Copper-Invar-Copper - CIC-Technique

Invar is a nickel iron alloy with ca. 1/3 nickel and 2/3 iron. Its coefficient of thermal expansion CTE is about 1.5 ppm/K. Implementation of CIC layers (copper-invar-copper-composite metal) into multilayer PCBs is used to contol in plane expansion (CTExy).

Thermal stress at higher thermal cycles leads to fatigue and fraction of solder joints.

Ceramic semiconductor components like leadless ceramic chip-carrier (LCCC) are used for high reliable electronics. Differences of CTE of the components of ca. 6-8 ppm/K im Bauteil versus ca. 16 ppm/K of the PCB substrate lead to damages at the solder joint if no CIC is applied.

The bigger the BGA the more carefully has to be planned the stack-up.

Normally two symmetrically Cu-Invar-Cu Layers of 0.15 mm thickness each are arranged directly beneath the external layers. Cu-lnvar-Cu layer can be connected electrically, but it is not recommended to use it as power planes due to its impact on drills. On the other hand, CIC can be used for heat spreading like power planes. Heat is spreaded by the copper share ot CIC. Invar does not play a significant roll in this view, because the thermal conductivity is only 3% compared to copper.