alloy on a surface where it does not belong and an unexpected change on the opposite side of the board. A cross section shows the material distribution inside a barrel.

Where wicking is present, the record should include the pad geometry, the via treatment and the profile, because the correction depends on which of them created the path. These records sit with the parameters described in manufacturing tolerances.

Design Measures

Keeping vias out of pads unless they are filled, using symmetric thermal reliefs and avoiding long traces from a soldered pad to a large plane are the practical measures. Each is cheap at layout time and expensive to correct later.

Where a joint must be protected from heat loss, a heavier preheat or a slower ramp reduces the gradient without changing the design. That choice depends on whether the board or the artwork can be changed first.

Process Control and Verification

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Is wicking always a defect? No. Alloy climbing a lead is normal within limits, and it is a defect only when it reaches a region where it interferes.

Does a thermal relief reduce current capacity? It reduces the cross section, so the spoke count and width should be chosen from the current requirement as well as the thermal one.

Can a via be left open next to a pad? It can, and it is a wick path, so tenting or plugging is the safer choice where the joint volume matters.

What is the quickest correction? Reducing the paste volume to the calculated requirement, which removes the surplus that finds the path.