Wave Solder Chip Wave Setup: Turbulence, Bridging and Thermal Load

The chip wave, also called the turbulent wave, is the small wave that runs ahead of the main wave to wet the surface and drive flux gases away from the joints. Its settings decide whether the board bridges on fine pitch parts, and they interact with the main wave far more than most setup sheets acknowledge.

What the Chip Wave Is For

The chip wave solves two problems at once. It applies solder to the board while the board is still moving fast relative to the wave, which produces a scrubbing action that helps the solder wet the pad and the lead. It also lifts and pushes the flux gases away, so that the main wave meets a surface that is already wetted rather than one that is still outgassing.

That second function is the one that matters most on a mixed assembly with surface mount parts underneath. A board without a chip wave relies on the main wave alone, and the gases released under a component have to escape through the joint, which produces voids and incomplete fill. The chip wave gives them a path before the main wave arrives. The chip wave is not a smaller version of the main wave and should not be set as one. It is usually a narrow, high velocity jet with a turbulent surface, and its height, shape and stability are separate variables from the main wave.

Wave Height, Contact Depth and Pump Speed

Chip wave height is set so that the wave contacts the board across its full width with enough interference to wet the whole underside but not enough to flood the top side. A typical setting is 0.5 to 1.5 mm of interference, measured with the board in place rather than with the pump running free. The pump speed controls the height and, at the same time, the roughness of the wave surface.

Contact depth interacts with conveyor speed. Where the conveyor runs fast, the contact time is short and the wave has to be set a little higher to deliver the same wetting; where the conveyor runs slowly, the same height produces excessive contact and can push solder onto the top side through a via. Set the conveyor speed first and then the height, in that order. The chip wave and the main wave have to be positioned so that the board passes from one to the other without losing contact. A gap between them allows the board to cool slightly at the joint, which increases the chance of a cold joint on a heavy thermal pad. Set the distance between the two waves from the conveyor speed, and record it as part of the setup.

Turbulence and Its Effect on Fill

Turbulence is a feature rather than a fault in the chip wave, but too much of it produces defects. A very rough wave entrains air into the solder and throws small amounts of it onto the board surface, which appears as solder balls after cooling. It also produces an unstable contact area, so the wetting varies from board to board.

Judge turbulence by the wave surface, which should be a fine, even chop rather than a boiling, spattering surface. Where solder is being thrown above the board plane, the pump speed is too high for the nozzle or the nozzle is partly blocked, and the blockage is usually the cause rather than the setting. The nozzle condition decides turbulence as much as the pump does. A nozzle with a worn or enlarged slot produces a slow, broad wave with little chopping action, so the operator compensates with more pump speed, which in turn increases the splashing. Inspect the nozzle slot width and straightness on a schedule, and replace rather than adjust when it is worn.

Bridging: Causes and Corrections

Bridging between adjacent pins is the classic chip wave defect and it has several distinct causes. The first is excessive exit angle between the board and the solder: if the board lifts out of the main wave at too shallow an angle, the solder does not drain away from the last pins. Increasing the conveyor exit angle or reducing the wave height fixes it.

The second cause is insufficient flux, or flux that has been consumed before the board reaches the wave. The chip wave relies on active flux at the surface, and where the preheat is too hot or the conveyor too slow, the flux is spent before contact. Check the flux specific gravity and the preheat temperature before adjusting the waves. The third cause is thermal: a heavy copper plane or a large ground pad conducts heat away from the pins, and the solder freezes before it can drain. Raising the preheat or slowing the conveyor for that product is the standard response, but the more durable fix is a pallet design that reduces the thermal mass, following the same pallet design principles used for shadowing control.

Thermal Load and Preheat Interaction

The chip wave adds heat to the board before the main wave arrives, and that additional heat has to be accounted for in the preheat setting. A board that is correctly preheated for a single wave will be hotter than intended when a chip wave is added, and the excess heat can activate the flux too early. Measure the board temperature at the point of contact rather than at the end of preheat.

