Hi,
Usually most of the PCBs have solder masks but recently i have seen a PCB with only few tracks(high voltage/current) with solder mask open. when do I have to keep the solder mask open for traces?

 

I think it was someone's idea to reduce the resistance of the track by covering it with a layer of solder. Maybe I'm wrong - but I know one thing for certain - If I'm wrong someone on this forum will correct me.

But don't forget that solder has ten times the resistivity of copper, so you would need 350μm of solder on top of the 35μm of copper to halve the resistance.
As the thickness of solder that would be applied by HASL is not guaranteed, then, if your think your track is a little too narrow for the current it is carrying, then make it wider. If that is not possible get the pcb made with thicker copper.

 

I think it was someone's idea to reduce the resistance of the track by covering it with a layer of solder. Maybe I'm wrong - but I know one thing for certain - If I'm wrong someone on this forum will correct me.

But don't forget that solder has ten times the resistivity of copper, so you would need 350μm of solder on top of the 35μm of copper to halve the resistance.
As the thickness of solder that would be applied by HASL is not guaranteed, then, if your think your track is a little too narrow for the current it is carrying, then make it wider. If that is not possible get the pcb made with thicker copper.

Here’s a video with more information, @Ian0

 

Here’s a video with more information, @Ian0

As I thought - "Too inconsistent to rely on"

 

Hi,
Usually most of the PCBs have solder masks but recently i have seen a PCB with only few tracks(high voltage/current) with solder mask open. when do I have to keep the solder mask open for traces?

That's some old school (cheapo) method of adding current carrying capability. If you need to carry high currents on PCB add more copper thickness.