When in doubt, start at the beginning...
Was there a lot of corrosion or was it light? You may want to focus your initial efforts on those areas. Also, it's not clear from your question but was the phone recognized by iTunes? If so, it's not Tristar (anymore).
Otherwise, when the phone is dead and you have no leads or heat spots, start by checking VCC_MAIN & BATT_VCC and then move on to the output voltages generated by the PMIC. Check to see if they are shorted to ground (careful with low resistance lines like PP_CPU & P_GPU) with an unpowered board and then power the board and measure the output voltages. If one of them is off or dead, that may explain your dead board.
Start with that and then come back and post the details.
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