Wave Soldering: Wave Solder Defect Troubleshooting
The wave process solders through hole joints by passing the board across a standing wave of molten alloy. It is fast and robust, and it has a narrow window: the flux must be applied correctly, the board must arrive at the right temperature, and the wave must contact each joint for the right time. When those conditions slip, the defects are visible and often clustered, which makes them easy to recognise and sometimes harder to attribute.
How the Wave Process Works
The board moves through flux application, preheat and then two waves in most machines. The first is turbulent and drives alloy into the barrels and up onto the topside; the second is smooth and shapes the fillet. The exit from the wave is as important as the entry, because the alloy has to drain from the joints before it freezes.
Every defect can be related to one of four things: what the flux did, how hot the board was, how the board met the wave, or how it left it. Framing the search that way prevents the common mistake of adjusting wave height for a problem that is really a flux or thermal issue.
Flux Application and Preheating
Flux must reach the underside of the board in the right quantity, and a spray fluxer with a clogged nozzle leaves dry patches that will not wet. Foam fluxers have the same failure mode in a different form, with foam density varying as the bath ages. Both are checked by flux deposition measurement rather than by looking at the board.
Preheat drives the solvent out and brings the board close to the soldering temperature. Too little and the wave has to do all the heating, which produces incomplete fillets; too much and the flux burns off before the wave arrives, leaving nothing to clean the surfaces. Our guidance on surface finishes is relevant here, because a finish that wets poorly shifts the whole window.
Bridging Between Adjacent Leads
Solder bridging is alloy that remains between two adjacent conductors after the board leaves the wave. It is caused by insufficient drainage, which comes from too much alloy on the joint, a wave that is too high, a conveyor speed that is too fast for the alloy to leave, or a flux that has stopped working. Excess alloy is the most common single cause.
Direction is another factor. Bridging tends to be worst where traces run parallel to the wave direction, because the alloy is drawn along the gap rather than drained away from it. Rotating connectors or changing the conveyor direction removes a whole class of the defect, and palliatives such as an angled wave or a hot air knife only treat the symptom.
Shadowing and Solder Skip
Shadowing occurs where a tall component or a wave pallet prevents the alloy from reaching the joints behind it. The wave is a moving fluid, so anything upstream casts a wake in which the joints see little contact time. Solder skip is the visible result: joints that are unfilled or partially filled while the rest of the board is good.
The fixes are geometric. Reorient the offending component so that its long axis is parallel to the wave direction, increase the wave height slightly, reduce conveyor speed to give the alloy time to flow, or change the pallet opening. Spacing the parts further apart helps, and the pallet design discussed below is often the deciding factor.
Icicles and Webbing
Icicles form when alloy continues to flow from a joint after it should have started freezing, which points to too much heat, a wave that is too hot, or an exit that holds the joint in contact too long. Web formation across adjacent leads is a variant that appears where flux residue has left a tacky bridge for alloy to follow.
Both are addressed by reducing alloy volume and improving drainage, by confirming that the flux is not leaving sticky residue on the exit side, and by checking that the conveyor is level so that the board separates from the wave uniformly across its width.
Dross and Alloy Condition
Dross is the oxide layer that forms on the surface of molten alloy. Some is unavoidable, but an excess indicates a wave that is splashing, a temperature that is too high or an alloy that is contaminated. Dross carried into a joint appears as rough, dull or pitted solder, and an alloy that has drifted out of specification changes the melting behaviour of every joint.
Alloy condition is a maintenance subject rather than a defect to be tuned away, and the tests and sampling intervals are covered in our article on plating thickness control. When defects appear across the whole board with no geometric pattern, alloy condition is a reasonable first suspicion.
Pallet and Fixture Effects
A wave pallet masks the areas that must not see alloy and holds the board flat, and it changes the thermal and fluid behaviour everywhere it touches. A pallet that is too thick holds heat and can cause icicles; one with openings that are too small causes shadowing; and one that warps prevents contact on part of the board.
Board flatness has the same effect. A board that bows during preheat will meet the wave unevenly, producing a defect pattern that follows the bow rather than the layout, and the measurement and correction methods are described in our article on PCB warpage control.
A Structured Troubleshooting Order
Work through four questions in order. Is the flux reaching the board and still active at the wave? Is the board at the correct temperature at the wave? Does the layout and pallet let the alloy reach and leave every joint? Is the alloy itself in specification? Answering them in that sequence resolves most defects without random adjustment.
Where the defect persists, the more general case library in our article on solder defects and board failures is a useful reference, because many wave defects are the same physical phenomena seen in a different process. Record the settings that produced a good result, so that a later change can be reversed without guesswork.
FAQ
Why did bridging appear after a design change? Because the change moved traces parallel to the wave direction or reduced the spacing between them. Check the orientation before adjusting the machine.
Does a hotter wave fix skip defects? Sometimes, but it also increases icicles and dross. The more common fix is a longer contact time or a layout change that removes the shadow.
Can nitrogen improve wave soldering? It reduces dross and improves wetting on marginal finishes, which helps with skip. It does not correct shadowing, which is a fluid and geometric problem.