The thermal load also depends on the board itself. A heavy copper plane near the edge of the board pulls heat away from the joints and produces a lower joint temperature for the same wave setting. Where the same wave settings are used for two products of different copper distribution, one of them will be outside the process window, and the profile should be verified per product rather than per machine. Record the measured board temperature at contact for each product, together with the wave settings. That single number explains more bridge and cold joint problems than a full parameter list, because it is the quantity the solder actually responds to.

Solder Pot Condition and Alloy Effects

The solder pot condition changes the wave. Oxide and dross on the surface reduce the wave height for a given pump speed and introduce intermittent contact, and copper contamination from the boards raises the liquidus so that the wave freezes sooner after contact. Both effects appear as defects under the chip wave before they appear under the main wave, because the chip wave is thinner and cools faster.

Check the pot level before each shift and skim the surface, and take periodic samples for a solder pot analysis as described in our solder pot maintenance guidance. A pot that is running at the copper limit will produce grainy joints and a dull wave surface, and neither is corrected by changing the wave settings. The alloy itself matters for the same reason. A lead free alloy runs hotter and has a higher surface tension, so the wave drains less readily and the bridging tendency is greater. Settings developed on a tin lead process cannot be transferred to a lead free process without re validating the wave heights and the conveyor angles.

Flux Application and the Chip Wave

The chip wave depends on active flux at the board surface at the moment of contact. Where flux is applied by spray, the deposit has to be uniform across the board, because a dry patch produces a spot where the chip wave cannot wet and the main wave then bridges. Verify the spray pattern by running a test board with flux sensitive paper before adjusting the waves.

The flux chemistry matters too. A no clean flux with a low solids content may have consumed its activator by the time the board reaches the chip wave, particularly where the preheat is on the hot side. Where bridging appears on a product after a flux change, check the flux activation window against the measured board temperature before touching the wave settings. Foam fluxers behave differently again, because the foam density sets the deposit. A foam that has become coarse applies an uneven film, and the resulting wetting variation looks exactly like a wave problem.

Setting Up a New Product on the Wave

Set a new product up in a fixed order rather than adjusting several parameters at once. Start from the conveyor speed, set the preheat to give the measured board temperature the flux requires, then set the chip wave height, then the main wave height and the exit angle. Change one parameter at a time and inspect the result on a coupon or a scrap board.

The inspection should cover both the joints and the top side. Solder thrown onto the top surface, flux residues left in place and solder balls around the wave exit are all evidence that the settings are outside the window, even where the joints look acceptable. Grade the joints against the solder joint acceptance criteria used for the product rather than by eye. Where a pallet is used, set it up before the wave settings, because the pallet changes the thermal load and the contact geometry. A pallet that has been added after the waves were set will shift the whole process.

Records and Drift Detection

Record the conveyor speed, the preheat zones and the measured board temperature, the chip wave and main wave heights and pump speeds, the nozzle identification, the exit angle and the pot analysis result with the run. When a defect appears part way through a shift, the record shows which parameter moved.

Look for drift rather than for failure. A wave height that has to be increased to hold the same result indicates a worn nozzle; a preheat that has to be raised indicates a changing thermal load or a pot that is loading with oxide. Trend both against the run count so that the maintenance is scheduled from the trend and not from a batch of rejected boards.

FAQ

What is the chip wave for in wave soldering? It wets the board and drives flux gases away before the main wave arrives, which reduces voids and incomplete fill under surface mount components. It also gives a scrubbing contact that helps wetting, and its settings are separate from those of the main wave.

Why does bridging happen on fine pitch pins? Most often because the board leaves the solder at too shallow an exit angle so the solder cannot drain, or because the flux has been consumed before contact. Check the exit angle, the preheat temperature and the flux specific gravity before changing the wave heights.

How high should the chip wave be set? Enough interference to wet the whole underside, typically 0.5 to 1.5 mm with the board in place, and not so much that solder is thrown onto the top surface. Set the conveyor speed first, then the height, and record the measured board temperature at contact.