Correlation Between Chemical Reaction and Brittle Fracture Found in Electroless Ni(P)/immersion gold–solder Interconnection

Abstract

Occurrence of brittle interfacial fracture at an electroless Ni(P)/immersion gold-solder joint has long been a serious problem not yet fully understood. In our previous report on the electroless Ni(P) [J. Mater. Res. 19, 2428], it was shown that crystallization of the Ni(P) film and growth of the Ni 3 SnP layer were accelerated after the intermetallic compound (IMC) spalling, and accurate failure locus of the brittle fracture due to so-called "IMC spalling induced microstructure degradation of the Ni(P) film" is presented for the first time in this communication. For Sn-3.0Ag-0.5Cu solder joints, (Ni,Cu) 3 Sn 4 and/or (Cu,Ni) 6 Sn 5 ternary IMCs formed at the interface, and neither spalling nor interfacial fracture was observed. For Sn-3.5Ag joints, Ni 3 Sn 4 compound formed, and the brittle fracture occurred through the Ni 3 SnP layer in the solder pads where Ni 3 Sn 4 had spalled. Since the Ni 3 SnP layer is getting thicker during or after Ni 3 Sn 4 spalling, control of IMC spalling is crucial to ensure the reliability of Ni(P)/solder system. Electroless Ni(P)/immersion gold (ENIG) has been used for under bump metallization (UBM) of flip chip and ball grid array (BGA) and for surface finish layer of printed circuit boards (PCBs). 1-9 Electroless Ni(P) is selectively deposited on metal surfaces with uniform thickness. 10,11 Immersion gold layer prevents the Ni(P) layer from oxidation and enhances wettability of solders. 7 However, the Ni(P) metallization has two problems against extensive commercial application. One is spalling of intermetallic compound (IMC) during wetting reactions. 9,12 Intermetallic compound spalling from thin-film UBMs has been found only after complete consumption of the films. The Ni(P) metallization, however, shows the spalling phenomena before a complete consumption of the film. The other is brittle fracture phenomena. 1-8 The brittle-fractured solder joint has been found after surface mounting of the BGA package on a printed circuit board (PCB) with ENIG metallization on the pads. The failure has been reported to be unpredictable and to occur at relatively low defect levels. 5 Interfacial P segregation, 2 void and crack formation during the crystallization of the Ni(P) film into Ni 3 P phase, 7 and hyperactive corrosion of the Ni(P) film during the immersion gold plating 3 have been suggested as possible causes of brittle fracture. The mechanism of the brittle fracture, however, is not yet well understood. In this study, we systematically investigated the relationship between IMC spalling and the brittle fracture of ENIG-solder interconnection.A specimen for lap-shear testing is composed of two FR4 PCBs having 16 solder pads 250 m in diameter. Sizes of the PCBs were 10 × 10 mm and 10 × 30 mm for the top and bottom substrates, respectively. The same UBM, Cu/Ni(P)/Au, was used for both sides. The P content of the Ni(P) film was 7 wt%. Schematic diagrams of test specimen and metallization scheme are presented in Figs. 1(a) and 1(b). The Sn-3.5Ag or Sn-3.0Ag-0.5Cu (all in weight percent un...

Failure Morphology after the Drop Impact Test of the Ball Grid Array Package with Lead-Free Sn-3.8Ag-0.7Cu on Cu and Ni Under-Bump Metallurgies

“…A number of papers demonstrated that with the thicker intermetallics, the solder joints tend to have a brittle failure mode, dependent on the detailed solder alloys composition. [13][14][15][16] Excessive growth of intermetallics can deteriorate solder joint ductility. For example, in thermal fatigue damage, where shear deformation dominates as a main factor, there are several failure modes, including intermetallics/solder failure and bulk solder failure, dependent on solder alloy composition.…”

Board Level Drop Reliability of Epoxy-Containing Sn-58 mass% Bi Solder Joints with Various Surface Finishes

“…During initial reflowing, the entire Au layer on the Ni layer dissolved into the molten solder, and the Sn-58Bi solder formed a uniform and continuous Ni 3 Sn 4 IMC (chunk-shaped) at the solder/Ni interface. 20,21) In a previous work, we investigated the interfacial reactions and joint mechanical reliability of Sn58Bi solder with three surface finishes, OSP, ENIG, and ENEPIG. 22) In the study, we successfully identified the interfacial products during the reflow reaction.…”

ENIG Corrosion Induced by Second-Phase Precipitation

“…In addition, mechanical handling and board attachment processes can also create brittle failures at the ENIG-solder interface. Although a number of potential root causes have been proposed, including: (i) hyperactive corrosion of the Ni(P) film during the immersion Au plating process; 7 (ii) interfacial segregation of phosphorus; 8 (iii) Kirkendall voiding; 9 and (iv) Ni 3 SnP formation, 10 the mechanism(s) responsible for this failure mode is not yet fully understood. Furthermore, a more complex mechanism for joint embrittlement has been proposed for devices with an ENIG pad finish on one side of the solder joint and a Cu-pad finish on the other side of the joint.…”