This work focuses on the development of foamed high temperature thermoplastic substrates for printed circuit boards. For this application it is necessary to achieve mean cell diameters smaller than 30 µm in order to be able to realize vias and high packaging densities (miniaturization). Different additives as nucleating agents, namely macro- and micro-crystalline talc, silica, calcium carbonate, and wollastonite, were melt-compounded with polyetherimide using a twin-screw extruder. Foamed samples are prepared by foam extrusion using a slit die and CO2 as physical blowing agent. The aim of this study is to analyze the influence of the mean particle size and the particle surface tension on the mean cell diameters. Therefore, the shape of the additives, the foam morphology, and the elongational viscosity were considered. The additives with a suitable particle size and surface tension exhibit a positive influence on the foam morphology, resulting in smaller cell diameters (<30 µm), a narrower cell size distribution and a foam density lower than 900 kg/m3. If the mean particle diameter of the nucleating agents is lower than 0.6 µm in this study, no nucleation effect could be observed. This is related to the fact that no heterogeneous nucleation occurs, if the particle diameter is too small. If the mean particle diameter of the used additives is larger than 1.5 µm, which could be demonstrated in this study in case of polyetherimide, then the additive acts as nucleating agent and heterogeneous nucleation occurs. Furthermore, it was observed that the mean cell diameter was affected by the different surface tensions of the studied nucleating agents.

中文翻译：

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成核剂类型对印刷电路板用挤出聚醚酰亚胺泡沫形态的影响*

这项工作的重点是开发用于印刷电路板的发泡高温热塑性基材。对于此应用，必须实现小于 30 µm 的平均单元直径，以便能够实现通孔和高封装密度（小型化）。作为成核剂的不同添加剂，即粗晶和微晶滑石、二氧化硅、碳酸钙和硅灰石，使用双螺杆挤出机与聚醚酰亚胺熔融复合。泡沫样品是通过使用狭缝模头和 CO2 作为物理发泡剂的泡沫挤出制备的。本研究的目的是分析平均粒径和颗粒表面张力对平均细胞直径的影响。因此，要考虑添加剂的形状、泡沫形态和拉伸粘度。具有合适粒径和表面张力的添加剂对泡沫形态有积极影响，导致更小的泡孔直径 (<30 µm)、更窄的泡孔尺寸分布和低于 900 kg/m3 的泡沫密度。如果本研究中成核剂的平均粒径低于 0.6 µm，则无法观察到成核效应。这与如果粒径太小不会发生异质成核的事实有关。如果所用添加剂的平均粒径大于 1.5 µm，这可以在本研究中在聚醚酰亚胺的情况下证明，则添加剂充当成核剂并发生异相成核。此外，观察到平均泡孔直径受所研究成核剂的不同表面张力的影响。