Through-Hole Hole Fill in Wave Soldering: Causes of Skip
Hole fill is the one wave soldering requirement that cannot be judged from the top of the board. Alloy visible at the top of a barrel can hide a void running half its length, and a joint that looks sound on the underside may be barely connected to the plating. This article explains what the fill percentage means, which process variables decide it, and how to verify it so that the answer is a measurement rather than an impression.
What Hole Fill Means and Why It Is Specified
Hole fill is the proportion of the barrel height filled with alloy after wave soldering, measured from the top side of the board downwards. Class 2 work normally requires 75 percent, and Class 3 requires the same 75 percent together with a visible fillet on the top surface, which means the alloy has to wet through the barrel rather than simply bridge the bottom. The acceptance rules are set out in our acceptance criteria guide.
Fill is not the same as the presence of alloy at the top. A joint can show a wet fillet while a void runs down one side of the plating, and it is the void that fails thermally. Microsection is the only method that separates the two cases.
Flux Activity and Its Limits
Flux has to reduce the oxide on the barrel wall and on the lead, and its activity has to survive the preheat. A flux that has largely evaporated before the board reaches the wave leaves the barrel oxidised, and the alloy then bridges the bottom without climbing.
Preheat is set from the flux chemistry rather than from a general rule. A typical no-clean flux wants a top side preheat between 90 and 110 degrees Celsius, while a water-soluble chemistry tolerates more. Measuring the top side temperature with a contact probe rather than reading the heater setpoint is what makes the setting meaningful.
Flux deposition is uneven long before it becomes obvious. A spray pattern that looks uniform on a bare panel can still leave thin patches over the barrels, which is why deposition is measured rather than inspected.
Thermal Relief and the Barrel as a Heat Sink
A through-hole connected to an internal plane conducts heat away faster than the wave can supply it, so the alloy solidifies before it has climbed. Thermal relief spokes, usually four at 0.3 to 0.5 mm wide, cut that conduction while keeping the electrical connection.
Where a full plane connection is required, the relief is the compromise that keeps the joint solderable. A solidly connected barrel on heavy copper often needs a longer contact time or a higher preheat on that area, and the change is confirmed by section rather than by looking at the top side.
Solder Skip and Its Relationship to Flux
Solder skip is a joint that receives no alloy at all, and it usually follows a flux deposition problem rather than a wave problem. A spray fluxer with a blocked nozzle leaves a stripe of unfluxed board, and every joint inside that stripe skips in the same way.
The diagnosis is a deposition measurement: weigh a coupon before and after spraying, or spray onto paper and compare the pattern with the layout. If the skip pattern follows the spray pattern rather than the board features, the fault is in the fluxer and not in the wave.
Contact Time, Wave Height and Direction
Contact time on a wave is normally two to four seconds, and fill improves with time up to the point where the assembly is overheated. Wave height matters as well, because a taller wave touches more of the board, and the working depth is measured from the board surface to the crest rather than from the nozzle setting.
The direction of travel relative to the hole pattern changes the fill on a dense connector. Where the connector runs parallel to the wave, the first row of pins shields the rows behind it, and rotating the panel on the pallet is often more effective than lengthening the contact time. Yield effects of that rotation are visible in the first pass yield record.
Barrel Geometry and Aspect Ratio
The aspect ratio of a hole, its depth divided by its diameter, sets how hard it is for flux gas and alloy to move through it. Above about 5 to 1 the fill becomes sensitive to every other variable, and above 8 to 1 a standard wave process struggles whatever the settings. The plating side of this is described in our hole copper notes.
Finished hole size matters for the same reason, because plating reduces the hole and the lead occupies part of what remains. A hole that is tight against the lead restricts the gas path, and gas trapped beneath the board holds the alloy out of the barrel.
The clearance between lead and finished hole is a design decision that the assembly house cannot recover, so it belongs in the stack-up review alongside the drill schedule.
Outgassing, Voids and the Top Side Fillet
Moisture absorbed by the laminate is released as the board crosses the wave, and the vapour has to escape through the barrel. If the escape path is blocked by a tight lead or by fast solidification, the vapour forms a void running the length of the barrel and reduces the effective fill.
Baking removes much of that moisture, and the schedule should follow the laminate thickness and the supplier data. Baking also changes the solderability of some finishes, so it is confirmed on a sample before being applied to a full batch.
Verification by Microsection and X-ray
Fill percentage is verified by microsection through a sample of barrels from each production lot, taken after the wave and before any rework. The section is cut through the centre of the barrel, and the measurement is taken at the highest and lowest points of the fill.
X-ray gives a rapid check on dense connectors, but it shows projected alloy and cannot separate a filled barrel from one with a void behind the lead. The two methods answer different questions, so the drawing should state which one applies.
Writing the Requirement on the Drawing
The drawing should state the fill class, the barrels that are exempt, the thermal relief pattern on plane connections, and the lead to hole clearance the design allows. Without those notes the assembly house works to its own standard, and the difference is discovered at section rather than at design review.
Where a barrel sits on heavy copper, the drawing should identify it, because it needs a longer contact time or a different preheat and the process has to be prepared before the run rather than corrected after the first panels are inspected.
FAQ
Is 100 percent hole fill achievable? Occasionally, on a thin board with generous clearance, but it is not a specification a normal wave process can hold, which is why the classes are written at 75 percent.
Can a skipped joint be reworked? Yes, by hand soldering with the barrel preheated, but the reworked joint should be re-inspected by section because a hand-filled barrel rarely reaches the same depth.
Does baking always improve fill? It removes moisture, which helps, but it also changes the solderability of some finishes, so the effect should be confirmed on a sample before it is applied across a batch.
