Scanning Acoustic Microscopy

Overview

One common investigation during a failure analysis project or reliability study is a package integrity evaluation - looking for delamination, popcorn cracking, or other anomalies that may reduce or even end the lifespan of a part. These sorts of anomalies can cause mechanical stresses on bond wires, provide a point of ingress for corrosive chemicals or other contaminants, or make the device unsuitable for use in its intended environment. With the huge variety of package types on the market, understanding which tests to apply can be difficult; fortunately, IAL has extensive experience with a wide range of tools and techniques to facilitate your package integrity analysis needs.

Fundamentals

Most package integrity analysis begins with an optical inspection to identify any gross anomalies (obvious cracks or voids, for example). Optical inspections take very little time, and can be done with no impact to the device under test; no reconditioning or special care is needed to use the sample in production if an optical inspection is the only test a device is subjected to. While this inspection is a good first step, the absence of visible defects does not necessarily imply good package integrity, as other anomalies may be lurking beneath the device’s surface.

  • Acoustic microscopy revealed severe delamination at the die-to-encapsulant interface on one corner of this device. This delamination may be a pre-existing condition that could cause reliability issues, or it may be indicative of a failure!
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Generally speaking, the majority of microelectronics are encapsulated using an epoxy mold compound that is formed over the semiconductor die and related interconnects (e.g. the leadframe of an SOIC, or the circuit board substrate of a BGA). The most common way of determining integrity of these types of packages is acoustic microscopy, a technique that uses focused ultrasonic waves to non-destructively probe a package for delamination or cracking. These defects commonly result from mechanical stresses, and as such are often screened for during reliability testing (such as Moisture Sensitivity Level qualification).

In other cases, it may be necessary to perform more destructive analysis in order to get a greater depth of detail in determining package integrity. Dye penetrant is one such destructive test; plastic encapsulated or hermetically sealed samples are immersed in a vat of dye, then subjected to vacuum and pressure to force the dye into any cracks or voids. The device is then fractured, and any dye incursion is documented. Cross-sectional analysis also provides excellent information, allowing cracks, delamination, or other anomalies to be directly viewed under a microscope.

Sample Types

Though there are a multitude of package integrity tests, not all tests are applicable to each type of sample; as an example, acoustic microscopy does not generally make sense to analyze hermetically sealed packages, as the thick ceramic layers and air gaps in the package do not allow good propagation of the ultrasonic waves. For more information about which tests may be applicable to your products, please feel free to consult with one of IAL’s engineers, who are glad to help determine an optimal course of analysis.

Applications

  • Moisture Sensitivity Level (MSL) qualification and screening
  • Finding voids or poor adhesion in die attach material
  • Inspecting the underfill and solder connections of a flip-chip BGA
  • Identifying leaks in a hermetically sealed package

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