Failure analysis diagnoses the cause of failure in electronic components. It usually relies on instruments and chemical methods to confirm the failure mode and propose corrective and preventive actions. When microscopic morphology has to be examined, and in particular when a cross section or the internal structure must be observed, ion milling combined with scanning electron microscopy (SEM) is the method of choice for the investigation.

The structure of an electronic component is extremely well hidden. Such latent defects are on the micron and nanometer scale and cause sudden, catastrophic failures, which makes them very harmful. Impurities, inclusions, voids and corrosion cracking in the lead frame cannot be removed in later processing and tend instead to cause even more severe damage.

Case analysis 
The case discussed here focuses on the oxidation state of the lead frame surface. Samples were prepared by ion milling and examined with a Hitachi cold field emission SEM equipped with EDS. By combining backscattered electron images, secondary electron images and energy dispersive spectroscopy, the morphology and phase composition at the failure site were clearly observed, and the element composition was used to analyze the failure of the component.

When analyzing the failure of an electronic component, the microscopic morphology of the failed part is observed first. The backscattered electron detector and the secondary electron detector of the Hitachi cold field emission SEM provide both compositional and morphological information in two imaging modes, the compositional mode and the topographic mode, each with its own characteristics and revealing different details.

After one reflow soldering pass, oxidation and other volatile contamination on the copper frame surface turn it dark red and greatly reduce solderability. The Hitachi cold field emission SEM can acquire BSD compositional images and SED topographic images simultaneously, so the morphology of the dark red area on the frame surface after the high temperature excursion is clearly visible and can be assumed to be a copper oxide layer. Energy dispersive spectroscopy shows that it consists mainly of copper, oxygen and carbon with a few other elements.

SEM plus ion milling 

To analyze the quality of a solder joint, the internal microstructure has to be observed and assessed, because internal voids and inhomogeneities cause failures in service. Observing the internal structure of the joint requires the joint to be cut.
Conventional cutting and polishing changes the cross-sectional structure of the joint, so the internal condition cannot be observed truthfully. Ion milling solves this problem: with a suitable ion gun, an ion beam is used for cross-section cutting or surface polishing and the original structure is preserved.
A Hitachi cold field emission SEM combined with an Oxford energy dispersive spectrometer provides both morphology and composition. In the solder cross section cut by ion milling, gas pores and intermetallic phases near the copper interface can be seen.

SEM with EDS analysis shows that the silver solder paste is bonded to the copper surface, that copper-tin intermetallic grains have grown at the interface, that the copper lead has a nickel plating, and that gas pores and a large amount of iron-rich inclusions are present in the normal region.