Conveyor Angle in Wave Soldering: The Exit Decides

The conveyor of a wave soldering machine is inclined, and the angle is one of the few parameters that affects bridging, icicles, barrel fill and solder usage at the same time. The incline is set so that the board meets the wave at a controlled point and leaves it so that the solder drains away from the joints rather than pooling in them.

The angle is easy to set once and then forget, which is why it is often the last parameter checked when a defect appears. Its effect is systematic rather than random, so reading the defect pattern is usually enough to decide whether the angle has moved: an angle problem produces the same defect on every board and on both sides of the panel.

What the Angle Controls

The incline changes the geometry of the contact between the board and the wave. A steeper angle makes the board meet the wave over a shorter length at any instant and changes the effective contact time, and it also changes the direction in which the solder leaves the joint. The exit geometry is the one that matters most, because drainage decides the shape of the fillet and whether a bridge survives.

The angle also affects the preheat, because the board spends its time in the tunnel at an incline and the thermal profile is measured along that path. Changing the angle without re-profiling shifts the peak that the assembly sees, particularly on a long board where the effect accumulates across the length.

The Standard Range and Why

Most machines are set between 4 and 8 degrees of incline, with 5 to 7 degrees covering the majority of production. The lower end of the range suits boards with heavy thermal mass and large holes, where a longer contact is needed, and the upper end suits dense fine pitch assemblies where drainage and bridging control are more important.

The angle interacts with the wave height and the pump speed. A high wave with a shallow angle produces a long contact and a large amount of solder on the board, while the same height with a steep angle produces a shorter contact and less solder. The three parameters are therefore adjusted together, and a record of one without the others is not reproducible.

Drainage and Icicles

An icicle is a spike of solder left on the underside of a joint where the alloy froze before it drained. It forms when the exit geometry holds a column of liquid metal long enough for it to cool, and it is characteristic of a conveyor that is too flat or of a board that leaves the wave too slowly. Raising the incline improves the drainage and reduces the frequency of icicles.

The temperature of the pot and the thermal mass of the board also contribute. A joint that is already cooling as it leaves the wave freezes sooner, so a board with a large ground plane can produce icicles at an angle where a thinner board does not. The two parameters, angle and thermal profile, are set together, and the profile side is described in the notes on preheat and flux activation.

Bridging and the Exit

A bridge is a connection of solder between two adjacent joints, and it forms either when the wave fills the space between them or when the solder fails to separate as the board leaves. The exit component dominates on fine pitch parts: a steeper angle separates the wave from the board more abruptly and pulls the excess solder away from the gap.

Bridging that appears on one side of the board and not the other suggests that the conveyor is not level across its width, or that the wave is not uniform along its length. Both are mechanical checks rather than angle changes, and both are worth making before the incline is adjusted, because a corrective angle change will mask a wave that is uneven.

Contact Time and the Wave Geometry

Contact time is the length of the board that is in the wave divided by the conveyor speed, converted through the geometry of the incline. A steeper angle reduces the contact for the same wave height, and a higher wave increases it. A typical window for a through-hole board is three to five seconds of contact, and the notes on wave height and contact describe how those two parameters trade against each other.

Fill of a plated barrel depends on the contact time and on the hole geometry, because the solder has to travel up the barrel against the escaping gas. A barrel that is incompletely filled at a short contact time may fill correctly when either the contact is lengthened or the angle is reduced. The mechanism and its other drivers are described in the notes on barrel fill.

Pallets and the Effective Angle

A pallet changes the angle that the board actually sees. A pallet with a warped base, a bowed board clamped into it or a fixture that lifts one edge presents the assembly at an angle that is not the conveyor angle. The result is a defect pattern that varies across the panel and that resists every adjustment made to the machine.

The check is to measure the board surface rather than the conveyor frame, with the pallet loaded as it would be in production. A pallet that does not sit flat should be repaired or replaced rather than compensated for by tilting the machine, which would move every other product on the line. Pallet design and its effect on the process window is covered in the notes on wave solder pallets.

Setting and Verifying the Angle

The angle is set with the machine at production temperature, because the frame expands as it warms and the cold setting is not the running setting. It is measured on the surface that carries the board rather than on the frame, with an inclinometer placed in the same orientation as the panels. The value is recorded with the wave height and the conveyor speed.

The first verification is a run of production boards with a profile and a visual check at the exit. The second is a sample inspection for bridging, icicles and fill on the parts that are most sensitive to drainage. Where a machine has more than one product, the angle should be recorded per product rather than changed between them without a profile.

Interaction With Flux and Preheat

The angle changes how far the board is from the preheaters at any moment, which changes the heat the board receives before the wave. A steeper angle can bring the board closer to the heaters at the entry and further from them at the exit, and the effect is largest on a long panel. Any significant angle change should therefore be followed by a profile.

Flux behaviour is affected in the same way, because the flux has to be dry before the wave reaches it. A change in the angle that shortens the time under the preheaters leaves the flux wet, and the result is voiding and spitting rather than a drainage problem. That is why the angle is treated as a thermal parameter as well as a mechanical one, and why it belongs in the process record with the profile.

Process Control and Records

The controls around the angle are the measured value with the machine hot, the wave height, the conveyor speed, the pot temperature and the profile, recorded per product. The angle is checked after any maintenance on the conveyor and after any change of pallet, because both can move the surface that carries the board without changing the frame setting.

The defect data closes the loop. If icicles and bridging rise together, the drainage has changed and the angle is a candidate; if fill falls while drainage is unchanged, the contact time or the wave height has moved. Recording the two defect counts separately is what makes that distinction possible, and it is the practical use of the angle record.

FAQ

Does a steeper angle always reduce bridging? It improves the separation at the exit up to a point. Past that point the contact becomes short and the barrel fill suffers.

Can the angle be changed without re-profiling? A small change can be, and any change that alters the distance from the preheaters should be accompanied by a profile.

Why does one side of the board behave differently? The conveyor is probably not level across its width, or the wave is not uniform. Both are mechanical issues rather than angle settings.