Reliability is a balance between force and strength, and when the force is greater than the strength of the copper and dielectric material, the PCB fails.
When exposed to thermal cycle testing, copper cracks that develop at an ever-increasing rate in the barrel of plated through holes (PTHs) represent an accelerated failure mode. This is a failure mode where material change plays a major role in how the circuits fail. The damage may have a delayed onset, but once initiated, tend to accelerate until failure as noted by a 10% increase in resistance is complete. Cracks in the barrel of the PTH advance through the metallization and the electrodeposited copper layers, propagating through copper crystals and traversing the barrel in a straight, horizontal path. The cracks are frequently found to be open upon microscopic review.
The resistance graph, FIGURE 1, demonstrates that at the beginning of the test, there was a degree of stress relieving or annealing until 150 cycles. It appears the annealing reduced the stress on the copper traces and interconnections, resulting in a small reduction in resistance for a number of cycles.
The damage accumulation was more or less linear until the resistance increased to 2%, after which, a non-linear acceleration was observed. There was an exponential accumulation of damage observed until failure at approximately 375 cycles. The damage profile in the resistance graph is typical for an acceleration type failure.
Dynamic mechanical analysis (DMA) is a thermal analysis method that measures a material’s viscoelastic properties. The test induces a mechanical displacement in a sample by flexing and measures the material’s resistance to the displacement and its ability to return to the original condition over a range of temperatures. This method measures the relationship between viscous and elastic characteristics of the material that may change in response to thermal cycles. A material that is elastic deforms in response to a force and returns to its original shape when the forces is removed. A material that deforms in response to a force and does not return to its original shape is said to be plastically deformed. Plastically deformed material around a failing PTH may prevent cracks from closing between thermal excursions. It is likely that the acceleration observed in this failure mode is in part due to the slowly accumulating plastic deformation of the dielectric material in response to thermal cycles.
A microsection of a failing PTH with an accelerating resistance profile frequently exhibits barrel cracks that are open at ambient with a gap between crack edges. Once the crack has initiated, damage accumulation begins to accelerate in response to the dielectric material losing elasticity and being plastically deformed. The slowly accumulating plastic deformation of the material, in response to thermal excursions, causes the copper cracks to open and become larger, expanding a little more with each thermal cycle.
Hysteresis is a condition where a system resists returning to the predicted or original state when a force is applied or removed. There is a physical hysteresis that is reflected in the deformation of the material preventing cracks from closing between thermal excursions. Developing cracks are expressed as a hysteresis in the resistance graph at ambient and at the maximum test temperature.
Conclusion
Reliability is a balance between force and strength. When the applied force is greater than the strength of the copper and dielectric material, the PCB fails. During thermal excursions, the PTH is under tension from z-axis expansion and, at the same time, an inward compression in the middle of the PTH due to x- and y-axis expansion. Once initiated, copper cracks develop rapidly to produce early failures. This accelerating failure mode is the result of the force, induced by thermal expansion of the material being greater than the strength of the copper interconnections and the dielectric material combined. At the same time, the material loses elasticity and undergoes plastic deformation. PCD&F
Paul Reid is program coordinator at PWB Interconnect Solutions Inc.; This email address is being protected from spambots. You need JavaScript enabled to view it..
