Buried engincoring changc ( H I i C ) is a method oflaying out engineering change ( I X ) wircs within a multi chip module (MCM) at original build time for later me.These BEC lines are used for repairs, ECs and temporary rues ( T X ) . Since repaw and TX connections are handled essentially as ECs, they will not be di~tittguished after this point.REC.' has posed several new chllcnges including how to elcctrically model, time and verify these complex multifacctcd structures, in the simplest way. BEC provides for 100% EC coverage on most MC'Ms. This paper discrcsses the modeling used to accurately predict delay, validate electrical integrity, nnd add a degree of flexibility to the wiring process. HistorylPurposePrevious I B M machine's M C M s were ECcd arid repaired exclusively with discrete jumper wires. Some of these discrele jumper wires were very long and OCCAsionally caused reliability problems, as well >IS electrorriagrrclic itrtcrf'ercncc ( K M I) prohlcins.'I'he UEC hardware methodology was dcvclopcd for improved rtAiability and rcduccd I