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Wave solder bridging is the most difficult defect to troubleshoot because it has a number of potential causes. The key is to understand the role of flux during wave contact—reducing the surface tension of the solder to reduce the tendency to bridge between pins as the board leaves the wave. Bridging occurs when the flux has been chemically depleted before exiting the wave. This can be a function of the flux and process.
With respect to flux, in general, fluxes with higher activity levels are more resistant to bridging. This means that no-clean fluxes can be more likely to result in bridging, especially with older formulations. Newer flux formulations are available that have higher levels of performance while still meeting the requirements of no-clean classification.
Regarding the process, the flux loading and total thermal input to the board are key factors. An inadequate amount of flux can result in flux exhaustion before wave exit. Excessively high thermal inputs can also lead to bridging with three factors to measure. Preheat temperature is the first, contact time on the wave is the second, and solder pot temperature is the third. The optimal values for each should be available on the data sheet for any flux, and they will be different for every flux formulation.
There are other reasons outside of flux and process that can lead to bridging. The design of masking pallets can cause bridging if there is inadequate clearance around the devices. The PCB layout can also cause bridging if rows of leads are unable to be processed through the wave in the proper orientation (orthogonally and not parallel to the wave). These factors may not be able to be overcome through process optimizations or flux selection.
Jason Fullerton is the customer technical support engineer at the Assembly Division of MacDermid Alpha Electronics Solutions.