Wicking: Preparation, Placement and Process Control
Solder wicking is the movement of molten alloy along a metal surface under capillary action. In a barrel it is what fills the joint, and on a stranded wire or a pad it is what carries alloy where it was never meant to go. The same physics produces both the good result and the defect.
The Capillary Mechanism
Molten alloy climbs a narrow gap because surface tension pulls the liquid into the space and the wetting surface keeps it there. The narrower the gap and the better the wetting, the further the alloy travels.
The travel stops when the heat is removed, when the gap opens, or when the alloy meets a surface it cannot wet. Every control on the process works on one of those three.
Heat, Time and Wetting
Alloy keeps moving as long as it is liquid and the surface is wetting. A long dwell therefore produces more wicking, and a hot, clean surface wets faster than a cold or contaminated one.
That is the whole reason that hand soldering a stranded wire needs a heat sink on the insulation. The alloy travels up the strands and stiffens the wire where it must stay flexible. Our article on hand soldering process control covers the tooling and the technique that limit it.
Wicking in Through Hole Joints
In a plated barrel the alloy must climb to form a fillet, and the amount is set by the barrel clearance. A larger clearance needs more alloy and more time, and a clearance that is too large produces an incomplete fill.
The barrel plating quality decides whether the alloy wets at all. A barrel that has been oxidised or contaminated during storage will dewet, and the joint will look as if the alloy refused to climb. Our article on solderability testing describes how that condition is detected before production.
Wicking Along Stranded Conductors
The strands of a wire form many narrow capillary paths, and flux travels along them ahead of the alloy. Once the flux has cleaned the strands, the alloy follows, and the only barrier is a heat sink or a mechanical stop.
A pre tinned wire is worse, because the tin coating is itself a wetting surface and the alloy travels further. The specification of the strip length and the use of a solder stop are the usual controls.
Wicking and Coatings
A conformal coating relies on the surface being covered, and alloy that has wicked into an area that is meant to stay flexible will crack the coating when the wire moves. The interaction is one of the reasons that the coating specification should include the assembly drawing. Our article on conformal coating inspection describes the defects that follow.
Design Choices That Reduce Risk
Thermal relief on a plane reduces the heat that is conducted away, which shortens the time the alloy stays liquid and therefore reduces the travel. A short barrel and a matched clearance do the same.
Where a wire must stay flexible, a mechanical crimp is a better joint than a solder joint, and the drawing should say so. Specifying solder where a crimp belongs is a decision that will be made again by every assembler.
Defects and Inspection
Alloy on a flexing wire, a stiffened strand area and a bridge along a row of pins are the visible results. The inspection should be done at the joint and along the conductor, because the defect is often outside the area that is normally examined.
A withdrawal force test or a bend test on a sample is more informative than a visual check alone, because the stiffened area may still look acceptable. Our article on joint acceptance criteria gives the framework for setting such limits.
Checks Before Release
A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made. Where the supplier and the user both measure the same property, they should agree on the method before the first delivery.
A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.
The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected. Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion.
The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.
Points to Confirm at First Article
Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to. Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts.
Verification and Records
The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.
Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.
FAQ
Is wicking always a defect? No. Wicking is how a through hole joint fills. It becomes a defect when the alloy travels beyond the area where the joint is meant to be, or when it stiffens a conductor that has to flex.
Does a higher soldering temperature increase wicking? It increases both the wetting rate and the time the alloy stays liquid, so it usually does. Reducing dwell and using a heat sink are more effective controls.
Can wicking be reversed? No. Alloy that has travelled into a stranded conductor cannot be removed without cutting the wire back, so the control has to be applied before and during soldering.
