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February 2015 • The PCB Design Magazine 25 beyond design EFFECTS OF SURFACE ROUgHnESS On HIgH-SPEED PCBS continues ness, but is the most expensive foil. ED copper has the roughest surface and depending on the application and speed requirements, may be perfectly adequate. RTF copper is smoother, and is much the same cost as ED. However, it does exhibit poor peel strength and is prone to de- lamination. So it is a trade-off between perfor- mance and price. Electrodeposited copper foil is the standard copper used in the laminate industry. ED foil is deposited from a copper solution, at a specific DC voltage, onto a moving polished titanium drum which forms the cathode. The foil is sub- sequently stripped from the drum. The grain construction formed by this process forms the dendritic "tooth" of the copper on the "bath side" of the copper. The drum side takes on the smooth texture of the polished drum surface onto which it is plated. Figure 3, illustrates a typical inner layer trace cross-section showing the roughness of both the upper and lower cop- per surfaces. Rolled copper is made by running a copper strip through successively smaller and smaller gaps in a rolling mill until it reaches the desired thickness. Rolled copper is smoother and can be made very flexible by annealing. Because it is smooth, its bond to laminates is totally depen- dent on the quality of the treatment it receives and the adhesive properties of the resin system employed. RA copper also has a different grain structure than ED copper and will etch at a dif- ferent rate. Much of the RA copper in the lami- nate industry is used in flexibly circuits, typi- cally bonded to a polyimide film with an acrylic adhesive. Reverse treated foils involve the subsequent treatment of the smooth side of the electrode- posited copper. Treatment layers are thin, rough coatings that improve adhesion of the base foil to the dielectric material. To the naked eye, copper clad laminate ap- pears smooth but at the microscopic level, all Figure 2: rolled annealed copper (left) and etched copper (right) under an electron microscope (courtesy of university of Cambridge). Figure 3: Inner layer trace cross-section [3] .