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58 PCB007 MAGAZINE I MAY 2018 alkaline cleaning will change the chemical na- ture of the surface by removing organic con- taminants, but will not affect the topography, whereas a persulfate etch will change both. In general, surface preparation is done to as- sure good adhesion of metal, dielectric, photo- resist, or soldermask to the prepared surface, although avoiding excessive adhesion could also be the object. Take the example of surface preparation before dry photoresist lamination. One can measure the surface profile of the cop- per surface using the technique of profilome- try. This technique measures changes in the structure or profile of a surface. The data col- lected gives information such as [1] : Ra—Arithmetic mean surface roughness: Ar- ithmetical mean of the sums of all profile val- ues. Rt—Total height of the roughness profile: Sum from the height Zp of the highest profile peak and the depth Zv of the lowest profile val- ley within the measured length (ln). Rzi—Maximum height of the roughness pro- file: Sum from the height of the highest profile peak and the depth of the lowest profile valley within a sampling length. Rz (max)—Maximum surface roughness: Largest of the five Rzi values from the five sampling lengths "i" over the total measured length. Rz—Surface roughness depth: Mean value of the five Rz values from the five sampling lengths over the total measured length. One must be careful about encountering dif- ferential etching. This occurs when spots of or- ganic residues have not been cleaned from the surface of the copper. One common source of these residues is epoxy or pre-preg dust that can deposit on the copper. In addition, the overall surface roughness of the material is often contributed by the glass fabric styles and thicknesses. This is especially notable since the industry has moved to thin- ner core materials and thinner copper foils. Be prudent when working with these thinner ma- terials as mechanical surface preparation may stretch thinner materials. In contrast, while chemical cleaning will not stretch the materi- al, excessive copper removal may impact sig- nal conductivity. Excessive copper removal or an overly roughened surface will impact signal integrity resulting in insertion loss at high fre- quencies. Several types of copper foil are used today. The two main categories are electrodepos- ited (ED) foil and rolled annealed (RA) foil. And within each of these categories are sev- eral variations. As an example, there are sev- eral foil types within the ED category. These differ based on the extent of the copper pro- Figure 1: Comparison of RTF to standard ED copper foil.