PCB Etchback is process performed to remove epoxy resin from the sidewalls of a drilled via hole to make sure it provides good electrical connectivity between the PCB layers. Maintaining good electrical connectivity between layers is very important for routing signals in multi-layer printed circuit boards.
During the via drilling process, if the drill temperature exceeds the glass transition temperature of the substrate, it causes the resin to melt followed by smearing inside the hole. This smear covers the terminal of the inner layer which is connected to the via and thereby inhibits the electrical connection. This smear is removed by a process called desmearing and is followed by an etchback process which either etches in to the via wall or extends the copper layer out in to the via hole to provide better connectivity between the via and the PCB layers.
There are two types of Etchback:
Negative Etchback: In the PCB Fabrication process, Once a Plated Through Holes (PTH) is drilled, it needs to be cleaned (desmearing) and plated. In the case of Negative Etchback, the copper land (i.e inner cooper layer) is ‘recessed in’ from the edge of the hole walls (See Image on bottom).
According to the IPC-6012C 3/A standard, negative etchback should not more than 0.5 mils. Interconnect stress testing is performed to check its reliability. An acceptable degree of the negative etchback depends upon the end product in which the circuit board would be used. Although negative etchback is cost-effective, it does not provide the best electrical connections and thus is usually not used in high reliability applications like Military, Aerospace and Medical devices.
Positive Etchback: In this process, the copper land protrudes out by crossing the edge of the hole walls, enabling a strong three-terminal contact. Such three-point contacts provide excellent electrical connectivity and therefore are used in high reliability applications like military/ aerospace and medical.(See Image on bottom)
Etchback Techniques
Desmearing and Etchback can be performed using chemicals or plasma. In the chemical process, an alkaline potassium permanganate-based etchant is used. In the plasma process, the gaseous free radicals are created, which attack the residue to form charged gases, which can be sucked away later on.
Chromic Acid: It gives uniform resin removal rate, but is an added factor for water pollution.
Concentrated Sulphuric Acid: It gives different resin removal rates based on its purity. It absorbs water from the air, hence has a limited life span.
Plasma (charged gas): It is expensive. Unlike acids, it can easily remove the smears in polyimide-based boards.
