Variable resistor SMD

how to know variable resistor (SMD) is broken and the reason. It is because of my way of soldering using hot air gun? When i checking the connectivity, i found out the pin 2 and 3 have connection on PCB. Then, i unsolder and again check the connectivity, also same, have connection between that pin.

It has plastic inside the variable resistor and you have melted the plastic.

thank you colin. how to prevent that? what the most suitable temperature(heater) to use for solder?

You have to solder delicate surface mount components by hand - such as LEDs, crystals, electrolytics, tantalums, trim pots, because if you are going to use non leaded solder, the temperature will be so high that there is no margin for error. Even the manufacturers and assemblers find it enormously difficult to solder the boards and prevent dry joints and prevent damage to components.

PCB Preheaters help a lot with hot air, and with lead-free solder if you're using it. They are usually very expensive for what they are though.

You set it to be 100C or 150C so the component and the PCB area around it is below the melting temperature of the solder (so the PCB doesn't suck heat away as much) so the hot air gun or iron only has to give the temperature a quick nudge to get it hot enough to melt. It also lets you set the temperature of the hot air gun lower since it doesn't need to be hot enough to heat everything up on it's own quickly anymore which reduces the maximum temperature the component might reach.

Thank you dknguyen for the information. Before this i use a bit high temperature which is 290c -to 300c. how about the air ( pressure) for the hot air, i used 30?

Shara Azhar said:

Thank you dknguyen for the information. Before this i use a bit high temperature which is 290c -to 300c. how about the air ( pressure) for the hot air, i used 30?

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I'm not the best person to help you specifics since I am not too confident in my hot air skills myself.