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40 The PCB Magazine • June 2016 Dry film adhesion tests for e'less copper HDI generated in mass production equipment are currently in progress and therefore cannot be discussed in this paper. However, all initial re- sults showed similar excellent performance as for e'less copper IC. Electroless Copper Adhesion on the Substrate New substrates introduced to the market for application with SAP technology are becom- ing increasingly smoother because of the de- mand for higher circuitry densities and signal frequencies. The influence of the substrate sur- face constitution on the adhesion of the elec- troless copper layer via mechanical anchoring and chemical interface bonding is significant. Weak adhesion of the copper layer could lead to spontaneous delamination failures (blistering) and line peel-offs that are not acceptable for customers in mass production. Internal stress characteristics of the electroless copper layer have been shown to significantly impact the tendency of a copper layer to delaminate from the substrate [1, 2] . The occurrence of compressive stress in the layer correlates to the probability of buckle driven delamination, namely blistering, whereas tensile stress suppresses the likelihood of blistering. The internal stress characteristics of the copper layer deriving from e'less copper IC have been analyzed using the in situ X-ray diffraction method (XRD). As can be seen in fig- ure 12 the internal stress is slightly more tensile compared to the reference electroless copper bath, one of the current standards in the mar- ket. The copper layer is therefore perfectly suit- ed to adhere on the substrate surface without any blistering issues for a representative deposi- tion thickness range. The blistering tendency of the copper layer is not only related to the internal stress charac- teristics but also to the peel strength of the layer. The peel strength is a well-known performance criterion in the industry and can be described as the clamp force of the copper layer to the substrate. Broadly speaking, blistering occurs when the delamination force due to the inter- nal stress characteristics exceeds the clamp force of the layer. The peel strength is dependent on the chemical bonding of the copper layer on the surface and the mechanical anchoring via the surface roughness. The latter is highly influ- enced by the conditions that are applied in prior process steps especially in the desmear and the Figure 12: Plot of the internal stress of copper films deposited from two different electroless copper baths against deposit thickness, as measured by in situ XRD. (Measurements in cooperation with Mt. Allison University, Canada.) HIGH-THROW ELECTROLESS COPPER—NEW OPPORTUNITIES FOR IC SUBSTRATES AND HDI MANUFACTURING