Lead-Free and Lead-Bearing Solder Intermetallic Fc rmation on Electroless Ni/Immersion Au Interconnects Affected by Black Pad

摘要

Black pad solder joint failures have been found to correlate with black pad corrosion, and are a result of an interaction between the process control of electroless Ni/immersion Au (ENIG) plating and other factors such as circuit board design. The mechanism of the defective solder joint formation which creates a susceptibility to premature, brittle, interfacial fracture, is not completely understood. Recent results have shown that black pad-affected interconnects and good interconnects have different ENIG microstructures before soldering. Both multilayered Ni/Au and spiked structures were detected in this work, and are discussed in this paper. Corroded and good structures behave differently during soldering. The intermetallic types and microstructures formed on both black pad-affected and good pads are described. The subject solder joints were made by soldering SMT pins using Sn-Ag-Cu and Sn-Pb solder pastes. The study included solder joint analysis befo e and after thermal aging at 125°C and 150°C. Shear tests were implemented for black pad detection and stress-related behavior analysis. A Ni-rich intermetallic, presumably Ni_3Sn, formd inside the corroded Ni layer. The Me_3S n_4 (Me = Cu, Ni, and Au) crystals precipitated simultaneously from the molten solder. The adhesion between these intermetallic layers was poor, and brittle cracks propagates between the intermetallic layers. The black pad failure occurred in lead-free solders as well as in Sn-Pb solder, and could not be prevented using the higher temperature lead-free reflow profiles. The similarities and differences between lead-free and lead-bearing solders in black pad intermetallic formation and failures are shown. Diffusional phase transformation occurs between intermetallic layers during aging, and therefore improves adhesion and joint strength.