PWB Barrel Cracks – Wear Out Failures

When exposed to thermal cycle testing, cracks that develop slowly in the copper barrel of plated through holes (PTH) is considered to be a wear out failure mode. This failure mode is often observed in boards that survive hundreds or thousands of thermal cycles and is the failure mode of robust PWBs. This is a failure that develops slowly over time. There is an initiation of cracks in the barrel of the PTH that can advance through the metallization and the electrodeposited copper layers. The crack typically propagates along the boundary between copper crystals and traverses the thickness of the barrel in a tortuous path. The cracks usually initiate at glass fibers that are on the inner edge of the drill hole and frequently the crack angle is 30° to 50° off horizontal. Once initiated the cracks slowly develop over hundreds of cycles. With a wear out failure mode the damage accumulates at a constant rate.

It is possible to create a graph of damage accumulation during thermal cycle testing by plotting resistance, taken at the maximum test temperature, for each cycle. The initial resistance measurement of the first thermal excursion, at the maximum temperature, establishes the baseline condition against which subsequent measurements are compared. It is assumed that this first resistance measurement is a reflection of an undamaged circuit, and, if there is an increase in resistance in subsequent thermal cycles it is considered to reflect damage accumulation as a result of cracks in copper or separations between internal structures. A 10% increase in resistance, typically measured in milliohms, is considered a failure. The top of the graph is a failure at 10% increase in resistance; the x-axis variable is thermal cycles.

Plotting a resistance graph of damage accumulation allows one to determine onset, rate of accumulation and if damage is accelerating. Different failure modes tend to produce different damage profiles. Armed with a resistance graph, and cycles to failure data, insight on the failure mode can be gained. Degradation of copper and material and stress relief may be inferred by the damage profile. A constant slow accumulation of resistance over the duration of the test suggests metal fatigue.

Concerns and Considerations - Frequently cracks are in the central zone of the barrel of the PTH. During thermal excursions the PTH is under tension from z-axis expansion. Above the glass transition temperature (Tg), typically between 130°C and 190°C in “FR4” material, the coefficient of thermal expansion (CTE) in the z-axis is typically around 150 to 350 ppm/°C. The x-axis and y-axis compression of the copper barrel is approximately 20 to 40 ppm/°C above Tg. The warp and the weft of the glass fiber restrain thermal expansion in the x-axis and the y-axis. The direction of the fiber glass warp exhibits a slightly lower CTE than the weft direction.

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