Failure Analysis Of Electronic Devices Using Scanning Acoustic Microscopy

Scanning acoustic microscopy, or SAM, is a non-destructive technique used in failure analysis of complex devices. SAM can provide a resolution down to sub-micron thicknesses. SAM is an efficient tool for analysis of adhesion between layers and presence of possible flaws in each layer. This can be used e.g. for investigations of sealing, coating, flip-chip underfills, BGA, QFN, wafer to wafer bonding etc.

In SAM ultrasound waves propagate through liquids and solids. Whenever there is change in acoustic impedance such as at interface/boundary of internal flaws, change of material or change of density in the same material, partial reflection and transmission takes place. The amplitude, polarity and the time of flight of the reflected signal provide important information.

Characteristic acoustic impedance, Z, of a material is given by

Z = r.v

Where, ρ = density of the material, and v = velocity of sound in the material.

Fig.1. Basic working principle of SAM. Delamination/air gap on the left side and good adhesion interface structures on the left are shown.

Fig. 2. Schematic presentation of pulse-echo mode operated SAM instrumentation.

Characteristic impedance of air is several orders of magnitude lower than that of solids, and this leads to nearly 100% reflection. This total reflection at air gaps/delamination is what makes SAM unique in interface quality investigations in all type of structures. In SAM equipment, a liquid bath – couplant – is used to transmit acoustic waves between transducer and the sample. Water is traditionally used as a couplant due to practical reasons. Fig. 1 illustrates the working principle of SAM.

P_R = │ (Z₂-Z₁) │/(Z₂+Z₁)   P_T = 2Z₂/(Z₂-Z₁)

Where P_I = incident wave amplitude, P_R = reflected wave amplitude and P_T= transmitted wave amplitude. As it can be seen from the reflection formula, higher characteristic impedance difference means higher reflection signal amplitude.

Schematic configuration of a scanning acoustic microscopy is shown in fig. 2. It consists of:

• A piezoelectric transducer that sends pulses of acoustic waves through liquid couplant into the sample. Between pulses, a receiver takes the echoes reflecting from the sample. In the C-SAM transmitter and receiver is the same piezoelectric transducer which electronically switches between two modes of pulse sending and echo receiving.
• A mechanical scanning unit which enables to focus the signals onto the interest area and raster scanning of the sample.
• A control unit or PC where the scanning and the data analyses are provided by the software system.

There are two inspection modes; pulse-echo and through-transmission modes. The reflection signal is used for pulse-echo mode imaging, while the transmitted signal is used for through-transmission mode imaging.

Pulse-echo mode can determine and locate the delamination, defect and voids in bulk material and provide high spatial resolution images. On the other hand, through-transmission images have a less spatial resolution and cannot locate the defect position in bulk sample. Through-transmission mode works as a complementary tool for pulse-echo mode findings and BGA scanning.

Spatial resolution in SAM increases with increasing frequency, but higher frequency means lover depth information at the same time. The trade-off between a low and a high frequency transducer is in the depth of penetration and resolution.

Case stories
Having described the principles of the scanning acoustic microscope we will now present some cases which are studied by SAM technique. Below are four distinct cases which demonstrate how SAM technique practically and effectively detects the failures. Due to confidential nature, limited explanation is given about sample details.

Voids in underfill material in flip chip devices
Company A encounters short failures in their flip chip devices mounted on PCBA. The customer believes that voids in the underfill material between the solder balls result in short circuit during the reflow process. X-ray analysis was used for investigation. X-ray detected existing short circuits, but not voids.

SAM technique was used for non-destructive analysis of the devices. SAM analysis was conducted through the epoxy mold side. Due to the two-sided and populated PCBA structure, scanning through the substrate side was not effective enough. A demonstrative SAM micrograph on a virgin sample is shown in fig. 3. The rectangular white texture between the solder balls in the rows shows voids.

Fig. 3. SAM micrograph of an original sample. The white rectangular texture between solder balls implies voids in underfill material.

Fig. 4 SAM micrograph after removal of mold, UHF transducer is used.

In a SAM analysis, un-curved and plane surfaces help achieve the best results by keeping the surface in focus and preventing diffraction of the signals during x-y plane scanning. In this case, the surface was rough and curved. High frequencies, such as 100 MHz and UHF (Ultra High Frequency) transducers, were therefore useless.

