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The term "desmear" refers to a stage in the PCB manufacturing process that's used to clean and prepare the holes drilled into the board for further processing. This step is especially important when preparing the holes for the plating process.

During the drilling process, heat is generated that can cause residues from the base material (commonly epoxy and glass for FR4 boards) to be deposited on the hole walls. These residues, often referred to as "smear", can be a barrier to the adhesion of the plating in the hole. The desmear process is designed to remove this smear and provide a clean, rough surface for good plating adhesion.

There are a few different methods commonly used for desmear:

Chemical Desmear

In this process, the PCB is immersed in a chemical solution (often a mixture of sulfuric acid and potassium permanganate) that dissolves the smear. The board is then rinsed thoroughly to remove any remaining chemicals.

Plasma Desmear

This process uses a plasma (ionized gas) to remove the smear. This is often used for high-density interconnect (HDI) boards and other applications with small or blind vias, where a chemical desmear process might not be as effective.

Mechanical Abrasion (Scrubbing)

This process uses a brush or other abrasive to physically remove the smear. This is often used in combination with a chemical or plasma desmear process for added effectiveness.

After the desmear process, the holes typically go through an etching process to further roughen the hole walls and improve adhesion for the plating process. The board then goes through a series of chemical baths to deposit a thin layer of copper onto the hole walls, which seves as the base for the electroplating process.

 

It's important to note that the desmear process is a critical stage in PCB manufacturing, as inadequate smear removal or hole preparation can lead to weak plating adhesion and potential reliability issues in the finished board.