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How Scanning Acoustic Microscopy Aids in Electronics Failure Analysis

Posted by James McLeish on Apr 25, 2017 11:20:00 AM

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Decapsulation and cross-sectioning are two failure analysis techniques frequently used to root cause failure issues in electronics.  However, these destructive methods that are expensive and time consuming,  can be made more effective by the use of Scanning Acoustic Microscopy (SAM)  to identify the most likely failure site for destructive methods to be focused on. 

SAM is used by failure analysts as a step in the root cause analysis of a failed product. It is typically used similar to X-Rays to identify the sites of potential abnormalities,  within a component or circuit board, to target specific suspect locations for destructive cross sectioning which is then used to identify the specific type of abnormality and its root cause. 

What is scanning acoustic microscopy?

 Scanning acoustic microscopy is a non-destructive analysis approach that uses high or ultra-high frequency ultrasound to penetrate solid materials.  It is used to image the internal features of a specimen to reveal internal defects like delamination, voids, cracks and bond failures.

In other words, SAM is typically performed as a precursor to a destructive analysis procedure, to locate an internal abnormality site to target the place to place a cross section cut.  This eliminates the need to perform multiple incremental section cuts in a trial and error approach to hunt for a failure site to identify the root cause. 

How does Scanning acoustic microscopy work?

In the broadest sense, Scanning Acoustic Microscopy uses focused sound waves (similar to sonar) to inspect internal structures. Is also known as scanning acoustic tomography. 

The scan requires that the component or item of interest is immersed in water, along with the tip of the acoustic transducer.  The water provides the media for transmission of focused sonic energy to a small point on the sample of interest.

Sonic pulses of different frequencies are used to penetrate various materials.  As a sonic pulse hits the sample of interest, it is either scattered, absorbed of reflected back to the transducer.  At interfaces between materials having different acoustic impedances, acoustic reflections (an echo) occurs  The 'time of flight' of the pulse, the time for a sonic pulse to be emitted by the acoustic source, scattered by an object and received back by detectors under and integrated in the transduced head is determined.  The intensity and polarity of this echo is recorded. 

Then the transducer is rapidly moved slightly and a new sonic pulse is generated.  This process is repeated in a systematic pattern until the entire region of interest has been scanned.  The value for each scanned point are assembled by the SAM computer into an colored sonic image of the interior of the object of interest. 

The SAM can use high or ultra-high frequency sound waves for more or less sample penetration and clearer resolution.  High frequency sonic pulses produce higher resolution images at shorter focal lengths that provide less penetration, used on thinner devices. Lower frequency sonic pulses produce lower resolution images are longer focal length that provide greater penetration, used on thicker devices.  So unlike X-Rays a SAM FA technician has the ability to adjust for the depth penetration appropriate for their imaging and information gathering needs.

In addition to frequencies the SAM FA technician can choose to apply scanning acoustic microscopy using transducers to scan electronics components and reflect ultrasound waves for varying degrees of spatial details. Transmission, amplitude and phase inversion imaging are also options with scanning acoustic microscopy that are particularly effective in detecting delamination or voiding.

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SAM mechanism diagram

 

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SAM image example - the colored hot spot denoted locations of internal voids or delaminiation in an integrated circuit

How does scanning acoustic microscopy benefit failure analysis in electronics?

An obvious benefit of using scanning acoustic microscopy is its inherent value as a non-destructive method for identifying failure sites in order to target where destructive failure analysis cross sections needs to be positioned to identify the failure type and root cause.

The component failures SAM is applicable to include:

  • Cracking
  • Delamination or voiding
  • Some electrical shorting or electrical opens within a component

The depth and accuracy provided by SAM makes it a useful technique for identifying the location of abnormalities within an electronic component. 
This makes SAM a preferred non-destructive precursor to destructive failure analysis method to make failure analysis faster and more effective.   

For information on design considerations for mitigating the effects of coefficient of thermal expansion (CTE) mismatch, watch our webinar, Solving Problems of Overly Constrained Boards. Click the button below to download your free copy.

Overly-Constrained-Boards

 

Topics: Failure Analysis