Mechanism of u-HAST failure caused by flux residue containing bromide

Fan-Out Panel-Level Packaging (FOPLP) partly uses a printed circuit board (PCB) technology and materials for cost advantage but finer line width and spacing than PCB requires better understanding of its reliability. The present study presents the effect of residual bromide (Br
^-
) on the reliability of FOPLP using an unbiased highly accelerated stress test (u-HAST). We prepared a package with three redistribution layers (RDLs) with under bump metallurgy (UBM) of which layers were connected through Cu micro-vias. The micro-via structure was made on Cu surface by sputtering of seed metals (Ti/Cu) followed by electroplating of Cu. We conducted u-HAST using two kinds of FOPLP products. One was processed by a flux with Br
^-
ions during solder ball attach and the other was processed by a flux without any halide. The microstructure and chemical composition were investigated using a focused ion beam (FIB) and a transmission electron microscope (TEM) equipped with an energy dispersive spectrometer (EDS). We observed that u-HAST failure occurred only in the samples treated with the flux containing Br
^-
ions for solder ball attach. The cross-sectional images of the failed samples show that there were delamination between the seed metals (Ti and Cu) along the via wall as well as at the bottom of the Cu micro-via. We detected bromine in the area where delamination occurred and found that the seed Ti was oxidized. We propose a failure model that delamination occurs through the combined process of forming a galvanic couple of titanium and copper in the presence of water, oxidation of titanium and diffusion of Br
^-
ions in the titanium oxide layer.