Plating and Recrystallization of Galvanic Cu Films on Roll Annealed and Polycrystalline Cu Foils and the Effect of Intermediate Electroless Cu Layers (2022)

Flexible printed circuit boards (FPCBs) are of increasing importance in the electronics industry. One approach to achieve a highly flexible copper foil is through a roll-anneal (RA) process as this creates a microstructure with large crystallites (of the order of up to 100 μm wide) along the plane of the foil (such foils are often known as highly annealed or HA type copper foils). These crystallites are aligned with their [100] direction along the rolling direction and their [001] direction perpendicular to the surface of the foil (cube texture). This microstructure prevents stress accumulation at grain boundaries upon bending, and therefore metal fatigue by minimizing initial cracking spots. In the production of FPCBs, the RA foils are laminated onto flexible polymers such as polyimide or other insulating dielectrics. To produce multilayer flex boards some applications may combine HA-Cu foils with polycrystalline Cu foils. Further, a sequence of electroless and galvanic Cu plating is also required, and here sophisticated plating processes are needed to fulfill the requirements for the different foil materials. In the case of an HA-Cu foil, an excessive epitaxial grain growth from the foil side into the galvanic Cu layer (known as bottom-up recrystallization) during electroplating must be avoided, as this can inhibit the shiny deposit which is desirable for reliable automatic optical inspection and further structuring [1]. Contrarily a high degree of bottom-up recrystallization is desirable on polycrystalline Cu foils to ensure the highest reliability requirements for stacked blind micro vias or through hole interconnections in HDI any layer PCBs [2]. In this context, we will present that a thin (0.3 - 0.5 μm) intermediate electroless Cu layer has a major effect on the bottom-up recrystallization of a galvanic Cu layer plated onto HA-Cu foils, and in this, the stabilizer system used in the electroless Cu plating process plays a crucial role. It is shown on single Cu crystals, that a stabilizer system not containing a specific component effectively suppresses the grain growth of (100) oriented crystals, i.e., HA-Cu crystals, but does not influence the bottom-up recrystallization of (110) or (111) oriented Cu crystals. Whereas electroless Cu plated with the component in its stabilizer system selectively suppresses the grain growth of the (111) oriented crystals but maintains the bottom-up recrystallization of the other low indexed Cu crystals. Accordingly, for both stabilizer systems a high degree of epitaxy is observable on polycrystalline Cu foils where all crystal orientations are available. This is accompanied by an excellent reliability performance for polycrystalline inner layer Cu/Cu/Cu-interconnections. It is remarkable, that the interruption of epitaxy on (100) or (111) oriented single Cu crystals is observable at the electroless Cu/galvanic Cu interface, whereas the electroless Cu layer itself is completely bottom-up recrystallized for both stabilizer systems. We also noted that some electroless Cu additives cause a distinct surface texture (surface facet distribution) and substructure on HA-Cu foils, which increases the codeposition of leveller additives during the initiation of galvanic Cu, and so, can reduce the subsequent bottom-up recrystallization of the galvanic Cu layer. An additional type electroless Cu additive system functions in a different way. Here, the electroless Cu layer non-selectively blocks the grain growth of all crystal orientations. This leads to two non-epitaxial interfaces at the Cu foil (both RA and polycrystalline)/electroless Cu interface and at the electroless Cu/galvanic Cu interface. In such cases, a significantly lower reliability even for polycrystalline inner layer Cu/Cu/Cuinterconnections is observed.