General principles of PCBs design

How to design PCBs correctly, to reach boards which are cost-effective to produce and to populate? Which are the most important PCB design principles? What about production technology of PCB? We have prepared for you a series of articles about the PCB´s design.

This article is presented to you by SOS electronic

PCBs are 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 late '60s, when there were the first rules for their design and production drawn up - IPC standards. The standard IPC-2221 "Generic Standards on Printed Design".

What is the first step when designing PCB?

In electronic praxis, 3 types of components are used and possibilities of their soldering are:

  1. components with leads "Through Hole" – TH (axial, radial), can be soldered by hand or by wave,
  2. leaded components for a surface mount – "Surface Mount Devices" – SMD, which can be soldered in a reflow oven or by a wave,
  3. no-lead components for a surface mount – "Surface Mount Devices" – SMD, which can be soldered in reflow oven.

With these three types of components you can create an electronic device in which these components are placed on a carrier - printed circuit boards (PCB). Components can be populated on a PCB either on one side (SMD, TH or a combination of both) or on both sides (TH only on a top side, SMT on both). When the PCB is designed, there must be placement of components on PCB and type of soldering technology considered, because different rules apply when soldering by solder wave and different for soldering in the reflow oven.

1. Soldering by solder wave

If we want to solder components on bottom of PCB by solder wave, we must ensure that during the soldering process components will not fall off. SMT components on the upper side will be populated into a solder paste, then soldered in a reflow open. SMT components on the bottom of the PCB will be bonded by a glue and then TH components will get into holes in a PCB. SMT components in a glue and TH components will be soldered by a solder wave.

To populate components with leads on one side of the PCB is easy process as only leads are sink into the solder wave and conductive joint is created. Interesting is it in case of SMD components, when whole components are sink into the solder wave and therefore must withstand temperatures of waves that may be up to 260 ° C. On a PCB side, which will be soldered by a wave, only such SMT components should be placed, which are recommended by producer to be soldered by a wave and they meet a condition at least 260 °C for 10 seconds heat resistance. Such as ceramic resistors, MELF, MINIMELF, monolithic capacitors, components in SOT, SOD, SOP packages with minimum pitch of 0.65 mm, in order to avoid a short circuit of the integrated circuit, as wave washes each pin of parts and may result in the creation of so-called bridge or resistive and capacitive trimmers if designed in a way to prevent solder ingress.

We need to put down components that can withstand temperatures of solder wave and have the necessary spacing of the leads. In respect to a way of soldering it´s necessary to take into account distances among SMT components, their orientation against a solder wave as well as a height of particular components. Components should be turned so that their leads created a right angle with a solder wave. This way a proper flush of leads by a wave will be ensured. However, this requirement can only by met if using components with leads on only two opposite sides (SO, SOP, SOIC...). In case of integrated circuits with leads on all 4 sides, we need to place them on top of the PCB and populate in a reflow oven. It is also recommended to create robber pads behind the integrated circuitto reduce solder bridging from the last leads pair. It may prevent from short circuit, but lessen the scope for conductive connection.
 

2. Soldering by solder paste

This is the most common way of soldering today. In this case, conductive connection is created by components placing into solder paste, which is applied before components populating. The joint is thus created also under components. When using a solder paste for soldering, we´ll avoid problems with components twisting. This method of soldering increases integration of components on a PCB. There´s no need to take in mind height relations of particular SMT components (tantallum capacitors, power MELF resistors, power transistors) and this method is also the only suitable for soldering of SMT components with a heatsink pad on a bottom side of a package and also for soldering of SMT components without leads.
Wave soldering can cause on some SMT components an unwanted effect called Tombstoning. By influence of imbalance of forces acting on both ends of a component, the component can „get up“ as a „Tomb stone“ (mainly at two-pin components like resistors, capacitors,.....). This effect is caused by irregular temperature spread on a PCB during reflow. This can be eliminated by correct applying of the solder past using a metal template.

