Roughness Asymmetry
Surface texture parameters quantify the vertical distribution of peaks and valleys across a manufactured board or solder joint. The metric of surface skewness indicates whether the height distribution of a surface is dominated by rounded peaks or by deep scratches. This parameter represents the asymmetry of the surface profile about its mean line.
Solder Performance
Interfacial bonding in solder joints depends on the specific topography of the copper pad and the finished surface. A negative value for this parameter describes a plateau-like surface with deep, narrow valleys that holds solder paste and promotes uniform wetting. Conversely, a positive value indicates a surface dominated by sharp peaks that can lead to localized stress concentrations and incomplete coverage during the reflow process.
In fine-pitch ball grid array assembly, these topographic variations can alter the contact angle of the molten solder and lead to voiding or poor intermetallic layer formation. Precise control of the chemical etching and plating processes is required to achieve the desired surface texture.
Profilometry Assessment
Quantification of this surface texture is achieved through non-contact optical profilometry on test coupons. The calculated value is a dimensionless number that assists process engineers in monitoring the wear of mechanical brushing rollers or the depletion of chemical micro-etch baths. This parameter thus provides a diagnostic tool for PCB surface finishes.
