Just as medical ultrasound equipment is used to examine bodily structures that are normally hidden underneath the skin, SAM uses high-frequency sound waves to image subsurface features of electronic components. By studying the characteristics of these ultrasonic sound waves after they interact with a sample, the failure analyst can detect physical anomalies ranging from air gaps or delamination in plastic encapsulant material to cracked ceramic dielectrics in a capacitor. Indeed, by using frequencies above 100MHz, defects as small as a crack in the die bumps of a flip-chip BGA can be identified.
To create and analyze these ultrasonic waves, piezoelectric transducers are used. Piezoelectrics are crystalline materials with a unique property; when an external voltage is applied to a piezoelectric, the material will deform. Similarly, when a piezoelectric material is subjected to physical stress, an electrical charge is generated. In a Scanning Acoustic Microscope, voltage is applied to the piezoelectric transducer, which emits an ultrasonic wave due to the deformation of the crystal; this ultrasound interacts with the device under test, and is then recaptured by the same transducer (or, depending on operating mode, by a secondary transducer) and converted into an electrical charge, which can then be processed to generate an image.
The frequencies used for Scanning Acoustic Microscopy are more than two orders of magnitude higher than the highest frequency of sound that can be detected by the average human ear. Since these ultra-high frequency sound waves cannot travel through the air for any appreciable distance, it is necessary to use a transmission medium like water to facilitate acoustic microscopy. The sample and piezoelectric transducer must be immersed in water in order to allow good transmission of the ultrasound waves; this means, of course, that the sample must be able to survive underwater for the duration of the scan.
There are several different analysis techniques that can be used with a Scanning Acoustic Microscope, each providing the electronic failure analyst with different information about the sample. An A-Scan can be used to examine the amplitude and phase of a reflected or transmitted ultrasonic pulse; with this information, an analyst can quickly identify the physical interfaces within a device and determine whether or not delamination exists between layers of a structure. The B-Scan uses the reflected ultrasonic waves to construct a “virtual cross-section” of the sample under test, allowing the analyst to indirectly view the device's internal construction, while the C-Scan allows the analyst to focus on a single interface, using reflected or transmitted ultrasound to create an image that can be studied to look for any packaging defects.
By using the appropriate mode of Scanning Acoustic Microscopy for electronic failure analysis, our analysts can get an accurate picture of the construction and integrity of a component. With this picture in hand, our engineers can provide the customer with valuable data and insight which can be used to better understand and correct the problem causing the failure.