What is the Purpose of Etchback?
Etchback and desmear serve many purposes. The first purpose of etchback is to assure the copper is clean. Assuring the copper is clean will then help form a metallurgical bond with the plating. The last purpose of etchback is to form a three-point contact around the inner layer. Why are each of these processes so important? Below we will go in more depth as to how each process effects the overall goal of etchback.
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Copper Cleaning, Plating and Three-Point Connection
The purpose of etchback or desmear is not just to roughen the resin surface, so that the plating can stick to it, but also to be able to form a bond between the plating and the resin. If you don’t have a roughened surface when you remove the excess of smeared resin, you’re going to have difficulty getting a good uniform deposit of copper to stick and thus have difficult forming the metallurgic bond.
If after the copper cleaning and metallurgical bond is formed with the plating, you are left thinking, “Why are we sill having issues?” you may wan to check the three point connection. Issues with etchback are often due to the three point connection. The connection is formed after we remove the resin from all sides that interconnect.
Typically, we like to see, if possible, two to three tenths of resin removal. We know not all of this is possible as we move up the resin reliability curve. The further up the resin reliability curve you go, the greater difficulty you are going to have to getting a three point connection.
Something to keep in mind when it comes to etchback and making the three-point connect: do not overdo it by making desmear aggressive. This can cause other problems, like for example, a over roughened whole wall or even a wedge defect.
When it comes to etchback, keeping in mind the purpose is clean the copper so it can form a metallurgical bond for plating, and thus form a three-point connection, By keeping the purpose in mind, you can continually improve your process to make it as efficient as possible.