Electroless Ni Layer Research Articles

The microstructure of eutectic Au-Sn solder bumps on Cu/electroless Ni/Au

In this work we studied the initial microstructure and microstructural evolution of eutectic Au-Sn solder bumps on Cu/electroless Ni/Au. The solder bumps were 150–160 m in diameter and 45–50 m tall, reflowed on Cu/electroless Ni/Au, and then aged at 200°C for up to 365 days. In addition, Au-Ni-Sn-alloys were made and analyzed to help identify the phases that appear at the interface during aging. The detailed interfacial microstructure was observed using a transmission electron microscope (TEM). The results show that the introduction of Au from the substrate produces large islands of-phase in the bulk microstructure during reflow. Two Au-Ni-Sn compounds are formed at the solder/substrate interface and grow slowly during aging. The maximum solubility of Ni in the—phase was measured to be about 1 at.% at 200°C, while Ni in the-phase is more than 20 at.%. The electroless Ni layer is made of several sublayers with slightly different compositions and microstructures. There is, in addition, an amorphous interaction layer at the solder/electroless Ni interface.

Interfacial reaction and microstructural evolution for electroplated Ni and electroless Ni in the under bump metallurgy with 42Sn58Bi solder during annealing

In the present study, several under bump metallization (UBM) schemes using either electroplated Ni or electroless Ni (EN) as the solderable layer are investigated. The EN and electroplated Ni are first deposited on Cu/Al2O3 substrates, followed by electroplating of thin gold coatings. Joints of 42Sn-58Bi/Au/EN/Cu/Al2O3 and 42Sn-58Bi/Au/Ni/Cu/Al2O3 are annealed at 145 C and 185CC for 30–180 minutes to investigate the interfacial reaction between the solder and metallized substrates. For 42Sn-58Bi/Au/Ni-5.5wt.%P/Cu/Al2O3, 42Sn-58Bi/Au/Ni-12.1wt.%P/Cu/Al2O3, and 42Sn-58Bi/Au/Ni/CU/Al2O3 joints annealed at 145 C, only Ni3Sn4 intermetallic compound (IMC) formed at the solder/EN interace. When annealed at an elevated temperature of 185 C, plate-like Ni3Sn4 IMC forms at the solder/Ni-5.5wt.%P interface, while a trace of (Ni, Cu)3Sn4 IMC is observed at the solder/Ni-12.1wt.%P interface and within the solder region. For the electroplated Ni-based multi-metallization substrate, the Ni3Sn4 IMC is present at the solder/Ni interface during annealing at 185 C for a short period of time. In the 42Sn-58Bi/Au/EN/Cu/Al2O3 joint, the EN spalls off the EN layer and migrates into the solder region when annealed at 185 C. The interface of the solder/electroplating Ni becomes saw-toothed as the annealing temperature is raised to 185 C. In addition, an enrichment of phosphorus is observed at the interface of the Ni-Sn IMC and EN.

Solder joint reliability of plastic ball grid array packages

Presents the results of a study of the effects of solder ball pad metallurgy, intermetallic compound (IMC) thickness and thermal cycling on the shear strengths of PBGA package solder balls. The study of the microstructures of solder balls revealed that only a very thin layer of intermetallic compound existed between solder balls and Ni or Ni alloy barrier layers immediately after ball placement and reflow. The protective Au layer was dissolved completely and a needle like AuSn4 intermetallic compound was then formed and dispersed evenly in the solder balls. The overall thickness of the IMC layers was thicker than 15μm after storage at 150°C for 1,000 hours. During the shear tests failure occurred at the interface of the two IMC layers. The fracture surfaces of solder balls with electrolytic Ni and thick Au layers were smooth and brittle fracture was observed. The ball shear strength decreased dramatically with the formation of IMC layers. For the solder balls with electroless Ni and thin Au layers, only a single IMC layer was formed at the interface and its thickness was only 2.5 μm after storage at 150°C for 1,000 hours.

Bonding mechanism of electroless Ni-P film with AlN substrate and Cu foil

Electroless Ni (EN) plating method is employed to metallize Y2O3-doped AlN ceramic substrates with different surface morphologies, including the as-received, polished, and etched AlN. The EN-plated AlN substrate is bonded with the Cu foil to form a sandwich-like AlN-EN/Cu/EN-AlN assembly by hot pressing in vacuum with a pressure of 6.5 MPa for 30 min. The bonding strength of the joint is determined by the adhesive abilities of EN/AlN and EN/Cu interfaces. Bonding between the Cu foil and the EN film is achieved by the interdiffusion of Cu and Ni atoms to form a solid solution interlayer, while the EN film is adhered on the AlN substrate mainly by the mechanical interlocking through the rough interface, etched holes, and open pores. At the bonding temperatures lower than 400/spl deg/C, no reaction occurs between the Cu foil and the EN film. Increasing bonding temperature enhances the interdiffusion of Cu and Ni to form a strong bonded solid solution interlayer. However, the depletion of the element Ni in EN film results in the formation and enlargement of the pores at EN/AlN interface, which makes a poor adhesion between EN film and AlN substrate. An optimum bonding temperature is observed around 600 and 700/spl deg/C for various surface conditions of AM substrates. Theoretical calculations of the residual thermal stresses in AlN, EN, and Cu layers with respect to the bonding temperatures are related to the variation of the bonding strengths. The relations of the adhesion strengths, interfacial morphologies, elemental distribution, and calculated residual stresses is proposed.