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78 DESIGN007 MAGAZINE I JUNE 2022 will be better than others for achieving a good bond. ere are also copper treatments used to ensure proper bonding when exposed to high temperatures. Additionally, there are other treatments that can be applied to the copper foil to support a reliable bond interface when considering long-term elevated temperature exposures. For most rigid PCB applications, ED cop- per is the most popular, however RA copper is used as well. Generally, RA copper is typically more expensive than ED copper. RA copper is oen not used unless there are specific prop- erties of rolled copper that can be beneficial to the application. Due to the nature of how RA copper is made, it has a very smooth sur- face. A smooth copper surface is advantageous for high frequency and very high-speed digi- tal applications, where having lower insertion loss is desired. Another property of rolled cop- per that is innate to the copper manufacturing process is the in-plane grain structure that is very good for applications where bending the circuit is necessary. One technical drawback to RA copper, which is also related to the grain structure, is that the etching of fine circuit features can be problematic. is can be over- come to some degree, with specific etching conditions tailored to RA copper. ED copper is widely used throughout the PCB industry. ere are many different types of ED copper and typically they are classified by surface roughness and/or treatment. IPC has different classifications for ED copper based on copper roughness. Examples of a few of the classifications are LP (low profile), VLP (very low profile), and HVLP (hyper very low profile). Copper roughness measurements can be confusing due to different methodologies. Generally, there are two categories for rough- ness measurements: contact measurements and non-contact measurements. e contact measurements use a physical probe, a stylus, to measure the peaks and valleys of the cop- per surface. e non-contact measurements are typically using reflected light or laser mea- surements to determine the peaks and valleys of the surface. e contact profilometers can be less accu- rate due to the stylus tip size, where the sty- lus may not be able to resolve the depth of a very narrow valley. Also, the stylus can "plow" through the peaks and not get an accurate mea- surement. ose who are experienced with this technique understand these issues and there are some adjustments that can be done to miti- gate the accuracy concern. However, as a gen- eral statement, the non-contact profilometers are typically more accurate than the contact profilometers for fine-grain copper types. ere are many ways to describe the surface roughness profile. Engineers working directly with the PCB fabrication will typically want to use the Rz profile, which is the peak-to-valley measurement over a sample area, defined by multiple line measurements. If the Rz profile is measured as an area (not a line measurement), the Rz profile would be designated as Sz. e engineers working with electromagnetic mod- eling soware for high frequency or high- speed digital (HSD) will oen want the Rq or Sq profile of the surface. e electromagnetic modeling industry has found good correlations between the Rq or Sq profiles and the surface impact on RF or HSD performance. e Rq and Sq are basically the root mean square of the surface roughness when measured with many samples. RA copper is often not used unless there are specific properties of rolled copper that can be beneficial to the application.