An Overview of Printed Circuit Boards: Design and Assembly

Design and assembly are two of the main processes involved in the manufacture of PCBs. To get a clearer picture of what happens in each of these processes, read on below.

Design

printed circuit boardsBack in the day, designing printed circuit boards was a totally manual process. Artwork generation was done on clear mylar sheets, oftentimes resulting to two to four times the desired size. First, the schematic diagram was converted into a layout of pins, pads and components. Next, traces were routed towards the necessary connections. Pre-printed grids helped in the layout, while rub-on dry transfers of usual circuit element arrangements helped in standardizing the layout. Self-adhesive tape was used to make traces. At the end, the layout “artwork” is reproduced photographically on resist layers of blank boards with copper-clad coating.

These days, however, the practice is a lot less labor intensive. Computers take most of the work, since they can perform a lot of the steps automatically. Here are the steps included in the basic progression of common commercial designs for printed circuit boards:

1. An electronic design automation tool creates a schematic capture.

2. Depending on the required case and circuitry of the PCBs, the template and dimensions of the card are decided. During this step, the need for fixed components (together with heat sinks) is determined.

3. The layers of PCBs are decided – often one to twelve, depending on the complexity of the design. The power and ground plane are also determined. Here, signal planes are found in the top layer and in internal layers.

4. With the help of routing copper thickness, dialectic layer thickness and trace width, the line impedance is determined. In case there are differential signals, trace separation is also considered. To route signals, stripline, dual stripline or microstrip is used.

5. Next, the components are placed. The vias and lands are marked into place, all the while considering geometry and thermal factors.

6. Signal traces are routed. High frequency signals help ensure optimal EMI performance by being routed in internal layers, between ground or power planes.

7. Gerber file generation comes next as a part of the manufacturing process.

8. The final step in design involves the power plane acting as the counterpart of the ground plane. The power plane acts as an AC signal ground and provides DC voltage for circuits which are mounted on the PCBs. On the other hand, if using electronic design automation tools, the power and ground planes are often automatically drawn as a negative layer that have connections and clearances to the plane (which was also automatically created).

Assembly

As the PCBs are finalized, the electronic components are attached, forming a working printed circuit assembly (PCA). For through-hole manufacturing, the leads of components are inserted through holes, thus the name. On the other hand, surface-mount manufacturing requires components to be placed on lands or pads found on the printed circuit boards’ outer surfaces. In either type of construction, the leads of components are both mechanically and electrically attached to the PCBs using a molten metal solder.

There are different ways of soldering used in PCBs. For creating a high volume of production, the surface-mount placement machinery, reflow ovens and bulk wave soldering are often employed. Meanwhile, for small volume production, experienced technicians are capable of soldering the tiniest parts by hand with the help of a microscope, a fine tip soldering iron, and some tweezers. A number of parts can be too difficult to work manually, including BGA packages.

As the printed circuit boards are finalized and done, they have to be tested in multiple ways. If they fail any of the tests, the technicians have to desolder them, and replace components that aren’t working. This is known as “rework”.