There are various ways to create a copper pattern, or mask, for a printed circuit board. The simplest method is to use a permanent marker. You can also use a silkscreen, a photoengraving method using a photoresist coating, and PCB milling. These methods are collectively known as subtractive methods of creating PCB patterns. A subtractive method will remove the copper substrate from the board and leave only the desired PCB pattern. This pattern is based on the circuit’s schematic design.
Here are the definitions for each method:
Silkscreen printing uses a special ink that is designed to resist the etching solution. The paint, or ink, masks specific portions of the printed circuit board, and protects the copper foil from being eaten away by the acid. All the exposed copper will then be removed by the etching process.
A woven mesh is needed in order to create a silkscreen. The desired pattern is cured and developed into the mesh, forming an ink-blocking stencil. The mesh will then be divided into ink-blocked areas and open areas. The open areas of the mesh will contain the copper connections and the vias (circular copper area that will be drilled). Inks can be transferred to these areas. You just put the silkscreen on top of the PCB, and apply ink to the screen. A squeegee is used to force the ink through the mesh openings. The final product is the PCB pattern.
This method is used in mass production. The silk screen is used to print one PCB layout pattern to multiple copper boards in the shortest possible amount of time.
A photoengraving process consists of a special-purpose printed circuit board called a photoresist, a photomask and a developer. This type of PCB is sensitive to light. This process uses a photomask and a developer. A photomask marks a portion of the copper that will be exempted from etching. A developer will then be used to remove the exposed areas of the photomask coating.
A photomask is a plastic film (example: over-head projector film) that contains the printout of a PCB layout. A photoresist has a thick cover that protects it from light. This cover must not be peeled off before the exposure process begins. This process involves the exposure of the photoresist to bright light. The protected areas (copper that should not be eaten by the acid), however, should be covered with the photomask.
Put the light source on top, the photomask at the middle, and the photoresist PCB at the bottom. The photomask and PCB must also be tightly pressed together. The exposure will depend on the intensity of the light source. After some time, remove the board from the light.
The developer is a powdered chemical that is mixed on water. The solution will dissolve the portion of the mask that was left exposed. The PCB must be submerged to a container that contains the developer solution. Shake the basin until an image appears. The image, usually of blue color, is the developed printed circuit board pattern. The part exposed to the light source contains bare copper.
This method is used by hobbyists and small businesses. It is still better to use this method, than using a permanent marker as a mask.
PCB milling does not create a mask. It uses a machine that physically removes the copper by drilling on the PCB itself. It is effectively the etched board itself. The miller usually is a two or three-axis mechanical drill. This machine operates like a plotter. It receives commands from a computer software, which controls the driller’s position in the x, y, and, if possible, the z axis. The data is retrieved as a file. This method is commonly used with computer-aided machinery. It is only used by large industries, since only few people or businesses can afford this machine and its corresponding software.
There are various ways to create a printed circuit board pattern. Your chosen method may vary, depending on your objectives. Having a good mask can save you problems in etching.
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