Wave Solder Pallet: Practical Checks for Production
A wave solder pallet is a carrier that holds the board through the wave soldering machine. It locates the board, masks the areas that must not be soldered and shields the side that has already been reflowed. It is a tool rather than a part of the product, and it is designed once per assembly, so a pallet that is wrong is a pallet that makes every board wrong.
What a Pallet Does
The first job is to carry the board. The pallet presents the board to the conveyor, the fluxer and the wave at a defined position, so that the process is repeatable from board to board.
The second job is masking. Any area that must not see solder is covered by the pallet, which is faster and cleaner than applying tape to every board.
The third job is shielding. A board with surface mount parts on the underside cannot be immersed in a wave, so the pallet covers those parts and lets the solder reach only the through hole joints.
The fourth job is support. The pallet holds the board flat, which matters for a thin board that would otherwise flex in the machine.
Material and Construction
The common materials are a composite such as a glass filled resin and a machined aluminium alloy. The composite is lighter and does not conduct heat as fast, while aluminium is stronger and more dimensionally stable.
The material has to survive thousands of cycles at soldering temperature without warping. A pallet that warps changes the position of the board and the depth of the mask, and the process drifts.
The openings are machined or moulded, and the edges have to be deburred so that they do not damage the board or the mask. A sharp edge is also a place where solder collects and later drops onto a board.
The pallet thickness is chosen so that the board sits at the right height for the wave, and the opening margins should follow the pad standards that the rest of the board uses.
A pallet also has to tolerate the flux and the cleaning chemistry, since a material that absorbs flux will slowly change its dimensions and its surface energy.
The pallet thickness is chosen so that the board sits at the right height for the wave. A board that sits too low is flooded and one that sits too high is starved.
Masking and Openings
The openings define the areas that will be soldered, and they are the most carefully made feature of the pallet. Each opening should expose the joint and the pad with a margin, and no more.
An opening that is too small shadows the joint and leaves it unfilled. An opening that is too large lets solder reach a part that was meant to be protected.
The mask has to clear the components on the shielded side by a small gap, because a pallet that presses on a part transfers the clamping force into the solder joints.
A pallet with too many separate openings is also expensive to make, so the layout should group the joints that need exposure. Grouping the selected pins at one edge of the board makes a simpler and cheaper pallet.
Where a connector has to be protected but its pins soldered, the opening is a slot around the pins rather than a window over the whole connector.
Thermal Mass and Profile
The pallet adds thermal mass and reduces the heat that reaches the board, so the profile measured on a bare board is not the profile the product sees.
The effect is largest for a thick aluminium pallet and smallest for a thin composite one. The choice of material is therefore a process decision as well as a mechanical one.
The profile should be measured with the production pallet, at the position where the product runs, and it should be recorded with the pallet rather than with the machine.
A pallet that is used for two products which have different thermal masses will need two profiles, and the two should be documented separately.
Shadowing and Fixture Fingers
A finger that crosses a row of pins blocks the solder in the same way as a tall component. The fingers should be placed between the rows of joints rather than across them.
Shadowing also comes from the pallet walls. A deep opening with steep sides can shelter the joint from the wave, so the opening should be shaped to let the solder flow in and out.
The direction of travel matters, since the shadow falls downstream of the obstruction. The pallet should be designed with the machine direction in mind, and it should not be reversible.
A pallet that can be loaded the wrong way round will be loaded the wrong way round, so a keying feature on the pallet and on the conveyor is worth the effort.
Handling, Wear and Maintenance
The pallet is handled by operators and by the conveyor, so it should have handles or clearance for the machine, and it should be light enough to lift comfortably.
The pallet should also be checked after any change to the board, because a layout revision that moves a connector can make an existing opening useless. A pallet that is reused without a review is a pallet that will produce a defect on the first run.
The openings wear as the boards are loaded and unloaded, and a worn opening becomes oversized and lets solder reach the protected area. The wear should be checked at a defined interval.
The pallet also accumulates flux and solder on the masked surfaces, and a build up changes the clearance to the board. Cleaning should be part of the routine rather than a reaction to a defect.
The condition of the pallet should be recorded as part of the quality record for the assembly, since a defect that repeats at one position on the board is usually a defect in the tool.
The pallet should be identified by a part number, so that a defect can be traced to the tool that produced it.
Practical Rules
Measure the profile with the production pallet in place, because the pallet changes the heat that the board sees.
Design the openings for the joint rather than for the connector, and keep the fingers between the rows rather than across them.
Make the pallet keyed so that it cannot be loaded the wrong way round, and mark the direction of travel on it.
Record the pallet number and the profile with the assembly documentation, and check the openings for wear at a defined interval. The solder defect guide lists the faults that follow a worn opening.
FAQ
Why does the profile change with a pallet? Because the pallet adds thermal mass and shields part of the board, so the heat that reaches the joints is lower than on a bare board.
What material should a pallet be? A composite is lighter and cooler, while aluminium is stronger and more stable. The choice depends on the number of cycles and on the thermal requirement.
How often should a pallet be replaced? When the openings wear beyond their tolerance or when the pallet warps. Both should be checked at a defined interval rather than when a defect appears.
