Hallo, I am having difficulties soldering frp connectors on motherboards. They wont stick to the motherboard. I check with a multimeter in diode mode that the pins have connection with the pads. But when I   connect a flex cable and then take it off, the frp connector comes loose. Do I need some special solder? The solder I use now comes from China and it was cheap. That is why I bought it. Or am I using wrong technique?