Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 12
Situational Awareness; Confusion; Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Procedure; Airspace Structure; Human Factors
Aircraft X (not tagged up and just on a 1200 code) called BFI on 118.3 about 12-13 miles north well outside the Class Delta inbound to land. I observed S46 working a VFR airplane Aircraft Y 8 miles north of my airspace NE bound and the aircraft did not have an altitude display; just RDR. Aircraft Y was on a direct intercept course for my arrival Aircraft X. This airspace is VERY busy and if they were going to let a VFR fly through it at a displayed VFR target southbound it's a safe bet the BFI might be talking with that aircraft. I had to call S46 and ask what altitude Aircraft Y before the targets merged. S46 is the RADAR facility and they should have reached out to BFI to ask if they were talking to the target.S46 is required to advise us if a known target in our Delta doesn't have any correct info in the display. This was definitely outside our Delta but an early call would have prevented wo targets merging. If BFI had a larger Class Delta this might not have been an issue.
BFI Tower Controller reported an airborne conflict that may have been prevented if Class D was larger or TRACON had been more proactive.