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The topic of intrinsic copper structure has been largely neglected in discussions regarding the PCB fabrication quality control process. At face value, this seems especially strange considering that copper has been the primary conductor in all wiring boards and substrates since they were first invented. IPC and other standards almost exclusively address copper thickness with some mild attention being paid to surface structure for signal loss-mitigation/coarse properties.
Yet we still lack standards references as to what the actual copper grain structure should look like to optimize microvia reliability, what a target pad and capture-annulus should look like after laser drilling and post-treatment, and what copper grain structure yields which etch rate for optimizing the substrate differential etch and resolution limit. These topics generally fall into the category of individual factory know-how.
In this article, we will present examples of applications where improved measurement and control of copper grain structure and topography provide significant gains in value to the PCB fab process.
In the case of microvias, in addition to the traditional chemical analyses, white-light microscopy inspections, weight gains/losses, and SIR readings associated with laser drill and the metallization of microvias, best practice has recently found that three different inspection steps for copper structural assessment to assure high-reliability results are also valuable to de-risking the process.
To read the rest of this article, which appeared in the October 2023 issue of PCB Magazine, click here.
