Although there can be several reasons for failure of plated through holes (PTH) in Printed Circuit Boards (PCBs), the primary reason and most frequent is thermomechanical fatigue. PCB Trace Technologies Inc. has analyzed the typical failures in PTH induced by thermal and mechanical reasons.
Apart from mechanical reasons such as twisting of a PCB resulting in corner cracks, mismatch between the coefficient of thermal expansion (CTE) of the materials in the PCB produces the most thermomechanical stress. Typically, most laminate materials for PCB fabrication, including other dielectric polymers, have a larger CTE as compared to copper traces, pads, and the plating of the PTH. Operational power dissipation and/or ambient temperature changes therefore, can induce cyclic stress due to the substrate material and copper features in the PCB expanding or contracting at different rates, leading to fatigue failures.
Moreover, PCB laminates exhibit different values of CTE in different directions. This large anisotropy in the CTE generates various types of PTH failures. Addition of reinforcement materials like glass fibers or aramid fibers embedded in laminates usually cause such anisotropic response. Reinforcement materials reduce the CTE in the X and Y directions significantly, but they have a minimum effect on the thermal expansion of the material in the Z direction. This leads to several common failures within the PTH structure.
Barrel Cracks
High CTE contrast in the Z-direction between the copper plating and the organic resin of the substrate can result in cracks appearing in the PTH barrel after thermal cycling. The high deformation in the Z-axis is due to the embedded reinforcement in the substrate producing a lower effect on the Z-CTE. With increase in temperature, the substrate expands more in the Z-direction, cracking the barrel as the PTH plating is unable to expand at the same rate.
The barrel crack failure starts typically at glass fibers present on the inner edge of the PTH. Once initiated, the crack advances through the copper plating, propagating inside the thickness of the barrel.
PCB Trace Technologies Inc. conducts thermal cycling tests to detect barrel cracks. If the temperature is below the resin glass transition temperature, the thermally induced fatigue crack typically propagates into the barrel of the PTH. According to our experience, boards with pre-existing damage in the material fail after only a few cycles, whereas, robust materials can withstand several thousand thermal cycles without damage.
Inner-Layer Cracks
Inner-layer cracks can cause inner-layer copper foil to detach from the PTH. This can result in the separation of electrolytic copper and copper flash, or the separation between the copper flash and the foil. Compared to high Z-axis CTE mismatch causing barrel cracks, in-plane resin expansion in the X-Y direction into the free space of the PTH causes inner layers to crack. The expanding resin deforms the copper barrel, forcing the copper foil to detach from the PTH wall.
PCB Trace Technologies Inc. uses good quality copper foil that prevents or significantly reduces this failure mode. We also ensure that the barrel thickness of the PTH is adequate. With a proper thickness, the barrel wall can deform elastically when the resin expands in the X-Y direction, thereby preventing inner layer cracks.
If the barrel wall is very thin, resin expansion in the X-Y direction causes it to deform plastically, leading to generation of large tensile stresses and failure at the inner contact points of the PTH.
Detecting Thermomechanical PTH Failures
Conventional visual inspections, even assisted with the aid of high magnification systems, will usually not reveal the real PTH status. Detection of PTH imperfections requires a mandatory microsection analysis. Apart from the assessment of the quality, microsection analysis provides critical information about the likely failure mechanism and causes. This makes the technique a valuable tool for analyzing failure. Comprehensive micro-sectioning inspections of PTH usually reveals a variety of defects and deviations.
PCB Trace Technologies Inc. regularly conducts internal microsection inspections through a PTH for detecting thermomechanical failures. The fail locations, as described above, are a clear indication of the cause of failure observed by electrical conductivity monitoring. For this, we combine micro-section inspection and fluorescence microscopy techniques. These techniques are effective in highlighting small anomalies and avoiding false rejections.
We have a specifically dedicated laboratory and Electronics Systems Association (ESA) certified staff for micro-section examination of PCBs. ESA recognizes laboratories with higher quality standards that perform micro-sections on PCBs. For its high-level standards, ESA recommends our laboratories at PCB Trace Technologies Inc. to conduct such metallographic inspections on SMT PCBs.
PCB Trace Technologies Inc. manufactures PCBs conforming to quality standards such as MIL-PRF-31032/IC, ECSS-Q-ST-70-60C Dir.1, or IPC-A-600 for the requirements of a functional PTH for ensuring the general quality of the board.
Conclusion
PCBs contain several joined materials that operate in environments experiencing cyclic temperature fluctuations. The CTEs of these materials are usually different, imposing cycling strain, resulting in thermomechanical fatigue and different forms of failure. Even small temperature changes can have a large effect, depending on the CTE difference of the materials and their joint thickness. PCB Trace Technologies Inc. uses high quality material combined with judicious selection of CTE for practically eliminating thermomechanical failure.
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