Scanning Acoustic Microscopy (SAM) is an often under-used process development and failure analysis resource. Part of the cause for this is related to the unknown or under-known science behind the method. Also, many engineers are simply not familiar with applications for acoustic microscopy in electronics manufacturing. SAM is an inexpensive and efficient tool for evaluating such things as printed circuit boards, underfills, BGAs, wire bonds, discrete components, and wafers. Acoustic microscopy holds an advantage over most evaluation methods because it is non-invasive and non-destructive, and unlike X-ray, SAM can evaluate low and high-density plastic materials. SAM is particularly useful for inspection of small, complex devices. It can detect sub-micron air gaps, as thin as 0.13µm (Newton Ring Experiment) and has a defect resolution of 5µm, allowing for inspection of interconnects.

Acoustic microscopy defined
Acoustic microscopy utilizes ultrasound to image a sample. An acoustic wave is transmitted through the device, and transmission and reflection characteristics are used to generate an image of the device. Ultrasound is defined as sound waves with a frequency greater than 20 KHz (greater than what can be heard by the human ear). Typical acoustic microscopes utilize frequencies in the MHz range. SAM systems use sound energy to image a sample through the use of transducers. Samples are submerged in a liquid medium such as distilled water or alcohol to ensure that the ultrasound waves are delivered to and from the samples. (Ultrasound waves propagate most efficiently through liquid and solid materials and vice versa). The transducer converts electrical energy in mechanical vibrations. The ultrasonic transducers send pulses into the liquid and through the samples. The transducer also receives reflected pulses (echoes) from discontinuities and disturbances from the sample. The transducer transforms the reflected sound pulses into electromagnetic pulses which are displayed as pixels with defined gray values thereby creating an image.

The penetration and reflection of the acoustic waves is highly material dependent. Each material has acoustic impedance as seen in Table 6-1. Echoes are created by differences in the acoustic impedance. For example, a small void in a plastic substrate will create an echo reflection because the acoustic impedance of the plastic material differs from that of the gas in the void.

Acoustic microscopy applications
Much like X-rays, scanning acoustic microscopy has an equal number of production and analytical applications. SAM can be used for process control, incoming material quality control, and qualification of components, assemblies, and processes. The method can detect delaminations, die tilt, solder ball joint geometry, underfill coverage, voids, and porosity. Modern high-speed acoustic microscopy systems are capable of imaging a large number of electronics packages during a single analysis. Advanced systems allow for automated inspection and imaging of components in JEDEC trays. This allows non-sacrificial examination of product prior to production, during processing, after testing, or just prior to final delivery.

Some of the more commonly examined components and packages include:

• Bare printed wire boards (PWBs)
• Bonded wafers
• Capacitors
• Ceramics
• COB (Chip On Board)
• CSP (Chip Scale Package)
• Flex circuits
• Flip chips
• MCM (Multi Chip Module)
• PBGA (Plastic Ball Grid Array)
• PDIP (Plastic Dual In-line Package)
• PLCC (Plastic Leaded Chip Carrier)
• PQFP (Plastic Quad Flat Pack)
• Smart cards
• SOIC (Small Outline Integrated Circuit)
• TSOP (Thin Single Outline Package)

Some of the SAM failure analysis and R&D applications include analysis of popcorn cracks, package voids, underfill delaminations, cracks, voids, bonded wafer voids, solder cracks and a number of other process originated defects. The method is particularly sensitive to unintentional air gaps in and between materials. This makes the method suitable for non-destructive examination of bonded layers.

The A-Scan display (Figure 6-1) is needed to properly set up the data acquisition for the test to be performed. This includes focusing the transducer and set data gates at the point of interest within the material. The data gates will detect the reflected echoes of the RF waveform. Detection modes can be Absolute Peak, Peak-to-Peak, Positive Peak or Negative Peak. The data collected within the data gate will be used to generate an amplitude and/or time-of-flight C-Scan image.

The B-scan mode (Figure 6-2) provides a side view of the sample (cross-section). In the B-scan mode, the A-Scan area of interest is gated. The positive amplitude echoes are plotted as bright spots, and the negative amplitude echoes are plotted as dark spots. This method is extremely effective for determining where layer (die layer, interconnect, encapsulate) defects such as voids and delaminations occur.

The most commonly used mode of acoustic microscopy is C-scan (Figure 6-3). C-scan provides a top-down view like an A-scan but generates an image from a scanning transducer. The C-scan is literally a compilation of A-scan amplitude and time-of-flight data taken at different positions along the X-Y axis. C-scans are often used to create interface scans to examine bonding, voiding, and interface integrity. Bulk scans and Time-of-Flight (TOF) scans can also be generated in the C-mode. Unlike interface scans that image specific planes through the sample, bulk scans are excellent for characterizing ceramic materials, voiding, and density distributions. It also allows for identification of defects whose location in the sample is unknown. TOF scans create 3D-like images based on the distance between the top of the sample and the defect.

Using acoustic microscopy for BGA and Flip Chip analysis
Flip chips are particularly well suited to SAM evaluation. The direct thermal expansion differential of the flip chip package means that the components are particularly sensitive to process defects. Polymer underfills are used to increase the mechanical strength and reduce the coefficient of thermal expansion differential between the component and the circuit board. Located under the component, underfills are all but impossible to image optically. The non-conductive polymeric nature of the underfill does not allow them to be effectively imaged using X-rays. SAM should be utilized to characterize the effectiveness of the underfill process (Figure 6-4 – The voids in the underfill are identified by differences in gray scale within the image). Changes in material properties and dispensing characteristics affect the amount of fill, voiding, and adhesion. Acoustic microscopy images underfill materials and other critical areas of silicon die flip chips including:

• Underfill delamination
• Underfill voiding
• Density distribution of filler particles (underfills)
• Interconnect delamination
• Interconnect voiding
• Interconnect cracking
• Die cracks

Plastic ball grid arrays are sensitive to moisture penetration. Trapped moisture in the component can expand and delaminate the component substrate (popcorning) or encapsulated materials. Popcorning is a leading cause of component warpage, inconsistent standoff height, and open solder joints. Acoustic microscopy is one of the only techniques capable of efficiently evaluating popcorning in PBGAs. Analysis of the PBGAs can also include acoustic scans of the die attach layer, solder joints, plastic encapsulation, and the circuit board underneath the BGA.

Conclusion
Acoustic microscopy can effectively evaluate hidden defects in component substrates and assemblies and should be considered when determining different failure analysis and quality control methods. The technique compliments X-ray, optical inspection, cross-sectioning, and decapsulation methods. The non-destructive nature of the test makes it suitable for inspection of expensive individual pieces of hardware as well as multiple lots of components and bare boards. Understanding the SAM test methods ensures the best possible information from the sample is obtained. Smaller component features, increased Pb-free reflow temperatures, a rise in the number of counterfeit components, and increased use of chip scale packages are all driving forces for increased use of acoustic microscopy in electronics manufacturing.

The EMPF would like to thank Matec Micro Electronics of Yardley, Pennsylvania for their technical data and images used in this article.