Wave soldering lead-free solder temperature

In solder wave applications, there are several ways to apply a flux to a PCB, the method selected depends on the flux system utihzed. A foam fluxer (Figure 43) can be used for rosin-based or noclean alcohol fluxes that meet the lead-free process temperature requirements. A foam fluxer is... 

Solder Float Resistance. This test addresses the thermal resistance of the laminate material floating on the solder bath. Becanse this method subjects the sample to a thermal gradient across the z-axis of the material similar to an actual wave solder operation, the resnlts of this test are particnlarly important and—as mentioned previously—either solder pot temperatures or exposure times should be increased if the laminates are intended for use in lead-free assembly processes. 

The impact of Pb-free technology on wave soldering has largely occurred in the equipment performance. It has been determined that the same solder bath temperatures that are used for Sn-Pb processes (250 to 270°C) are suitable for the Sn-Ag-XCu Pb-free alloys. Therefore, excessive dross formation and flux residue removal have not become a significant problem during equipment operation. The lack of shiny fillets with the Sn-Ag-XCu alloys has been addressed by modified alloys having Ni and Ge additions that alter the solidification process, which leads to shinier fillet surfaces. 

When adjusted properly, the board meets the crest of the wave and disrupts the oxide skin. In doing so, the fluxed components and board are immersed in the flowing, oxide-free molten solder. If all steps are carried out properly, the solder alloys with the fluxed, oxide-free component leads, component pads, and PTH barrels. Upon exiting the wave solder machine, the assembly cools below the solder liquidus temperature and solder joints are formed. The more the system is used to solder boards, the faster the contamination and dross build-up. 

For wave soldering, since the solder pot temperature is typically already quite high for the tin-lead solder, the increase needed for the lead-free solder is generally less as compared with reflow, and the dwell time with the solder pot is also generally very short (a few seconds). 

The preheat temperature for lead-free wave soldering is not very different than that used with tin/lead soldering. The typical top-side temperature for lead-free wave soldering ranges from 180° to 225°C, depending on the flux in use. 

The wave-solder-pot temperature for lead-free soldering should range from 265° to 270°C. The specific temperature to use depends on board layout and if pallets are used, and should be adjusted to maximize wetting. The dwell time in the wave should... 

FIG. 11 Comparison of wave soldering profile for eutectic Sn-Pb and lead-free solders. The main differences are increased preheat and soldering temperatures. (From Ref 53.)... 

SMD-component, lead-free wave soldering is that components experience a solder immersion temperature that exceeds 250°C which may result in component damage. 

Similar to traditional Sn-Pb wave soldering, metals dissolve relatively quickly during lead-free wave soldering operations. The rate of alloy or metal dissolution of exposed pads on PCBs or component leads depends upon their composition, solder composition, bath temperature, and the flow velocity of the wave solder. The rate of dissolution of a specific metal is lower if this metal is already present in the lead-free solder bath. Of particular concern is the dissolution of copper from circuit boards when exposed to lead-free solder waves. 

In eutectic Sn-Pb wave soldering, formation of solderballs on the surface of a PCB is a low-occurrence defect. For lead-free wave soldering, however, the number of solderball defects is substantially higher than with eutectic Sn-Pb. The higher operating temperatures required in lead-free soldering softens the solder resist on boards which makes solderballs more prone to stick to the board surface. In addition, the reduced wettabihty and increased oxide formation found with many lead-free solders increases the chance of solderball formation. 

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