For sharp images, destructive removal of the top epoxy mold was conducted by gentle grinding. This enables high frequency transducer usage and results in much better resolution.

By subsequent destructive analysis it was verified that all underfill voids were being detected by optical inspection after removal of the die, had been detected in the SAM analysis.

Delamination study in round objects
Company B had an issue with one of their sub-components. The reason for this issue was attributed to delamination between epoxy encapsulation and metal deposited ceramic electrode. The device has a round structure and length of a couple of tens of millimetres, while having a circumference around 10mm. Destructive analysis techniques, such as cross-sectioning, were used successfully for detection of delamination, but customer B wanted a non-destructive analysis option.

Even though SAM is conventionally used for flat-surfaced objects, usage for round objects is also gaining ground. A narrow scan line approximately 0.5 mm wide along the device was executed at several locations by turning the device using a 50 MHz transducer. The resulting SAM micrograph is given in fig. 5. The device in the image was expected to have partial delamination by the customer prior to SAM analysis in DELTA.

Fig. 5. SAM micrograph (A) of a partial delamination expected device with corresponding B-scan (cross-sectional scan), Phase Gate mode image of corresponding scan micrograph (B) is shown.

As it can be seen from fig. 5, different modes of scanning possibilities were used and they showed consensus about the delaminated and the not-delaminated regions. It is important to confirm the results with a different mode of scanning in SAM, as this enables the operator for correct interpretation of the results before making the final conclusion.

In order to verify SAM results, dye penetrant test was used to highlight the delaminated areas and observe visually. Resulting stereomicroscope image is given in fig. 6 for partially delaminated device. The yellow line on the image shows the scan line along the device.

Fig. 6. Optical micrograph of partially delamination expected device, yellow line shows a demonstrative scan line along the device. Red area at the scan start shows the delaminated part of the interface in consensus with SAM results given in fig. 5.

Coating quality – voids in solder mask study
SAM technique is often used for coating quality studies such as for thickness measurement, intrusions of contaminants, delamination and trapped air bubbles/voids.

Customer C would like to investigate the quality of the solder mask for voids which work as a moisture trap and in the long term cause moisture-related corrosion failures in PCBA assemblies.

In fig. 7 SAM micrograph is given with cross-sectional SEM (Scanning Electron Microscopy) image of the same sample. The SEM image was obtained after destructive cross-sectioning the sample. For the purpose of the analysis, SAM detected all voids in one scan in minutes. The alternative method used is cross-sectioning at random location, subsequent SEM inspection for voids, and then interpolating the result for the whole surface area. SAM provides this type of analysis – voids in coating – for transparent and opaque coatings non-destructively.

Fig. 7. SAM micrograph of PCBA (A), and SEM image of cross-sectional analysis of PCBA (B). SAM shows total void population in one scan in minutes. Alternatively SEM is used combined with cross-sectioning and interpolating result to the whole surface area of the PCBA.

Obsolete components – re-tinned
Company D had stored components from ‘last time buy,’ but needed to re-tin the devices so they could be used in a lead-free process. Components are subsequently tested for functionality. Even though the components passed functional tests, the customer requested SAM analysis. The obtained results are given in fig. 8.

Fig. 8. SAM micrographs for three obsolete SMDs. Device top and rear side SAM micrographs are presented together. Phase gate mode is used for highlighting delaminated interfaces for easy understanding of the scan results. Rear side micrographs were provided after removal from PCBA.

Conclusion
SAM is a non-destructive technique used in the failure analysis, and here its versatile usage for different purposes is presented. Compared to alternative methods for the same purposes, SAM is practical and less time consuming. In this article successful SAM analyses of different subjects and failures were demonstrated for:

• Voids in underfill material in flip chip ICs
• Delamination between epoxy shell and metal electrode for round objects
• Coating quality of solder mask for air bubbles – for transparent and opaque coatings
• Delamination and pop-corn failures in re-tinned obsolete components

Acknowledgement
This article is issued under the project ‘Physics of Failure based reliable product development’ which is carried out by DELTA Denmark.