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.
return to failure modes