Stress of electroless copper deposits on insulating and metal substrates

• We report stress of electroless Cu deposits on insulating and metal substrates.
• The final deposit stress is substrate-independent.
• The final deposit stress and the X-ray diffraction based strain agree.
• The stress change due to the thermal contraction of the substrate is observed.
• Plating bath type, substrate and surface preparation alter the stress.

In the fabrication of printed circuit boards, electroless copper is plated on insulating substrates. However, data for film stress by substrate bending are frequently obtained with metal substrates. We compare the stress evolution on an insulating substrate (acrylonitrile butadiene styrene) with results from commercial Ni–Fe and Cu–Fe alloy test strips, as well as X-ray diffraction based strain data. Tests were done with two plating bath formulations, one with and one without added nickel. Substrate type and condition determine the stress near the beginning of plating. Stress of the Ni-free films depends more strongly on the substrate material. Further, when the samples are cooled from the bath operating temperature to room temperature, the thermal contraction of the insulating substrate compresses the plated thin copper film. The measured stress change agrees with the change predicted by calculation. Data correction methods are discussed, and other substrate materials can be tested readily with the method employed here.