Highly Accelerated Life Testing (HALT) is designed to induce product failure to determine failure site, failure mechanism and root cause. Omitting a thorough post-HALT root cause analysis prevents improvement in design margins and, overall, eliminates the value of the HALT process.
To use HALT to its best advantage, take a systematic approach to root cause analysis, proceeding from the least destructive to most destructive methodology until root causes are conclusively identified and the failure mode, site and mechanism are categorized. Here’s how:
The crucial first step in any failure analysis effort, information gathering needs to be done in an organized way, using standard nomenclature.
Crucial failure data to collect revolves around:
- Failure history: when the failure occurred; the stresses being applied to determine possible failure location; and, any changes the device being tested underwent
- Failure mode: the failure behavior experienced by the observer
- Failure site: the presumed location of the failure
- Failure mechanism: the factor causing the failure (like vibration or temperature cycling), not considered a root cause
Non-Destructive Evaluation Methods
Non-Destructive Evaluation (NDE) is designed to provide maximum information with minimal risk of damaging or destroying physical evidence.
A broad range of NDE methods are available, including:
- Electrical Characterization is considered the most critical step in failure analysis, especially for failures induced during temperature step stress testing. Electrical characterization is the only way to determine recoverable failures (operational limits) and permanent failures (destruct limits). There are a number of common electrical characterization techniques:
- Joint Task Action Group (JTAG) boundary scan: Accurately identifies failure site
- Oscilloscope: Useful in probing operational circuitry
- Resistance measurements: A binary approach primarily used on electrical opens
- Isolation of attached components: Applying electrical characterization without component removal
- Parametric characterization: Comparison of performance to datasheet specifications
- Curve tracer: Characterizes diode, transistor and resistance behavior
- Time Domain Reflectometry (TDR): Measures phase shift of return signal to locate electrical opens
- LCR Meter: Measures inductance, capacitance and resistance of electronic components; in simpler versions, impedance is measured internally and converted for display to the corresponding capacitance or inductance value
Regardless of electrical characterization technique chosen, reintroduction of environmental stresses is sometimes required for proper failure analysis, especially in elevated temperatures.
- Visual Inspection is exactly as the name implies – the product is observed in its physical state to detect defects.
- X-ray Microscopy has the greatest potential for finding solder joint failures under area array devices. It detects voiding or poor wetting, providing evidence of root cause failures during the application of cyclic stresses. A major detractor, however, is difficulty related to identifying micro-cracks that occur during solder fatigue or fracture.
- Thermal Imaging is best used when individual components fail, especially when there is elevated current leakage. Although rarely used, the process can provide evidence of excessive temperature rise and identify root cause of failures occurring at temperatures below expectations.
- Superconducting Quantum Interference Device (SQUID) Microscopy uses a highly sensitive magnetic detector to find faults in circuits. SQUID requires that the device being tested is a flat shape, and is often preferred to thermal testing because it causes very little damage.
- Acoustic Microscopy uses high or ultra high frequency ultrasound to penetrate solid materials and reveal visible images of internal defects like cracks, delaminations and voids.
Destructive Evaluation Methods
Destructive Evaluation (DE) is used when information obtained during NDE is insufficient to conclusively identify the root cause of failure.
DE primarily consists of two techniques:
- Cross Sectioning identifies the failure site and removes a sample using band saws with diamond-encrusted blades to minimize damage to the printed circuit board assembly (PCBA). The sample is epoxy-mounted near the plane of interest to prevent relative movement and improve handling during grinding and polishing, which is done to reveal microstructural elements.
- Decapsulation uses acid to remove encapsulant and expose internal workings of integrated circuits. It analyzes on-die failures primarily created by hot step stress tests, find wirebond interconnect issues, and determine if electrostatic discharge (ESD) and/or electrical overstress (EOS) may have affected the die surface.
Sherlock Automated Design Analysis™ Software
Sherlock Automated Design Analysis™ software can be used to asses the validity of the failure mechanisms and provide mode shapes and stresses predictions during vibration, provided the vibration loads were sufficient to fail the interconnect and the defects were identified during failure analysis. If not, there may be an issue with the testing procedure.
Combining Sherlock with HALT root cause analysis can be powerful. Contact us today if you are interested in learning more, or check out our Common Mistakes by Electronic Design Teams webinar by clicking the link below.