SAM: Non-destructive inspection of plastic COTS parts and beyond

Suitability of advanced acoustic microscopy techniques for screening plastic COTS devices susceptible to experiencing catastrophic failures.

The applicability of this technique in the inspection and failure analysis of systems besides COTS includes PCBs, metallic assemblies, hermetic packages, electronic potting, RF devices, and others.

During this webinar, we discuss how the Sam technique is the best technique for inspecting commercial EEE components for implementation in high-reliability applications. It provides a deep analysis of the devices’ internal structure, integrity, stability, and defect identification.

We also present a study of ALTER Technology carried out in collaboration with the University of Seville, in which we relate SAM results with Electrical Measurement failure. We also talk about the latest projects in the Scanning Acoustic Microscopy Lab.

Questions & Answers

• Could you please describe the differences between the two modes of SAM – C-SAM and T-SAM?

Confocal inspection (C-Scan mode). This scanning mode analyses the internal structure at a fixed depth within a selected thickness, generating a stratified image of the component. In the resulting image, the higher intensity of the acoustic waves reflected at air interfaces and defects provides a clear contrast. Thus, this scan mode is particularly sensitive to air voids and delaminations and permits the defection of sub-micrometric-thick features.

Through-transmission mode (Th-SAM). Through-transmission mode (Th-SAM) is a scan mode that analyses the intensity of the ultrasound beam that propagates through the entire thickness of the sample. This scan mode is used to thoroughly inspect the whole system in a single scan. In addition, this inspection complementary to confocal reflection analyses (C-SAM) is very effective in the detection of hidden defects at the die attach and to confirm the interpretation of C-SAM findings. The image below compares C-SAM and Th-SAM images that were simultaneously acquired. This comparison shows how the delaminated areas produce a strong reflection (very bright zones in the CSAM image) and low intensity in the transmitted signal.

• Electrical measurements after burn-in + Delamination inspection of packaged ICs: Why are low temperatures worse than high temperatures?

We hypothesize that, in general, such responses to low and high temperatures are due to thermo-mechanical stress, which is accentuated by the presence of delaminations. In low temperatures, this process is aggravated by the moisture that can be trapped in the delaminations. However, a more detailed study based on component specifications would be necessary to prove this hypothesis.

• So is T mode the default go-to, and is it used first? What does the C-Mode give?

C-mode provides phase contrast by highlighting the defective parts, confocal, and higher lateral resolution. C-mode is complemented by the T-mode that provides lower quality images that detect anomalies in the whole volume (more like an X-Ray image).

• Can you tell us the precautions to inspect in water, ics with encapsulated non-hermetic?

The samples are carefully placed into a sample holder and introduced in deionized water. To perform the SAM test, it is necessary to introduce the samples into a coupling medium. By choosing deionized water, we ensure that the components are degraded as little as possible. After the test is performed, the parts undergo a drying process that is agreed with the customer to best suit the needs of the component. This drying can be done baking at a low-medium temperature or “air-drying.”

• What is the minimum size of a defect you can measure laterally and thickness?

All our transducers are capable of detecting defects on the nanometre scale. In particular, the longest-range transducer is capable of detecting defects down to 40µm.

• Which transducers do you have at Alter? That is, which frequencies do you have?

We currently have four different transducers in our lab of 25 MHz, 50 MHz, 75 MHz, and 100 MHz. We are planning to acquire at least one more of 200 MHz.

• Is SAM used as part of space evaluation/qualification activities of devices as well as failure analysis?

Yes, some of the tests flow where we currently perform SAM are Destructive Physical Analysis, Screening, Constructional Analysis, Failure Analysis, Qualification test, etc.

• The 3D X-ray tomography of the part may be needed more exposure to X-ray. Is this more exposure does not affect the part performance?

X-Ray is considered a non-destructive technique, so it should not affect the part performance. Here you have more information about this technique: https://www.doeeet.com/content/testing-eee-parts/inspection-electrical-verification/radiographic-xray-inspection/

• Is it possible to use any other liquid ( apart from water)?

There is the possibility of using other liquids; however, we used deionized water because it recommends it.

• Would it be 1-hour max in DI water for parts with MSL6, or does it need to be less?

We always inform our customers about the test requirements, and we always consult which treatment we should give to the samples. For MSL 6, we keep the samples as little time as possible it is recommended that baking is performed after.

• Have you screened lots for flights? And mounted boards for flight?

Yes, we have both assembled and unassembled components. Since SAM is a non-destructive technique, all the samples can fulfill their function.

• Do you have any statistical data on how this technique is effective? It also probably depends on calibration, right?

We don´t have statistical data. However, we usually check the presence of anomalies by performing cross-sections. On some little occasions, the defects cannot be verified but attribute this to other factors such as low SEM resolution or defects on the nanometre scale filled with dragged material during sectioning. They are not considered as a “false positive.”

• Is it feasible to perform SAM simultaneously with thermal cycling (i.e., inside a thermal chamber)?

We do not perform these tests simultaneously, the SAM equipment is too big to fit inside of a thermal cycling chamber, and also the SAM test should be performed at room temperature to ensure the water evaporates as little as possible, and also that the water at high temperature does not degrade the samples.
However, it is widespread that we perform SAM before and after thermal cycling since this process usually originates from delaminations.