A　surface　integral　equation　(SIE)　formulation　under　the　magneto-quasi-static　assumption　is　proposed　to　efficiently　and　accurately　model　arbitrarily　shaped　interconnects　in　packages.　Through　decently　transferring　all　electromagnetic　quantities　into　circuit　elements,　the　loop　analysis　is　used　to　carefully　construct　matrix　equations　with　an　independent　and　complete　set　of　unknowns　based　on　graph　theory.　In　addition,　an　efficient　preconditioner　is　developed,　and　the　proposed　formulation　is　accelerated　by　the　pre-corrected　fast　Fourier　transform　(pFFT).　Four　practical　examples,　including　a　rectangular　metallic　interconnect,　bonding　wire　arrays,　interconnects　in　a　real-life　circuit,　and　the　power　distribution　network　(PDN)　used　in　packages,　are　carried　out　to　validate　its　accuracy,　efficiency,　and　scalability.　Results　show　that　the　proposed　formulation　is　accurate,　efficient,　and　flexible　to　model　complex　interconnects　in　packages.