Recently, a customer contacted the EMPF helpline to conduct solderability testing per J-STD-002C Test S (4.2.5) on several ball grid array (BGA) components for analysis. .

Using the appropriate stencil thickness for the BGA lead pitch, and the temperature/time reflow parameters for the IR/convection reflow oven, testing simulated actual surface mount component performance in a reflow process. At the conclusion of testing, an inspection revealed failures which required the use of several analytical techniques to determine the source of the problem.

To pass optical inspection at 10x magnification, all leads must have a continuous, 95 percent defect-free solder coating in the critical areas. In addition, the BGA leads must be wetted in a consistent and unified manner, with no indication of surface oxidation anomalies. Optical microscopy, shown in Figure 2-1, revealed the existence of black residue around the periphery of one solder ball, and a smaller solder ball affixed directly to the main solder ball body. The presence of this defect and the residue were indicative of a possible contamination issue which required additional analysis.

A Scanning Electron Microscope (SEM) was used to examine the solder ball arrays under enhanced magnification. By using backscattered electrons (BSE), regions with elements having a higher atomic number appear brighter than regions with lower atomic number elements. BSE analysis of the failed component revealed contrasting bright and dark regions on the ball, indicative of different elements present on the surface (Figure 2-2). As a control, components that had passed the solderability testing criteria were also examined. These ball arrays displayed uniformly dark colored regions, indicating a homogeneous chemical composition.

To characterize the elemental compositions present, energy dispersive x-ray spectroscopy (EDS or EDAX) analysis was required. EDS is a technique using the electron beam in the SEM to determine the identity and the relative quantities the elemental constituents. The EDS analysis of the darker areas of the control component determined that both
carbon and oxygen were present (Figure 2-3), an indication of an organic contaminant. The contrasting brighter region displayed the expected presence of tin and lead on the solder ball. Additional analysis of the dark regions also indicated the presence of sodium, calcium, fluorine, magnesium, silicon, aluminum, chlorine, iron, and potassium. After cleaning several of the components to remove the residue, the tin-lead microstructures became readily apparent; further demonstrating that the dark regions may be indicative of a cleanliness problem.

To identify the organic contaminants, Fourier Transform Infrared (FT-IR) spectroscopic analysis was performed. Through adsorption of infrared radiation at specific frequencies, chemical functional groups in a sample can be determined. FT-IR can rapidly produce an infrared spectrum of a sample by using interference techniques and mathematical analysis. By comparing the unknown spectrum to a database of known chemical spectra, identification of chemical compounds can be determined. In this case, the FT-IR spectrum of the sample had peaks similar to a surfactant used in a variety of cleaning products.

Through analysis involving solderability testing, optical microscopy, SEM/EDS, and FT-IR, it was determined that the BGA components had solderability problems due to contamination. The presence of salts and organic contaminants is a reliability issue which may lead to component failure, performance problems, and an abbreviated life span. Changes in the cleaning process, such as using different cleaning materials or improved rinsing, were recommended.

The EMPF offers a variety of analytical instrumentation and techniques for failure analysis. Ion chromatography, solderability testing, optical microscopy, ROSE testing, FT-IR, and SEM/EDS capabilities are all available to investigate possible contamination issues and determine their root causes. For more information, please contact the EAB Coordinator, Ken Friedman at 610.362.1200, extension 279 or via email at kfriedman@aciusa.org.