| LGA stands for Land Grid Array,which is mainly used as a physical package for advanced microprocessors.LGA package is related to pin grid array(PGA)and ball grid array(BGA),but unlike PGA or BGA,land grid array package only has the pads at its substrate underside rather than pins or solder ball,LGA can be electrically connected to a printed circuit board(PCB)by the use of a socket that is notable for having the pins.The contacts are to be connected to a grid of contacts in the socket.Therefore either pad surface finish quality of LGA package or pin quality in the socket is critical for electrical connection.Electroless nickel immersion gold(ENIG)is a type of pad surface plating used for LGA package in the field of semiconductor assembly.ENIG has several advantages over more conventional and cheaper surface finishing such as organic surface protection(OSP)and hot air solder leveling(HASL),including good oxidation resistance,long storage life,excellent surface planarity,etc.This paper goes deeply into the investigation of discoloration related failure mechanism on ENIG pads.In contrast of normal ENIG pad,the discolored pad caused by severe oxidation of the Ni not only results in cosmetic reject at customer side but also in extreme severity leads to electrical failure due to increased contact resistance between pad surface and socket pins.To identify the root cause and take effective preventive action is much helpful and meaningful to eliminate ENIG quality issue.Two failure mechanism of discoloration issue on ENIG pad surface finish is identified with the help of a variety of material analysis technologies,one is induced by the contamination of tin oxide from solder paste on LGA pad surface during assembly process,and the other one is caused by the nickel oxide contamination through the pinhole in Au layer of ENIG pad under the influence of the joint of oxygen,high temperature and heavy moisture,which is a substrate ENIG process related issue.The corresponding preventive actions are also addressed to eliminate the reject rate by the discoloration issue during LGA assembly process and improve the overall outgoing yield. |
