Copper Fatigue in PTH and Microvias

The density of printed circuit boards is constantly increasing. Plated through holes (PTH) have been used for many applications to connects between board layers. These PTH structures can fail due to copper fatigue induced by temperature cycling.

The coefficient of thermal expansion of the board in the out-of-plane direction is usually much higher than that of the electroplated copper in the PTH. The mismatch in CTEs between the two materials causes the board to stretch the copper when the temperature increases and a reverse motion when the temperature decreases. This is the main driver for fatigue fracture in the copper barrel.

This is also the main driver for fracture in the PTH flange. The board expansion creates a bending moment at the corner of the PTH flange. This bending is reversed at when the temperature decreases. The fracture initiates at the inner corner of the flange.

In high density boards drilled plated through holes are commonly being replaced with stacked microvias. Stacked microvias are also made of electroplated copper and are just as susceptible to fatigue fracture. The CTE mismatch between the copper and the board is still the driving force but the different geometry means that the fracture will appear at different locations. 

The upper corner flange crack is in a similar location to the PTH flange crack location. The lower corner via crack initiates in the location of geometric discontinuity. The sharp change in cross-section from the landing pad to the bottom part of the via causes a stress concentration along the perimeter.

With current manufacturing techniques, the stacked microvias can be more reliable than plated through holes. With smaller diameters of microvias, their aspect ratios will increase. The board copper and laminate layer thicknesses will have to be considered carefully to avoid long term reliability concerns. 

Copyright Gil Sharon April 13, 2016  . All rights reserved.