Solder joint integrity of various surface finished build-up flip chip packages by 4-point monotonic bending test

Board level solder joint integrity was evaluated using the 4-point monotonic bending test. Ball grid array (BGA) pad surfaces of electroless nickel immersion gold (ENIG) and solder over copper (SOC) in combination with solder compositions of eutectic SnPb (63SnPb), Cu-doped solder (63SnPb0.25Cu), and Pb-free solder (Sn3Ag0.5Cu) were tested on circuit boards with OSP pad surface finish to evaluate the solder joint integrity at the time of board mounting. The interface and intermetallic compound (IMC) layers were analyzed with SEM and EDX. Test results indicated that BGA solder joints had higher fracture strength when assembled onto a SOC package substrate surface finish than when fabricated on an ENIG surface finish. For solder balls attached to an ENIG plated substrate, fracture strength increased and brittle fracture rate decreased when a solder which includes Cu was used. Failure analysis indicated that for ENIG samples, SnPb eutectic has less remaining IMC at the Ni layer surface and a thicker phosphorus rich (P-rich) layer than either Pb-free or Cu-doped solder. The combination of surface finish and solder type affects not only the IMC/P-rich layer formation, but the fracture strength and failure mode as well