Chosen Solution
Got a iphone 6s to repair from water damage. After cleaning and removing a shorted cap near RF_PMU iphone went back to life, everything works except mobile network, it keep searching for signal and a message appears saying: “Mobile data update failed” It have the correct IMEI set but Modem firmware is blank. I reballed RF_PMU and baseband, no damage nor bad tracks, the operation resulted in same behaviour. Is there any other IC responsible to Modem Firmware?
I had this problem once on a water damaged iPhone 6 after running the jumpers and re-balling the baseband CPU it still had no firmware. I replaced every chip under the bottom shield and it didn’t help. So I would defiantly say this is the baseband CPU or the traces under it. I wouldn’t spend any more time on this.
