Commercial Fixed Wing
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 11
Time Pressure; Situational Awareness; Workload
ATC Issue All Types; Conflict NMAC; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action
Airspace Structure; Chart Or Publication; Procedure; Human Factors
ORD was on a very busy east flow arrival push.  The weather was severe VFR and there were numerous aircraft on 1200 codes flying underneath the ORD finals.  I was working the north final.  A VFR aircraft who I believe departed ZZZ1 climbed up to 3900 feet which is 100 feet below the base of the Class B.  I did not see the aircraft approaching my final because of all of the overlapping data blocks from the south two finals.  When the conflict alarm sounded; I saw the traffic and issued a traffic alert to Aircraft X who climbed 800 feet to avoid a collision.The Class B airspace around ORD does not match the current arrival/departure procedures.  On East Flow; the low-side runway is required to turn final at 4000 feet in accordance with the facility Standard Operating Procedure.  The floor of the Class B airspace is 4000 feet; so there are constantly conflicts with VFR aircraft climbing up to 3500 feet or higher in that area.  When doing simultaneous approaches; there is nowhere to go when there is a conflict. The floor of the Class B airspace west of ORD should be 3;500 feet until 25 miles west of ORD.
C90TRACON Controller reported they did not notice an unidentified VFR aircraft climb to 100 feet below the Class Bravo airspace on a conflicting route with air carrier traffic conducting a Visual Approach resulting in a NMAC.