Analysis of 16 QFN Device I/O Pads for Solderability Failures

In-process failures were experienced during printed circuit board (PCB) SMT assembly of a 16 Quad Flat No Leads (QFN) device. The failures appeared to be solderability related with QFN unit I/O pads not soldering robustly and sometimes leading to QFN detachment following board mounting. When assembly did take place on affected QFN units, the resulting solder joint was observed to be weak. This paper reports on very systematic analyses of the QFN device I/O pads using optical inspections, AES surface, AES depth profiling, SEM/EDX, SIMS, FIB and TEM cross-sectional measurements to determine the root cause of the failure and the failure mechanism. The detached QFN units, suspect and good unsoldered units, passing and failing units obtained from customers were examined. The industry standard surface mount solderability testing was performed on good and suspect parts, and all were observed to pass as evidenced by >95% coverage of the I/O pads. Optical inspections and a wide variety of physical analysis of the pads on fresh parts showed no anomalies with only the expected Au over Pd over Ni found. AES analysis was performed including depth profiling to look for any issues in the NiPdAu over base Cu plating layers that could be contributing the solderability failures. The AES depth profiling indicated AuPd film on the Ni under layer for the I/O pads as expected. No unexpected elements or oxide layers were observed at any layer. Then, one failing and one passing units were compared by doing FIB cross-section, FIB planar section and TEM cross-section analysis. The cross-sectional analysis showed rough Ni surface for the failing units, while the Ni surface was relatively smooth for the passing unit. Further, finer Cu grains and Ni grains were observed on the passing units. Additionally, the lead frame fabrication process mapping showed rough Cu, Ni “texturing” and use of low electro chemical polishing (ECP) current on the bad units compared to that of the good units. All affected bad units were confirmed coming from a second source Cu supplier with the rough Cu. The weak and irregular NiSn IMC formation on the bad units caused IMC separation and possible spalling during board solder reflow primarily due to the rough base Cu and irregular grain sizes and resulting lower ECP lead frame plating current. A possible final factor was marginally low Pd thickness. In conclusion, the 16 QFN device solderability failure root cause summary and the lessons learned from a wide variety of analysis techniques will be discussed.