The Impact of Hydrogen Gas Evolution on Blister Formation in Electroless Cu Films

Abstract

Electroless Cu plating is a key process to provide electrically conductive layers on insulating substrates for the subsequent Cu electroplating of printed-circuit boards (PCBs). Recently introduced substrate materials are prone to spontaneous delamination failure (blistering) of the electroless layer during deposition. A higher Ni content in electroless Cu baths prevents delamination failure by increasing the internal tensile stress in the Cu layer. This effect is achieved by suppressing Cu self-diffusion through the incorporation of small amounts of Ni in the grain boundaries and the grain boundary junctions. Whether the hydrogen gas evolved inevitably during electroless Cu deposition and potentially trapped in the deposit may cause blistering is still under debate. Here we test this hypothesis by quantifying the generated hydrogen gas volume during Cu deposition in baths with Ni concentrations between 0 and 1000 ppm, and observing the rate of blister formation in the deposit. We find a constant molar ratio of 0.65 ± 0.05 mol H2/mol Cu independent of the Ni content in the bath, in contrast to a systematic and substantial change of the blister probability. From this lack of a correlation between the blister rate and hydrogen evolution, we rule out hydrogen evolution as a determining factor for blister formation.

Effect of Ni on Electroless Cu Plating Rates in the Presence of 2,2′-Bipyridyl and 2-Mercaptobenzothiazole

“…The inclusion of Ni 2+ in formaldehyde electroless deposition baths is frequently used to control deposit stress, to avoid blistering, and possibly reduce hydrogen incorporation from formaldehyde. [16][17][18] Ni in solution will co-deposit with Cu as metallic Ni creating a Cu(Ni) alloy deposit containing between 1%-6% Ni. 16,19 PHI-TRIFT-TOF-SIMS data indicates that Ni will aggregate at Cu grain boundaries.…”

“…[16][17][18] Ni in solution will co-deposit with Cu as metallic Ni creating a Cu(Ni) alloy deposit containing between 1%-6% Ni. 16,19 PHI-TRIFT-TOF-SIMS data indicates that Ni will aggregate at Cu grain boundaries. 16 The mechanism for stress mitigation in the presence of Ni has been suggested to involve diminished Cu diffusion into and out of the grain boundaries due to aggregated Ni incorporation at those grain boundaries.…”

“…16,19 PHI-TRIFT-TOF-SIMS data indicates that Ni will aggregate at Cu grain boundaries. 16 The mechanism for stress mitigation in the presence of Ni has been suggested to involve diminished Cu diffusion into and out of the grain boundaries due to aggregated Ni incorporation at those grain boundaries. 16,18 Cu electroless rates increase in the presence of Ni only when certain stabilizing additives are present, and Ni has no effect if these additives are not present.…”

“…16 The mechanism for stress mitigation in the presence of Ni has been suggested to involve diminished Cu diffusion into and out of the grain boundaries due to aggregated Ni incorporation at those grain boundaries. 16,18 Cu electroless rates increase in the presence of Ni only when certain stabilizing additives are present, and Ni has no effect if these additives are not present. 13 The mechanism of the increase in Cu deposition rate from Ni-containing baths is not fully understood, especially the dependence of this rate increase on the additive identity.…”

Quantification of the Nano-Voids in Electroless Deposited Copper Layers Using SiNx TEM Grids

“…The nano-voids are formed when hydrogen gas released by a reducing agent is trapped inside the deposited layer, or when growing copper grains are inappropriately joined, creating an empty space between them. [2][3][4][5] Generally, the electroless plated copper layer is much thinner than the electroplated copper layer; thus, it has received relatively little attention and its impact on reliability has been overlooked. However, as the number of stacked PCB circuits increased and the circuit became more complex, the size of the vertical interconnects access (via) decreased, and accordingly, the influence of the thin electroless copper layer on the via-crack increased.…”