pcb desing rules

Each PCB is different as for each design in establishing a separate set of regulations.

PCB Design – General Principles of PCB Design

How to design PCBs correctly to reach boards that are cost effective to produce and plant? What are the most important principles of PCB design? How about PCB production technology? We have prepared for you a series of articles on PCB design.

PCBs are an integral part of all electronic devices. Their basic function is to create a conductive connection between the pins of the individual components. PCBs first appeared in the 1960s, when the first design and production rules were developed – CPI standards. The standard for PCBs is the IPC-2221 “Generic Standards on Printed Design” standard.

What is the first step in PCB design?

In electronic practice, 3 types of components are used and their gluing possibilities are:
• components with “Through Hole” – TH terminals (axial, radial), can be glued manually or by wave,
• components with Surface Mount Devices – SMD, which can be glued in a reflow or wave oven,
• components without surface mounting terminals – “Surface Mount Devices” – SMD, which can be glued in the reflow oven.

With these three types of components you can create an electronic device in which these components are placed on a support – printed circuit board (PCB). The components can be planted on a PCB, either on one side (SMD, TH or a combination of the two), or on both sides (TH only on the top, SMT on both). When the PCB is designed, there must be the location of the components on the PCB and the type of soldering technology taken into account, as different rules apply to wave soldering or reflow soldering.

1. Soldering by soldering wave

If we want to solder components on the bottom of a PCB, we need to make sure that the components do not fall during the soldering process. The SMT components on top will be planted in a solder paste, then glued in an open reflow. The SMT components on the bottom of the PCB will be glued together with an adhesive and then the TH components will enter the holes in the PCB. SMT components bonded with adhesive and TH components will be bonded by a soldering wave.

Planting the components with terminals on one side of the PCB is an easy process, requiring only immersion in the solder wave and a conductive node is created. In the case of SMD components, when all components are immersed in the solder wave, they must withstand the wave temperature, which can be up to 260 ° C. On the side of the PCB, which will be soldered in a wave, only such SMT components can be planted, which have the manufacturer’s recommendation that they can be soldered in a wave and they must meet the requirement to withstand at least 10 seconds at 260 ° C. . These can be ceramic resistors, MELF, MINIMELF, monolithic capacitors, components in SOT, SOD, SOP capsules with a minimum pitch of 0.65 mm, to avoid a short circuit of the integrated circuit, because the wave covers each terminal of the parts and may involve the creation of a so-called resistive or capacitive bridge if they are designed in a way that prevents the soldering solution from entering.

We need to put down the components that can withstand the temperatures of the soldering wave and have the necessary distance between the terminals. For correct soldering it is necessary to take into account the distances between the SMT components, their orientation towards the soldering wave as well as the height of certain components. The components must be so arranged that their terminals create a right angle with the soldering wave. This will ensure proper gluing. However, this requirement can only be met if components with terminals on two opposite sides (SO, SOP, SOIC…) are used. In the case of integrated circuits with terminals on all 4 sides, it is necessary to place them on top of the PCB and glue them in a reflow oven. It is also recommended to mount brackets behind the integrated circuit to reduce the soldering point of the last pair of terminals. Short-circuiting can be prevented, but the possibilities of connecting the conductors are reduced.

2. Soldering with solder paste

Today, this is the most common way to glue. In this case, the conductive connection is created by the components placed in the solder paste, which is applied before planting the components. The connection of the components is thus created. When using a solder paste for soldering, we avoid problems with twisting the components. This soldering method increases the integration of components on a PCB. We do not have to consider the height of certain SMT components (tantalum capacitors, MELF power resistors, power transistors) and this method is also suitable for soldering SMT components with a radiator at the bottom of the capsule and also , for soldering SMT components without terminals. The soldering wave can cause some unwanted SMT components called “Tombstoning”. Under the influence of the imbalance of forces acting at both ends of a component, the component can “rise” like a “Tomb stone” (mainly to two-pin components, such as resistors, capacitors,… .. ). This effect is caused by irregular temperatures spread on a PCB during leakage. This can be removed by correctly applying the solder paste using a metal template.

General rules for the design of PCBs in terms of production technology

First, you need to know where the board is made. The basic information we should know is:
• Minimum route width – W,
• Minimum isolation difference – I,
• Minimum diameter of Via (opening in metal) – D.

These parameters determine the density of PCBs that can be produced – the so-called accuracy class. Currently, it is necessary to use a standard accuracy class of 6 or higher. This information must be applied when setting the design rules. The system does not allow the creation of a line that does not comply with these criteria.

 

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