Trelic’s picture of the week – 04/2017

Can you guess what does this picture represent?

The last week’s picture gave a short introduction to printed circuit boards (PCB) and plated through holes (PTH) used to connect different layers in PCBs. PTHs are formed when drilled holes between different layers are plated with copper. This week’s picture is digging deeper the same topic and showing a typical PTH failure.

The picture shows a cross-sectional image of a barrel crack in PTH structure of a PCB after thermal cycling test. Thermal cycling is known to cause cyclic stress to the copper structure of a PTH due to differences in coefficient of thermal expansion (CTE) of the used materials.

CTE of copper is 17 ppm/°C. The CTE of a fibre-reinforced, polymer-based PCB material, on the other hand, is not unidirectional. In the x-y plane the CTE is relatively close to the CTE value of copper. However, in the z-direction the CTE is much higher, 52 ppm/°C for the FR-4 material used here. Ultimately, z-directional stress caused by CTE differences between materials leads to a crack and increased resistance of the PTH, or even open circuit.

It is noteworthy, that cracks usually initiate from a flaw in the copper. Therefore, the quality of the PTH drilling and plating process are essential for reliable products.