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It is a reliable technique conducive to high volumes and short cycle times.

Among the many reasons we are seeing increased use of pallets in wave soldering is miniaturization. While even more SMT is introduced into board designs, through-hole components remain situated in-between. For mixed technology boards, selective soldering is the best option. However, not everyone has the capital to invest in new machines or the volume of such assemblies to justify a switch to selective soldering. Manual soldering is also ruled out for several industries such as automotive.

Wave soldering with pallets that cover SMD components is a good alternative. It is a reliable technique and supports high volumes; additionally, cycle times are short.

Pb-free processing raises soldering temperatures; consequently, boards will exhibit a tendency to warp during soldering. A pallet provides maximum support to the board during soldering and can prevent warpage.

Also, many odd-shaped boards appear for all manner of applications in automotive and consumer electronics. Sometimes it is very hard to transport these oddly-configured assemblies by chain or belt conveyor. Pallets, conversely, make it possible to transport and safely process all sorts of assembly shapes and configurations (Figure 1). Pallets also enable selective soldering in a wave solder machine by protecting wave-side components.

Due to the thickness of most pallets (sometimes up to 15 mm), no flooding of solder will occur on the top side of the assembly. The oxides that might be present on the liquidus solder just before the pallet arrives will be washed off by the leading edge of the pallet and the wave will be clean when soldering begins.

Stiffener rods can be added to carriers to provide increased rigidity during high-stress processing. Heat sinks, component holders and other dedicated tools can be mounted on the top side.

Using pallets helps to standardize product line widths and to solder different assemblies on one manufacturing line using bar codes or other identification devices to switch recipes on the fly.

Pallet materials. For a maximum lifecycle, pallets should be constructed of material unaffected by the high process temperatures (and overall tougher processing conditions) of wave soldering, particularly with respect to Pb-free solders.

The materials should have the following characteristics:

• Good dimensional stability.

• Good thermal shock resistance.

• Remain flat after multiple cycles.

• Good chemical resistance (flux and cleaning agents).

• Should not absorb moisture.

High-quality pallets will be able to withstand over 3,000 soldering cycles.

Process challenges. The flux application system should be able to supply the flux to the entire assembly. A poor pallet design may result in "shadowing" effects during fluxing. Some areas may have an insufficient amount of flux or no flux at all. The flux must be sprayed on the board and flow into the barrels by its capillary force.

Before the pallet enters the liquidus solder, it must be preheated using an adequate preheating unit. Typical preheat system configuration for pallets is a combination of calrod and forced convection heaters. If temperatures drop before touching the wave(s), selective pallets can generate a heat sink effect, sending the process out of control.

Pallets require a wave height up to 0.5". When using pallets for Pb-free wave soldering, the support of a turbulent "smart wave" is recommended to facilitate through-hole penetration. Nitrogen is recommended to reduce or minimize dross formation at high pump speeds.

Pay special attention to ensure that the board is flat mounted in the pallet. If gaps exist between board and pallet, flux will flow between them and solder will follow on the wave. This will result in unwanted solder residue on the board material (Figure 2).

Design recommendations. Avoid large components in the neighborhood of through-hole components as this may result in shadowing and makes good through-hole filling difficult.

Leave adequate space around the leading and trailing areas of through-hole components to make solder flow-off possible. Solder channels promote better solder flow. The channels will guide the solder to the pallet openings.

Design the openings in the pallet to be as large as possible to facilitate solder flow (Figure 3). This will eliminate defects such as bridging and solder balling. Through-hole penetration will also benefit from a larger opening since more energy will get into the soldering area.

Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV (vitronics-soltec.com); gdiepstraten@nl.vitronics-soltec.com. His column appears monthly.

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