ENIG Corrosion Induced by Second-Phase Precipitation

Abstract

Materials analysis of a flip-chip package lot with solder bump interconnect failures revealed a new mechanism for corrosion of electroless nickel immersion gold surface finish. Detailed scanning and transmission electron microscopy (SEM and TEM) in conjunction with focused ion beam microscopy and electron dispersion analysis of the unsoldered ball grid array substrate pads on packages that exhibited flip-chip solder bump interconnect failures revealed an unusual and subtle defect in the original Ni(P) layer, which was ultimately responsible for flip-chip joint failure. Detailed TEM analysis of the defect regions showed that they consisted of Ni(P) particles of slightly different composition than the bulk Ni(P) layer. Microstructure changes around these incorporated particles indicated that the second-phase particles were deposited from the plating bath during the Ni(P) growth stage. The second-phase particles provided additional surface area for nucleation and growth of Ni(P). Ultimately, a low-density boundary region in the growing Ni(P) layer formed where the particle-induced growth front and the planar Ni(P) film growth front intersected. This low-density interface eventually terminated at the surface of the Ni(P) layer. In addition the growth from the second-phase particle created localized surface topology that was different than that of the surrounding Ni(P) layer. The low-density interfaces as well as the surface topology led to enhanced corrosion of the Ni(P) layer when exposed to the immersion gold plating process. In some cases the corrosion was severe enough to create voids in the Ni(P) layer. The exposed, oxidized Ni(P) surfaces in and around these enhanced corrosion regions did not wet when exposed to solder. This led to degradation in the strength of the solder joint and subsequent solder interconnect failure.

Numerical investigation of thermal fatigue crack growth behavior in SAC305 BGA solder joints

“…Also, nickel corrosion in the BGA pad (ENIG) can also create a void. The oxidized nickel surfaces in and around the enhanced corrosion regions did not wet when exposed to thermal cycling, leading to microvoid formation (Osenbach et al , 2009). This leads to degradation in the strength of the solder joint and subsequent solder interconnect failure.…”

Black Pad Susceptibility of the Electroless Ni Films on the Cu UBM

“…This is consistent which the result of the corrosion couple experiment carried out on two Ni(P) films with varying P content where the lower P side served as the anode. 9 Nodular variations of the P concentration were ascribed to differences in the Ni(P) film surface curvature. Large nodules with small surface curvature had higher P concentration and remained intact, while small nodules with large curvatures contained less P and corroded.…”

“…The ENIG film has good electrical conductivity, solder wettability, and corrosion resistance, 1 but is susceptible to the black pad phenomenon during the soldering stage. [2][3][4][5][6][7][8][9][10][11] Characteristics of black pads include presences of mud cracks, major and/or minor spikes, 2,6 P enrichment in affected areas, etc. 7 As root causes of the black pad, several mechanisms have been proposed; 6-10 however, difficulty of reproducing the black pad has been the main barrier to the full understanding of the phenomenon.…”

Effect of Gold on the Corrosion Behavior of an Electroless Nickel/Immersion Gold Surface Finish

“…[15][16][17][18][19][20][21] Regarding the corrosion properties of ENIG, black-pad has been studied as an important failure mode in electronic applications. 12,22,23 However, its corrosive properties in electronic packaging remain the key issue, especially when exposed to an oceanic environment. With an ENIG layer, variations in alloy quality and corrosion resistivity can be achieved during the nickel phosphorus plating process.…”