The Scanning Electron Microscope has for many years been a mainstay of flip chip failure analysis, capable of revealing a wide range of potential problems. Internal slices of a sample for SEM cross-section inspection must first be “potted” by enclosing them in hardened plastic to permit handling and sectioning.
Cross-sectioning is mechanical, with some combination of sawing, lapping, and polishing revealing a planar section at any desired point in the sample, such as a cross-section view along the center of a row of solder bumps. SEM inspection of that view can reveal voids, cracks, delaminations, intermetallic layers, grain structure, and other problems or potential problems. Figures 1 and 2 below are examples of SEM cross-sections.
Mechanical cross-sectioning reveals the innards of the sample, but also may create its own problems. In particular, if the cross-section includes both hard and soft materials, such as metals and organic substrates, the mechanical operations of exposing them may smear a layer of the softer material across the surface, obscuring small but crucial details. The polishing step that normally follows exposing the desired surface removes some of the smearing, but polishing as a mechanical process again can create its own smear.
Previous techniques to remove smear include chemical etching of the surface, or removing material by exposure to a Focused Ion Beam, a machine intended for elemental identification in small areas of samples.
Wet chemical etching can remove undesired material after polishing. However, wet chemical etches are narrowly selective in which material they remove, and must be optimized for specific materials, making them cumbersome and expensive for broad use. In addition, wet chemistry adds handling and environmental drawbacks to a mechanical process.
Focused Ion Beam (FIB) equipment has been used as an alternative to remove obscuring materials. However, by design FIB has a small focal point, covering a relatively tiny area. This suits its primary purpose of isolating and identifying materials in small areas of a sample. But trying to clean an entire cross-sectioned sample this way is like sweeping a ballroom floor with a whisk broom – slow, painful, inefficient, and costly.
An Improved Method
A new, fast, less costly and more revealing approach to smear removal has been introduced by Chip Targets, a leading electronic analytical laboratory. They have adapted an Ion Milling system, generally used to create ultra-thin Transmission Electron Microscopy samples, to selectively remove SEM sample smear. Chip Targets uses the Bal-Tech RES 101 Ion Mill System, with a two ion beam gun, to readily de-smear samples up to 25mm in diameter. The beam angle can be adjusted from 0 to 45 degrees during removal, and specimen rotation can be continuous or oscillating.
Figures 1 and 2 compare a SEM cross-section of a lead-free BGA connection, before and after ion milling by Chip Targets. Figure 1 shows a metallographic cross-section after standard mechanical preparation with diamond lapping films and polishing with 0.3 µm alumina. The view, from the top down, includes part of the device, the device bond pad, the connecting layer between lead-free ball and the device pad, the ball, the layer connecting it to the substrate pad, and, at the bottom, the organic substrate, including a crossing metal track.
Figure 1. Before ion milling. (courtesy Chip Targets)
Figure 2. After ion milling. (courtesy Chip Targets)
Figure 2 shows the same view after ion milling. Details such as grain structure, attachment, and layer thickness that are obscured by smear in Figure 1 are clear in Figure 2. If you don’t see the differences, put your eyeglasses on.
In other applications, sample preparation with ion milling has allowed Chip Targets to identify delamination between die and die attach, to locate otherwise-invisible hairline cracks, to accurately measure bond line thickness, to examine the grain structure and to locate Kirkendall voids on samples otherwise obscured by smear.
Ion milling is a valuable surface preparation tool offering new capabilities and providing higher accuracy in failure analyses.
For More Information
The Chip Targets web site at http://www.chiptargets.com provides further information, examples, and technical reports.