Wave Solder Pallet Design And Use

A wave soldering process brings the board into contact with a standing wave of molten solder. For a board that carries surface mount parts on one side and through hole parts on the other, the fixture that carries the board is what makes the process possible, because it protects the side that must not be soldered and holds the board flat as it crosses the wave.

This article looks at how that fixture, usually called a pallet, is designed and what its material and geometry do to the soldering result.

Why A Pallet Is Used

The first function is protection. A board that has already been reflowed carries components and paste on the top side, and those parts must not come into contact with the solder. The pallet covers the top side while leaving openings that expose the joints to be soldered on the underside, so the solder reaches only the intended areas.

The second function is mechanical. A thin board sags when it is heated, and a board that is not flat does not present a uniform surface to the wave. The pallet supports the board and holds it against the conveyor, which keeps the contact consistent along the length of the panel. The third function is to carry identification, because the pallet can be labelled with the board part number and a bar code that the machine reads to select the correct programme.

Material And Thermal Mass

The pallet is made from a material that withstands the soldering temperature repeatedly without deforming, and that does not absorb solder or flux. A composite such as a glass reinforced resin is common, and the material is selected for its temperature rating, its dimensional stability and its low thermal conductivity.

The thermal mass of the pallet matters because it draws heat from the board. A thick pallet acts as a heat sink and makes the joints harder to bring to temperature, so the fixture should be no thicker than the mechanical requirement demands. The pallet also insulates the board from the preheat, so the preheat profile has to be set with the pallet in place rather than on a bare board.

Masking And Openings

The openings in the pallet define which joints see the solder. They are machined or moulded to match the through hole pattern, and their edges should be chamfered so that the solder does not accumulate on a sharp lip and transfer to the wrong place. The opening also has to be large enough for the solder to reach the joint with a proper fillet, and small enough to keep the solder away from the neighbouring surface mount area.

The mask that covers the rest of the board has to seal against the surface without pressing on the components. A mask that presses on a tall component can damage it or lift it, and a mask that does not seal leaves a gap through which solder can travel. The components that stand tallest therefore determine how the pallet is relieved, and the relief has to be designed from the component list rather than from the outline.

Solder Shadow And Flow

Solder shadowing is the effect of a pallet edge on the flow of solder behind it. An opening whose edge is close to the direction of travel creates a region where the solder is shielded, and a joint in that region receives less solder or none at all. The orientation of the board in the pallet therefore matters, and where a shadow cannot be avoided, the opening is enlarged or the travel direction is changed.

The flow is also affected by the depth of the opening. A deep pocket allows the solder to fill and remain in contact for longer, which improves the fill of a heavy joint, while a shallow opening contacts the board for a shorter time. The pocket depth and the conveyor speed therefore work together to determine the contact time, and both should be part of the process development.

Fixture Accuracy And Board Support

The board has to be positioned in the pallet repeatably, because the openings are fixed and the joints have to line up with them. The board is located on pins that pass through tooling holes, and those holes have to be present in the design. A board that is loaded with a small offset will expose part of the surface mount area, which is a defect that appears without any change to the process parameters.

The support pins that hold the board flat should be placed under areas of the board that carry no components on the underside and that are stiff enough to transmit the load. A pin under a large opening supports nothing, and a pin that presses on a component damages it. The arrangement is part of the pallet design, and it is confirmed on a sample board before the pallet is released. The tooling holes and the space they occupy belong to the considerations described for board outline and mounting design.

Maintenance And Life

A pallet wears in three ways. The mask surface erodes where it contacts the board, the openings accumulate flux and solder, and the locating pins wear and lose their fit. Each of those produces a loss of accuracy that appears as a variation in the soldering result rather than as a sudden failure.

The maintenance programme should therefore include a dimensional check of the locating features and a cleaning of the openings on a schedule. A pallet that is cleaned only when the solder result degrades is usually found to have been out of tolerance for some time, and the boards produced in that period are suspect. The soldering conditions themselves are covered in the comparison of leaded and lead free processes, and the place of the fixture in the assembly sequence is described in the development process.

Additional Considerations for This Build

Practical attention to selective masking pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating selective masking explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, selective masking is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

FAQ

Can a pallet be used for more than one board design? Sometimes, if the openings and the locator pins are compatible. In practice a pallet is usually dedicated to one design, because the openings are matched to the joint pattern.

Why is the preheat set with the pallet in place? Because the pallet insulates the board and adds thermal mass. A profile developed on a bare board will under heat the board once it is loaded into the fixture.

What causes solder shadowing? An edge of the pallet that shields the solder flow from a joint behind it. The fix is to change the orientation of the board or to enlarge the opening.