With the miniaturization of electronic products and the environmental demands for lead-free and halogen-free materials, PCBs are evolving towards higher density, increased Tg values, and environmental sustainability. However, due to cost and technical constraints, numerous failures arise during PCB production and application, leading to quality disputes. Conducting failure analysis on these cases is essential to pinpoint the causes, find solutions, and assign responsibilities.
Basic Procedure of Failure Analysis
To accurately determine the cause or mechanism of PCB failures, it is crucial to adhere to basic principles and a systematic analysis process. Neglecting these steps may result in missing valuable failure information, hampering the analysis or drawing incorrect conclusions. The general process starts with identifying the failure location and mode based on the observed failure phenomena, gathered information, functional testing, electrical performance evaluations, and initial visual inspections.
Locating failures is straightforward for simple PCBs or PCBA, but for complex structures like BGA or MCM packages, microscopic observation may not reveal defects readily, necessitating alternative methods for detection.
Then, the failure mechanism must be analyzed. Various physical and chemical methods are used to investigate the causes of PCB failures or defects, such as virtual welding, contamination, mechanical damage, moisture stress, medium corrosion, fatigue, CAF (Conductive Anodic Filament) or ion migration, and stress overload.
Next, the cause of failure is analyzed. Based on the mechanism identified and the process analysis, the root cause of the failure is determined, with necessary test verifications. Emphasis should be on thorough testing to accurately pinpoint the induced failure cause.
This analysis provides a targeted basis for subsequent improvements. Finally, a failure analysis report is compiled based on the test data, facts, and conclusions drawn during the analysis process. The report demands clarity, strict logical reasoning, and well-organized presentation, avoiding speculative claims.
During the analysis process, adhere to basic principles: start with simpler methods, progress to more complex ones, and prioritize non-destructive techniques before resorting to destructive ones. This approach minimizes the risk of losing critical information or introducing new artificial failure mechanisms.
Optical Microscopy is primarily employed for PCB visual inspections, identifying failure points and physical evidence, and initially determining PCB failure modes. Visual inspections focus on PCB contamination, corrosion, board burst locations, circuit wiring, and failure patterns (batch or isolated occurrences, concentrated areas).
X-ray Inspection
For parts inaccessible to visual inspection and internal defects in PCB through-holes, an X-ray fluoroscopy system is indispensable. This system utilizes principles of X-ray absorption/transmission varying with material thickness or density to produce images. It is particularly effective for inspecting PCBA solder joint defects, through-hole internal defects, and locating faulty solder joints in high-density packaging such as BGA or CSP devices.
Sectional Analysis
Sectioning analysis involves obtaining cross-sectional PCB structures through sampling, embedding, slicing, polishing, corrosion, and observation. This method provides detailed microstructural information (e.g., through-holes, plating quality) crucial for quality improvements, albeit destructively.
Scanning Acoustic Microscopy
C-mode ultrasonic scanning acoustic microscopy is pivotal for electronic assembly analysis. It utilizes high-frequency ultrasonic waves to detect amplitude, phase, and polarity changes reflecting material interface discrepancies. This method scans along the Z-axis for XY-plane information, detecting defects like cracks, delamination, inclusions, and voids in components, materials, PCBs, and PCBAs.
Micro-Infrared Analysis
Micro-infrared analysis integrates infrared spectroscopy with microscopy to analyze material compound compositions based on differential infrared absorption (primarily organic matter). This method is essential for identifying trace organic contaminants affecting PCB pad or lead pin solderability, crucial in resolving process-related issues.
Scanning Electron Microscopy (SEM)
SEM is pivotal in PCB failure analysis, providing high-resolution electron microscopy for topographical observations. It’s instrumental in analyzing pad surface topography, solder joint metallurgy, intermetallic compounds, solderability coatings, and tin whisker formation. Unlike optical microscopy, SEM images are monochromatic and require conductive samples, necessitating gold or carbon coating for non-conductive surfaces to prevent charge accumulation.
Thermal Analysis
Differential Scanning Calorimetry (DSC) measures heat differences between test and reference materials under controlled temperature programs. This analytical method determines material thermodynamic properties crucial for evaluating PCB polymer material curing and glass transition temperatures, vital for subsequent process reliability assessments.
This revised version maintains the original structure while enhancing readability and clarity throughout the technical details.
