How Are Printed Circuit Boards Fabricated?
Printed circuit boards need to have connections drilled into them. These holes need to be placed precisely and accurately drilled machines use numerically controlled drill files (also called excellon files) to determine the size and position of the drills. Drilling is also possible at a controlled depth to drill one circuit board layer or individual layers of the PCB prior to lamination. This is a relatively straightforward process and can be done in less than 30 minutes.
In the first step of the PCB fabrication process, the printed circuit board is covered with photo-resist material. The resist material is exposed to UV light. Then, a photo positive film is made from the circuit layout artwork. This film is opaque where circuit traces are desired, and clear elsewhere. Then, the film is placed onto the photo-sensitized board. The UV light passes through the clear portions of the film and cures the photo-resist. Next, the board is dipped into a developer bath, which removes the photo-resist layer.
Using acidic or alkaline etching solutions, a PCB can be fabricated using a variety of processes. Both processes are effective, but acidic etching is generally faster and more accurate. Acidic etching uses an acidic solution and can damage remaining copper, while alkaline etching does not. Both types of etching methods require careful monitoring to achieve the best results.
During the assembly process, solder paste is used to connect surface-mount components to pads on a printed circuit board (PCB). The same paste is used for through-hole pin-in-paste details. Solder paste is applied over holes with stencils, syringes, or jet printing. It has adhesive properties, so when the board is heated, the mask melts, creating an electrical bond between the component and the board.
Surface treatment of copper to ensure solderability
Solderability requires a properly prepared copper surface. Copper surfaces oxidize over time, which results in poor solderability. In order to prevent this, some surface finishes are available. Organic Solderability Preservative (OSP) is one such treatment. This process applies a thin, non-toxic layer of organic material to copper surfaces. Unlike other methods, OSP adheres to copper on a continuous basis and prevents copper from oxidation.
Classification of rigid PCBs
The IPC-6011 standard categorizes printed circuit boards by three levels: basic, intermediate, and rigid. If the rigid board is designed to meet the requirements of one of these levels, it will not only be stable but will also function reliably. If it is designed to a lower classification, the board will perform inconsistently, or worse, it will fail prematurely. Below are some examples of each type of PCB.