改善印制板表面平整性的新型表面涂覆技术

最近一种新型表面涂覆技术在印制板工艺中得以应用,即在减成法蚀刻图形线路之间填充树脂,以改善印制板表面平整性。在该技术中,首先PCB预固化,以避免树脂起泡;然后,UV固化至B-阶段;最后,用抛光轮慢慢磨掉线路表面的树脂,以获得树脂/Cu的平整表面,这可用于加成法镀铜。平整的表面有下列几个要点: (1)改善质量; (2)提高设计自由度; (3)改善元件安装。 与传统的PCB制造工艺及表面涂覆液态感光阻焊剂相比,该技术可有效改善阻焊裂纹问题。由于该技术与加成法镀铜工艺有相似之处,所以采用减成法可形成更为精细的电路,而且成本更低。需要特别指出的是,改善表面平整性,使得表面元器件安装密度更高和安装高度更为一致。将来该技术的主要课题是优化涂层厚度以控制特性阻抗,为埋入器件PCB提供有效可靠的平整表面。

An Advanced Coating Process for the Surface Planarization of PCB

An advanced coating process has been developed wherein screen-printing is utilized to fill the spaces between subtractively etched Cu circuit trace with resin. In subsequent processing, the PWB is pre-heated, enabling and accelerating the release of air bubbles that may be trapped in the deposited resin. Next, the PWB surface is exposed to UV energy until a B-stage cure is achieved. Finally, the resin on the surface of the trace is gradually removed using a buff polishing wheel, resulting in a resin/Cu co-planar surface equal to the planarity that is typically achievable using additive Cu plating. The planar surface achieved using the process described herein provides three benefits: improved quality, greater freedom of design, enhanced component mounting. In comparison to PWBs that have been manufactured in the traditional manner and coated with liquid- photoimageable solder resist, for example, the quality of protection from Au plating and resistance to cracks in the resist can be improved via this planarization process. Because of the similarity to planarity achievable using the additive Cu plating process, finer circuitry can be formed using a more cost effective subtractive processing. On a related note, denser and more consistent component mounting can also be expected as a positive by-product of en- hanced surface planarity. Future research and development objectives include optimization of coating thickness for impedance control and providing a useful, reliable platform for embedded component PWBs.