General rules for PCB design in terms of production technology

First of all you need to know where the board is manufactured. Basic information, we should know are:

  1. The minimum track width - W,
  2. The minimum isolation gap - I,
  3. Minimum diameter of a Via (metal through-plated opening) - D.

These parameters determines the density of PCBs that can produce - so called accuracy class. At present it is necessary to use a standard accuracy class 6 or higher. This information need to be applied to design setting rules. The system then does not allow make a line that is not in accordance with these criteria.


PART 2

The layout of components is an important step that aims is to minimize interfering voltages, radiation and overall immunity. What are the main principles? More in our next article about PCB´s design.

The layout of components is an important step that aims is to minimize interfering voltages, radiation and overall immunity. The basic principle is to place highly logic closer to the connector and a slower logic away from the connector. Sometimes meet all the recommendations is not possible, but always seek for a compromise and meet at least one rule, respectively as much as possible, if possible.

To basic principles of components placing belongs above all:

  1. Placing components from higher to lower bandwidths.
  2. Mutual physical separation of particular functional blocks (analog, digital, I/O, power line).
  3. Minimising of distances with aim to eliminate current loops.

Grounding is very important. There are two types of grounding given components leads to a common potential (GND):

  1. Single point – A
  2. Single point parallel – B
  3. Multi-point – C

Multipoint grounding is suitable for high frequency thus also digital applications. A multi-layer board is supposed to be used. Order and thickness of particular PCB layers is determined by overall PCB impedance (usually Z = 50 Ω). It supposes existence of a continuous conductive layer- GND at least in one separate layer of PCB. The principle relies upon a fact, that every pin of a component, that has to be connected to a GND potentials connected with a conductive layer in a shortest possible way. The same recommendation should be applied at connecting the power supply pins of components. Usage of a conductive layer for a common signal (GND) results in minimizing of current loops and decreasing of a parasitic inductivity of tracks.

Filtering of power supply in electronic circuits belong together with grounding k to the most important rules, which should be taken in mind at PCB design. Necessity of decoupling capacitors usage results from a assumption, that "every power supply is far from a load". Let´s suppose that pulse power consumption of a HCMOS gate is 15 mA for 3,5 ns, delay of a signal on a PCB (and also a supply current) is higher than 0,1 ns/cm, reaction time of a regulator on a step change of power consumption is roughly ~ 1 µs. Then it´s necessary to deliver power to a gate from a very near and fast voltage source, what a capacitor is. Proper functionality of decoupling capacitors depends on their capacity, ESR and its position on a PCB.

According to the function we differentiate three types of decoupling capacitors:

  1. Filtering (Bypassing) – serve as a wide band filter for power supply of a whole PCB or its part. Eliminates influence of power supply leads inductance. (C1, C2, C8; C1 and C8 ≈ 10µF až 1000µF). If possible always choose the largest capacity as possible.
  2. Local (Decoupling) – sserve as local energy sources for components and they also reduce pulse currents, that would go through a whole PCB. These capacitors must have excellent high frequency properties. It´s necessary to place them as near as possible to a component pin (C4, C5, C6, C7 ≈ 100pF to 0,1µF)
  3. Group (Bulk) – serve as an energy source for a simultaneous charging of several capacitive loads. Near a microprocessor it is C3 ≈ 10µF.

Decoupling capacitor must be always placed on a path between source and a load. All connections must be designed in a way to minimize current loops surface. Minimize tracks impedance (above all parasitic inductances L1 to L4) by shortest possible tracks and by usage of conductive surfaces. Parasitic inductances L5 and L6 are automatically eliminated.

What in case of digital circuits?

Pre-requisites for a quality PCB design for digital applications start already at a circuit design (schematics design). It is necessary minimizing of pulse currents because there is quick switching gates by digital circuits. Try for the least possible amount of simultaneously switched gates of digital circuits. It's more about software. Also choose a suitable logic as for input capacities and pulse current demands, calculate decoupling capacitors (pulse consumption, noise immunity, outputs load), treat unused inputs (avoid undefined status).

