can anyone suggest me some good surface mount equipment for mounting PHEMT on substrate. I have used a solder iron 15W , but i guess the heat generated is damaging the devise internally and i am not able to generate the required IV curves as in the data sheet.
You may be blowing up the device with static or line-power leakage. Are you using a grounded soldering iron? Have you tested the resistance from the soldering iron tip to the grounding lead?
Sometimes you can use a temperature controlled hot plate to reflow the solder. Sometimes you can heat up your circuit to close to but just under the soldering temperature with a hot plate, and then finish the soldering with a hand held iron on a specific component.