Through Hole Solder Barrel Fill And Wicking

A through hole joint is judged by two things that are visible and one that is not. The visible ones are the fillet on the solder side and the fillet on the component side; the invisible one is how much of the barrel between them is filled with solder. The requirement for vertical fill is written into the acceptance standards and it is the criterion most often missed, because a joint with two perfect fillets can still have an empty barrel in the middle.

This article explains what barrel fill means, why it fails, how the soldering method affects it, and how the result is inspected and improved.

What Barrel Fill Means

The barrel is the plated wall of the hole, and the fill is the fraction of its length that contains solder rather than air. A joint that is filled from the solder side to the component side is 100 percent filled, which is what the standard requires for most classes of product, with a lower minimum allowed for some. The fill is not the same as the fillet, which is the visible curve of solder where the barrel meets the pad.

The reason the requirement exists is mechanical and thermal. The barrel carries the current between the layers and the heat away from the joint, and a partly filled barrel has less cross section for both. It also has a cavity that can trap flux, and that flux residue can later cause a leakage path or, in the presence of moisture, an electrochemical failure between the layers.

Why A Barrel Fails To Fill

The commonest cause is that the gas cannot escape. Solder entering a hole from the bottom pushes air and flux vapour ahead of it, and if the top is sealed by a component body, by a mask or by a tented via, the gas has nowhere to go and the column stops rising. That is why a joint on a pad with a tight annular ring and no vent path is more likely to be incomplete than one on an open pad.

The second cause is that the solder solidifies before it reaches the top. A thick board, a large copper plane attached to the pad and a short contact time all remove heat from the joint while the solder is still rising. The third is contamination or a poor plating in the barrel, which prevents wetting and leaves a surface that the solder will not climb along, however long it is heated.

How The Method Affects The Result

Wave soldering fills a hole from the bottom in a single pass, and the wicking of solder up the barrel depends on the wave height, the contact time, the hole diameter relative to the lead and the amount of copper attached to the pad. Selective soldering gives the same mechanism with a smaller nozzle, and the contact time is easier to control because the nozzle position is programmed rather than set by a conveyor.

Hand soldering fills a hole from the top or the bottom depending on how the iron is applied, and the result depends heavily on the operator. Feeding solder into the top of the hole while heating the bottom pad will fill the barrel; heating the top pad and feeding solder onto it will produce a fillet and an empty barrel, which is a common and avoidable defect on hand assembled boards.

How The Board Design Affects It

Design choices that are made for other reasons change the fill. A pad attached to a heavy plane conducts heat away and needs more contact time; a pad with a thin annular ring leaves less metal to be heated; and a hole that is much larger than the lead leaves a gap that the solder has to bridge before it can climb. The relationship between a hole, its plating and the pin that will pass through it is discussed under via in pad or plated through.

Thermal relief on a through hole pad is the classic trade: a solid connection solders less easily but fills better where the heat is available, while a relieved connection solders easily and may not fill. The alloy also matters, since a lead free solder with a higher melting point and different wetting behaviour behaves differently in a barrel of the same size, as described under lead free versus leaded solder. The rules that bring these together are collected under PCB design and fabrication.

Inspection And Acceptance

The fill is measured by X-ray on a sample, because it is the only method that can see inside the hole without cutting the board. A section provides a definitive answer and is used to settle a dispute or to qualify a process. Visual inspection finds the gross cases, such as a joint with a fillet on one side only or a visible depression at the top of the barrel.

The acceptance criterion is usually expressed as a percentage of the board thickness, with a lower figure for a class 2 product than for a class 3, and with exceptions for a small number of joints. The criterion and the measurement method should be stated on the drawing, because a requirement that is not stated cannot be inspected and a requirement without a method cannot be measured consistently.

Process Fixes

Where the fill is consistently low, the first change is usually the thermal one: more preheat, a longer contact time or a higher wave. The second is the venting, and it is often the more effective, because a hole that cannot breathe will not fill no matter how hot it is. Leaving a small opening at the top of the hole, using a mask opening that is larger than the pad, or avoiding a tented via beside the joint all give the gas somewhere to go.

When the problem is wetting, the answer is usually in the plating or in the storage of the boards rather than in the solder. A barrel with a thin or contaminated plating, or one that has oxidised during a long storage, will not wet properly, and the fix belongs upstream. Measuring the plating thickness on a coupon and reviewing the storage conditions is normally enough to identify which of the two is responsible.

Venting, Mask And Component Bodies

A hole fills from the bottom upwards because the solder is pushed into it by the wave, and anything that stops the gas leaving the top stops the column. A mask opening that is the same size as the pad, a via tented on the component side next to the joint, and a plastic connector body sitting flat over the pad all act as a lid. Leaving a deliberate opening, or moving the vent to a point where the gas can escape, is often a bigger improvement than any change to the solder pot.

The component body contributes in a second way, because a large plastic part absorbs heat from the pad and from the lead. A connector with a wide body and many pins acts as a heat sink for every joint along its length, and the joints at the ends of the row are usually the ones that show the lowest fill. Preheating the assembly and lengthening the contact time address both effects at once, and neither requires a change to the board.

FAQ

Is a 75 percent fill acceptable? It is acceptable for some classes of product and for some joints, and the figure comes from the applicable standard or from the customer drawing rather than from a preference. The class should be stated on the drawing so the shop knows which figure applies.

Can a low fill be repaired? A joint can be reheated and topped up from the component side, and this is a normal rework operation. It works because the rework provides both a heat path and a vent that the original process may not have had.

Does a large hole always fill worse than a small one? Not necessarily, but a large hole has more volume to fill and more gas to displace, and the lead must be centred for the solder to bridge the gap, so it is less forgiving than a hole that closely matches its pin.