When preparing design of PCB we should minimising current loops surfaces by suitable concept of buses and power line and their placing on terminals, usage of SMT components (they´re smaller than TH components), selection of components with power pins on opposite sides – possibility of decoupling by SMT capacitor directly in a place of power supply leads of a given integrated circuit. Don´t use sockets at very fast circuits.

What needs to be taken into account when designing the PCB to avoid interference?

PCB need to be designed to keep the principle of electromagnetic compatibility (EMC). Electromagnetic compatibility of electric device is based on a fact, that the electronic device mustn´t interfere by its operation with other surrounded devices and that the device must be immune to interference from environment. PCB design from electromagnetic compatibility point of view is activity starting already at electrical schematics design of a design. It can be roughly said, that an electronic device, which „doesn´t radiate“ is also immune to interference.

Among basic rules of a PCB design from EMC point of view belongs mainly:

  1. Minimising of currents in electronic circuits – by a choice of suitable types of components, selection of circuits in respect to input impedances etc.
  2. Minimising of a frequency spectrum – not to use unnecessarily fast components (rise and fall edges). Uselessly fast data communication.
  3. Filtration and protection of input/output terminals – ESD protection (Electrostatic Discharge) and solution of transition effects, limitation of radiation from leads.
  4. Minimising of current loops and track lengths – current loops can be minimised by proper components placing, track placing, earthing, power supply tracks, proper usage of filtration capacitors.
  5. Shielding – suppression of radiation and at increasing of immunity at the same time.

PCB design from the perspective of EMC is a relatively complex operation. In general, if we comply with certain recommendations, we can say that we have done everything possible to make it right.


PART 3

An electronic device which includes a PCB may not operate, so it needs to be somehow revived - tested.

At a PCB design it´s also necessary to consider possibilities of electrical testing. In such cases it is necessary to create some space where we can connect the sensor and measure the necessary parameters (voltage, current, etc.). This means that we need to test electrical functionality of the PCB. Proper sizing of conductive tracks width and the width of the isolating gaps between conductive connections has a significant impact on the current load capacity and voltage load capacity of the PCB conductors. Therefore it is necessary to know what is on that circuit current load, so there will not be overheating. It is also necessary to know what will be the largest tension between the two pads to avoid sparks and creating short circuits. These parameters need to also be applied to the design system.

For example - current load capacity is relatively high in comparison to wire leads, because a flat conductor features much higher cooling surface than a wire conductor. Copper wire with a 0,07 mm2 cross section will get melted at a current of 15 A, while a copper foil of the same cross section will get melted at a current of 60 A. This value approximately responses to 850 A/mm2 current density. However a continuous current load capability is lower, approximately 100 A/mm2. Maximum operating temperature of a PCB depends on a so called base material softening point what is for the most often used material FR4 approx. 125°C. From this reason, it´s necessary to size the track widths to meet this condition.

Value of a maximum allowable voltage among PCB tracks depends on many factors: isolation gap width, type of a used base material, usage of silkscreen and last but not least – operational and safety requirements for a PCB usage. Silkscreen helps maintaining basic PCB properties when exposed to harsh environment like dust and humidity. We differentiate breakthrough voltage and a maximum operation voltage.

At this parameter we should know where the board will be given to use and under what conditions. Values of these voltages and methods of their testing are subjects of regulations.

How to test the PCB?

It´s worth to implement test pads (points), where testing jigs can be connected. If an automated test is possible, testing points should be placed in a matrix of a testing adapter. Placing components on a PCB should also suit needs of a visual inspection. For this purpose it´s necessary to maintain minimum components distances.

Recommended minimum distances between components on a PCB must be kept also because of a possibility of automated components placing by machines (minimum distance to a PCB border to grip the board in a machine) or a possibility of a manual placing.


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