What does upper right hand corner of iPhone 4 logic board do???

i replaced screen once successfully, luck have (or clumsiness) dropped , cracked again. bought screen iphone4parts, install went fine 1 tiny exception: upper right corner of logic board (right head phone jack) cracked off , stayed beneath little overhang. there hole screw go through @ broken corner. hole rimmed in black on underside , copper on side. tried attach image won't work using ipad. phone put , when powers goes off(black) lit black. subtle, can tell lit black if in pitch black room. phone rings phone calls, touch screen unresponsive. can answer calls in car using bt or if plug in mic'd ear buds , answer call button on ear buds. can hear text alerts-but drives me crazy because don't know i'm missing, or whom. suspect it faulty lcd screen. before replace screen yet again, want know tiny corner of logic board responsible for, worth fix 32 gb iphone 4. (oh, , phone functional cracked screen before replaced it-ugh!). thanks!

 

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