Over the past year there has been consistent growth in the use of high Phosphorous - electroless Nickel / immersion Gold (HP - ENIG) as a final finish. The finish is now frequently being used for PBGA, CSP and QFP, and more recently gathered considerable interest as a low cost under-bump metallization for flip chip bumping application.Corrosion will interfere with the electrical behavior either due to electrical conductivity of corrosion products creating a shortage, or damaging the final coating as a contact surface for key pads or contact switches. Due to the nature of the electroless plating process, the HP-ENIG coating has a Phosphorous content in the plated electroless Nickel layer of about 9.5 to 13 wt.％. This high Phosphorous content leads to an amorphous structure, with no grain boundaries, which therefore influences the formation of the IMC between Nickel and solder. The high Phosphorous content of the bulk electroless Nickel layer seems to control the diffusion rate of the Nickel into the solder, resulting into a dense and uniform IMC formation. This paper describes the influence of co-deposited P (from low to high P) within an electroless Nickel layer, regarding "Brittle Fracture" and the reliability of the solder joint integrity, using solder mask defined BGAs as test vehicle.