Solder Wave Height and Contact Time Control

Wave height is the depth of solder that stands above the nozzle lip as the board passes over it. Set correctly, the wave touches the underside of the board across its full width and the board floats on the crest with a small, controlled contact depth. Set too low, contact is intermittent and holes do not fill; set too high, the board is lifted, solder floods the top side, and the wave becomes unstable. Wave height is the single setting most often adjusted by feel and least often measured.

Why Wave Height Matters

The height determines how deeply the board is immersed in the crest and therefore how long any point on the board is in contact with liquid solder. Contact time is the product of the contact length and the conveyor speed, so the height and the speed together decide the thermal and mechanical exposure of every joint.

It also decides whether the wave is stable. A solder wave that stands far above the nozzle has a long free surface that is easily disturbed by the board, by the conveyor and by the pump, and the disturbance is transferred to the joints as turbulence and as dross being carried onto the board.

Contact Time and Wave Geometry

Contact time in a wave soldering machine is typically between two and four seconds for a board of moderate thickness. Longer contact fills holes and heats heavy joints better but increases the thermal load on the components and the dissolution of copper into the solder.

The contact length depends on the nozzle design and on the board. A wide nozzle with a gentle crest gives a longer contact length than a narrow, steep one, so the same height produces different contact times on different machines. The figure that should be recorded is the measured contact time for the product rather than the nozzle setting.

Conveyor Angle and Board Exit

The conveyor runs at a slight angle, typically between five and eight degrees, so that the board leaves the wave at the back edge and solder drains away from the joints rather than collecting on them. The angle interacts with the height: increasing the angle reduces the effective contact depth for the same nozzle setting.

Drainage is what the angle is for. If the board leaves the wave too flat, solder is drawn along the underside and forms bridges between adjacent joints and icicles at the trailing edge. If the angle is too steep, contact becomes uneven across the width and the leading edge of the board may not touch the wave at all.

Dross Formation and Wave Stability

Dross is the oxide skin that forms on the surface of molten solder, and it is produced continuously. It is also carried away by the wave, so a stable wave with a smooth, fast-moving surface produces less accumulation on the nozzle than a slow or turbulent one.

Excessive wave height increases the free surface available for oxidation and gives the dross more places to collect. Where dross is being deposited on the board, the height is one of the first settings to check, along with the nozzle condition and the solder level in the pot.

Nozzle Condition and Pump Settings

The nozzle determines the shape of the crest and the flow across its width. Erosion of the lip, a chip in the weir or a partially blocked slot all produce an uneven wave, so part of the board gets more contact than the rest. The symptom is a defect pattern that follows the board position rather than the joint type.

The pump setting controls the flow that produces the height. Increasing the pump speed raises the wave, but it also increases the turbulence and the rate at which the solder oxidises. Where the height has to be increased to compensate for a worn nozzle, the nozzle should be replaced instead.

Wave Height and Hole Fill

Hole fill depends on the contact time, the board temperature at the wave and the barrel geometry, and wave height enters only through the contact time it produces. Increasing the height to fill a hole that is not filling is a common mistake, because the real cause is usually insufficient preheat or a hole whose solderability has been lost.

The diagnostic is to look at the fill on a sectioned coupon against the contact time. Where fill is complete on thin boards and incomplete on thick ones at the same setting, the thermal budget is the problem rather than the wave height.

Bridge Defects and Wave Dynamics

Bridges form during the exit from the wave, when the solder film between two conductors is thick enough to remain connected as it solidifies. A wave that is too high, a conveyor angle that is too shallow and a separation that is too slow all make the film thicker and hold it in place longer.

The dynamics of the wave itself matter as well. A wave that has a fast-flowing surface behind the contact point promotes drainage, while a still or turbulent one leaves solder behind. Where bridging appears on one part of the board and not another, the nozzle profile and the flux webbing pattern should be examined before the solder is blamed. Solder spitting at the entrance of the wave is a related symptom, and the controls for flux spitting in wave soldering overlap with the settings that control height and drainage.

Setting and Verifying Wave Height

The height is set with a gauge block or with a fixed probe that is placed on the conveyor and passed over the wave, and the setting is confirmed by the contact pattern on the underside of a test board. A board with a faint, continuous band of solder across its width and no flooding on the top side is the practical evidence that the setting is correct.

The verification should be part of the daily check rather than a setting left alone for months. Solder level in the pot, pump wear and nozzle erosion all change the height over time, and each of them changes it in a different direction, so the gauge reading is the only way to know where the process actually is.

Records and Daily Checks

The record should carry the measured wave height, the conveyor speed and angle, the solder pot temperature and level, and the pump setting. Those five values describe the wave well enough for another shift to reproduce it, and they are what makes a defect pattern interpretable.

Where the board requires cleaning after wave soldering, the amount of residue left on the underside depends on how much flux was carried through the wave, which in turn depends on the contact time. The cleaning load and the wave setting are therefore linked, and a change in one should prompt a check of the other.

FAQ

What contact time should wave soldering use? Two to four seconds for a board of moderate thickness is typical. Thick multilayer boards with heavy copper need the longer end of that range, and thin boards with heat-sensitive parts need the shorter end.

Why does raising the wave height not fix an unfilled hole? Because hole fill is limited by heat, not by solder supply. An unfilled hole usually means insufficient preheat or a barrel whose solderability has been lost, and raising the wave adds defects elsewhere without filling the hole.

How is wave height measured in production? With a gauge block or probe carried on the conveyor and passed over the wave, confirmed by the contact pattern on a test board. It should be checked daily, because the level in the pot and nozzle wear both change it.