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Nano void Formation at Cu/Cu/Cu Interconnections of Blind Micro Vias: A Field Study
Keywords: Blind Micro Via, reliability, Nano void
The electrical reliability of multilayer HDI/BGA-PCBs is mainly affected by the thermo-mechanical stability of stacked micro via interconnections. Here, a critical failure mode is the stress related crack between the electrolytically filled via and the target pad, commonly known as target pad separation. The junction includes two Cu-Cu-interfaces, one between the target Cu pad and the thin electroless Cu layer and the second between electroless Cu and electrolytic Cu. Large nano-voids (with dimensions of several ten to several hundred nm) and inhibited Cu recrystallization across the interfaces are the two main indications of a weak link to the target pad, and are well observable via standard inspection tools such as FIB/SEM*. Recently, closer inspections of such junctions were performed via transmission electron microscopy (TEM) and a substantial density (up to 30000 voids/µm3) of circular shaped small nano-voids with diameters below 10 nm were observed [1][2]. The Nano-voided regions were assigned to the electroless Cu layer. To evaluate the impact on the junction reliability and to find probable root causes of this newly discovered void-phenomenon, an extensive field study of more than 400 TEM investigations, incl. TEM-lamella preparation, HAADF-TEM, XEDS and EELS-measurements*, was performed. TEM in combination with XEDS enables the localization of the electroless Cu layer in the junction by detecting the codeposited Ni (typ.: 0.2-1.5 at%) and a precise assignment of the nano-void affected interfaces or layers. This nano-void field study comprised the investigation of HDI and BGA-multilayer PCBs with stacked vias produced by several industry partners and single layer reliability test boards produced in industry like on-site facilities (Atotech Techcenters), accompanied by some laboratory plating tests. Different types of industrially relevant electroless Cu pretreatments, Pd-activator systems, electroless Cu baths and filling Cu-electrolytes were included in the test matrix, as well as the impact of a subsequent thermal treatment (one to ten IR-reflow cycles). In this paper we will discuss the following main results of this field study, with regards to root causes and probable failure mechanisms: • Two types of nano-voids are identified o Thermally induced nano-voids which are not visible immediately after electroplating and only become apparent after additional thermal treatment. o Plating induced nano-voids, that are visible immediately after electrolytic plating and do not form as a result of additional heat treatment. • For thermally induced nano-voids – The tested electroless Cu pretreatments and varying amount of adsorbed Pd by different Pd-activator systems have no direct impact on nano-void formation. Interestingly, the adsorption of Pd-nanoclusters (50-100nm diameter) on the target pad of a BMV, e.g. from Pd precipitations in the activator solution, causes the formation of both small and large nano-voids. • For plating induced nano-voids – The type of electroless Cu, i.e. the used stabilizer system, is the main impact factor on nano-void formation at the target pad/electroless Cu interface. This void-type is observable without subsequent thermal treatments • The absence of nano-voids in the electroless Cu layer on top of the Cu-clad panel (capture pad) compared to the high void-density at the BMV-bottom (target pad), points to Cu-depletion as an prerequisite condition to trigger the nano-void phenomenon in BMVs. • Nano-void formation close to the electroless Cu/electrolytic Cu interface is related to the combination of the stabilizer system of the electroless Cu and the type of electrolytic Cu, e.g. the type of used Leveler system. A suited combination of electroless and electrolytic Cu is able to produce a completely nano-void free Cu/Cu/Cu-junction. • Despite a substantial higher nanovoid-density in case of eless Cu type A compared to eless Cu type C, both electroless Cu baths examined show comparable and acceptable reliability performance through standardized thermal cycling test and quick via pull tests. *TEM-Transmission Electron Microscopy, HAADF-High Angular Annular Dark Field mode of TEM, XEDS-X-Ray Energy Dispersive Spectroscopy, EELS-Electron Energy Loss Spectroscopy
Tobias Bernhard, Scientist R&D
Atotech Deutschland GmbH
Berlin, Berlin
Germany


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