The soldering time is related to the contact length of the solderwave(s) and the conveyor speed. The contact length is affected by the solderwave height and the distance between the PCB and the nozzle rim.
The contact length is created as soon as the PCB enters the wave. If a PCB enters the main wave, the wave height will increase automatically. Due to the fact that the entering PCB will reduce the free flow of the wave, the wave height will rise since the solder has now to flow through a smaller gap between the PCB and the nozzle rim. The amount of solder that is pumped up can only flow through the smaller gap with an increased flow speed. The flow speed can only increase when the pressure on the liquid will increase also, for that the wave height will rise, resulting in the higher wave.
To prevent solder overflow over the PCB during entrance in the wave, the nozzle backplate should be set at a level that the solder will start to run over the backplate as the solder surface of the wave is raised for about 1 mm.
When the last part of the PCB passes the nozzle rim, the wave will start to reduce the wave height, back to the original level. This can mean that the contact length and the solder drainage conditions for the last part of the PCB in the solderwave will be different in respect to the fore going part of the PCB.
The real effect on the wave height is also depending on the width of the PCB or pallet in relation to the width of the wave.
Finally the lead protruding length in combination with the layout will have an effect on the soldering time for a specific joint.
This means that in reality the soldering time can vary per joint, having all process parameters constant.
ITW EAE Support Center