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Upcoming electronic devices are required to be further miniaturized. The microelectronics industry and packaging technology focus efforts on optimizing adhesion between plated copper and printed circuit board (PCB) substrate (here epoxy/glass buildup composite), while keeping smooth surfaces for high-frequency application. We propose herein to review and deepen the basic understanding of the sequential buildup process employed worldwide for the past several decades for multilayered PCB manufacturing. This multiscale and interdisciplinary study aims to establish the relationships between degrees of precuring (α), oxidative etching (permanganate desmear wet treatment), and copper adhesion. The epoxy curing states on industrial coupons were evaluated by diffuse-reflectance infrared Fourier transform spectroscopy. Then, atomic force microscopy (AFM) described the desmear performances through topography evolution with α between the different sequence steps. Finally, polymer−copper adhesions were investigated by using peel strength tests, AFM, and X-ray photoemission spectroscopy. We remark that high adhesion strengths were obtained for very smooth surfaces. This study outlines the contribution of the polymer network viscoelasticity (relaxation dynamic) on the polymer−copper adhesion. We observed that faster polymer relaxation rates tended to increase polymer−copper adhesions.

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Curing kinetics of an industrially important printed-circuit board (PCB) base material (epoxy–phenol/glass fillers) were studied by isothermal differential scanning calorimetry (DSC) measurements between 150 and 190°C, as relevant curing temperatures for the PCB industry. The extent of cure was calculated by integration of the exothermic peak and normalization by the total heat of reaction (obtained by nonisothermal DSC). Although the cross-linking was completed above 180°C, the kinetic profiles show two regimes: one fast and one slow. The kinetic parameters have been elucidated using an isoconversional model-free kinetic method, with the exact method of Friedman, to give to the PCB manufacturers a road map to predict curing behavior of base material. The linearity of Arrhenius plots was satisfactory. The apparent activation energy of curing reaction has been found to increase with the degree of conversion. The elucidation of the kinetic parameters allows us to propose an accurate and predictive description of the curing kinetics within the fast regimen of reaction (i.e., without vitrification). Finally, we discuss how these kinetic measurements and models can be completed and optimized.

The electrical reliability of multilayer high density interconnection printed circuit boards (HDI-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. In this paper we will show that state-of-the-art electroless Cu plating processes are able to provide solid, completely recrystallized and highly reliable stacked via junctions. Defect free interfaces were achieved by using ionic Pd-activators and electroless Cu baths with a cyanide based stabilizer system. Cyanide free electroless Cu baths tend more to the formation of nanometer sized defects, discovered via Transmission Electron Microscopy (TEM). In this case a precise adjustment of single stabilizer components is mandatory to achieve defect free layers. The defects are hollow and were identified as “nano voids”. A critical density of these nano voids weakens the interface, predefines the crack path and reduces the overall reliability of the junction. A precise localization of the nano voids within the junction was enabled by detecting the Ni-containing electroless Cu layer via TEM-Ni mapping. Slower volume exchange of the electroless Cu solution within the blind micro via (BMV) substantially increases the nano void density. The ability of nano voids to migrate and coalesce at elevated temperatures was investigated as well.