Circuit Boards With Advanced Technologies
New technologies continue to raise the bar for these products, and electronic circuits are becoming more complex. Electronic equipment components such as printed circuit boards (PCBs) must also adhere to the requirements. So PCB designers should know what solutions they can use, what the advantages and limitations are, and how to make the best use of them.
We present here the advanced technologies and requirements used, for example, in HDI circuits, as well as the application of nonstandard machining methods to additional external coatings. Throughout the article, the author explains what parameters define the quality of Advanced technology PCBs.
Preservation and protection
The PCBs often work under harsh environmental conditions, which requires them to be coated with additional coatings (i.e. masks) to protect them from ambient factors and improve their reliability. Some PCB areas are only temporarily protected, e.g. during assembly only, and this is particularly useful when a PCB passes several cycles of automatic soldering. Figure 5 shows an example of how peelable masks are applied to protect these areas. A special coating that protects certain areas on a PCB (and the holes inside them) from direct contact with solder and chemical processes, such as during automatic assembly of PCBs. It can be easily removed after processing and leaves no residue behind.
PCBs that have manual controls, keypads, LCD displays, etc. It is made from silk screened and cured graphite paste that is electrically conductive. The solutions are sometimes used as an additional and simple conductor layer along the PCB outer edge.
In some PCBs, the vias must be permanently protected from environmental conditions. The vias are closed to prevent soldering, shorting, etc. when components are assembled directly over the vias, causing contaminants or solder to enter. A BGA circuit is particularly susceptible to this problem. Currently, designers have many options available for obscuring and filling vias. What is the best method will depend on what risks need to be avoided. Soldering masks obscure vias in covered vias, which is the easiest solution. It does not, however, guarantee a proper fill. It only covers the inner walls and the ring. This provides a basic level of protection that is mainly effective during soldering, since the solder cannot wet the via. A more advanced method involves plugging vias with special filling compounds that are cured during production, then covering them with a mask layer. This method prevents all the hazards described above from affecting the vias.
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Press-fitting and Microvias
PCBs are becoming increasingly complex, so techniques that increase PCB usage are needed, such as microvias, which are holes 0.15 mm in diameter or smaller. The use of blind vias and buried vias as microvias is covered in “Elektronik” issue 6/2013. These vias provide the advantage of increasing the interconnection density without adding more layers or increasing the PCB dimensions. In some cases, packing density is also required due to the EMC requirements of the designed device.
Devices often come with connectors that allow them to be electrically connected to other devices and allow them to be swapped out for servicing, for example. If the connectors are not temperature resistant, then they cannot be soldered. Press-fit connectors are assembled. The connectors are installed by pressing them into specially made vias that have a diameter tolerance of *0.05 mm (in standard). Even though no specific design approach is required for press-fit assembly, design engineers should ensure that the PCB manufacturer knows which holes to dedicate for this process.
