Preparation And Properties Of Electroless Ni-Fe-P Coatings For Power Electronics Packaging

The excessive growth of interfacial intermetallic compounds(IMCs)at solder/Cu interface significantly deteriotrates the reliability of power electronics packaging.To inhibit the growth of IMCs,a metallization layer(UBM)is prepared on copper substrate.Electroless Ni-P coatings are the most commonly used UBM so far.However,Ni-P coatings will generate columnar crystal Ni₃P phases at elvated temperature.The holes of Ni₃P grain boundaries will provide channel for element diffusion and deteriorate barrier performance.Thus Ni-P coatings are not suitable for use in power electronics packaging.Ternary Ni-Fe-P coatings prepared by adding Fe into Ni-P alloys exhibit good conductivity and anti-electromigration performance.Thus Ni-Fe-P coatings show potential using in power electronics packaging.There are some researches revealing that structure of coatings do effect their properities a lot.While there are few studies on the relationship between the structure and properties of Ni-Fe-P coatings,as well as the interfacial reaction between Ni-Fe-P coatings with different structure and solder.Those restrict the use of Ni-Fe-P coatings as UBM.In this study,electroless plating was used to prepare Ni-Fe-P coatings with different compositions and structure.The relationship between composition and structure of the Ni-Fe-P coatings was analyzed.First,the thermal stability and crystallization behavior of amorphous Ni-Fe-P coating were studied.And the wetting properties and corrosion resistence of Ni-Fe-P coatings with various crystal structure were researched by cradle drop method and electrochemical experiments,respectively.At the same time,the diffusion barrier behavior of Ni-Fe-P coatings with different structure to Sn-Ag solder was studied systematically.And the growth mechanisms of IMCs at the Sn-Ag/Ni-Fe-P interface under different reflow time were analyzed.This study proved that the Ni-Fe-P crystal structure significantly affected their performance and the solder/coating interface reaction behavior,and revealed the underlying mechanismes.This research has high theoretical and practical value for the design and application of the ternary UBM material system.The results showed that the addition of Fe siginificantly increased the crystallization temperature of amorphous Ni-Fe-P coating.With the increase of Fe content,the structure of the electroless Ni-Fe-P coatings changed from amorphous structure,mixed crystal structure to crystal structure.The XRD results showed that the alloy phases of Ni-Fe-P coating with various structure differed a lot.The crystal structure Ni-Fe-P coating was composed of Fe Ni₃ phase and Ni phase,the mixed Ni-Fe-P coating was composed of a small amount of Fe Ni₃ phase,and the amorphous coating was composed of metal phosphide.Among the Ni-Fe-P coatings with various crystal structure,the mixed crystal Ni-Fe-P coatings exhibited the worst wetting performance,and the crystalline Ni-Fe-P coating showed the best corrosion resistance.The research of Sn-Ag/Ni-Fe-P interface reaction showed that the Fe Sn₂compounds forming at the interface of Sn-Ag/Ni-Fe-P with crystalline structure exhibited the smallest and most uniform thickness.While the IMCs betweent the Sn-Ag solder and amorphous Ni-Fe-P were consisted of columnar Ni₃Sn₄ and massive(Cu,Ni)₃Sn₄ which exhibited bad diffusion barrier performance for metal atoms in solder joints.Aggregated(Cu,Ni)₃Sn₄ and a small amount of Fe Sn₂ compounds were found at the interface of the Sn-Ag/Ni-Fe-P mixed crystal structure.Comparing the growth rates of different IMCs,it could be seen that the growth rate of Fe Sn₂was much lower than that of Ni₃Sn₄,which proved that the crtstalline Ni-Fe-P coating could effectively block the diffusion of Sn element in solder joint and showed the best diffusion barrier property.