8 Considerations For PCB Design

Table of Contents

circuit-board design

Every PCB design is unique and comes with its own set of challenges. These challenges vary from one design to the other, depending on the requirements of the project, the resources available, and the experience of the designers. However, a well-designed PCB design can go a long way in improving the reliability and performance of a product. Here is everything you need to know about designing a PCB.


1.1 PCB size and shape

The welding processing size of PCB sheet shape is width (200mm ~ 250mm) * length (250mm ~ 300mm).

If the long side of the PCB is less than 125mm, or the short side is less than 100mm, the panel method can be used.

This size helps avoid problems with wave soldering and reflow soldering processes.

If it is not rectangular, when the PCB is processed and welded by the conveyor belt, it will cause problems such as unstable transmission, flipping of the board when plug-in, and soldering on the component surface when passing through the wave crest tin bath.


If it is not a rectangle, the irregular-shaped PCB is assembled into a rectangle by using a craft panel.

Especially the 4 corners, if there is a gap, fill it up into a rectangle;

For PCBs with only SMD components, gaps are allowed, but the gap size should be less than 1/3 of the length of the side.


1.2  PCB substrate

In the design of the circuit board, the requirements for the PCB board must be put forward and marked in the technical requirements of the circuit board design document, including:

PCB board and grade (usually epoxy resin glass fiber cloth base FR-4, FR-5);

Flame retardant grade (UL94-VO, UL94-V1 or green flame retardant);

Plate thickness, nominal specifications are 0.8, 1.0, 1.2, 1.6, 2.0, 2.5, 3.0, 3.5 (unit mm);

Sheet thickness tolerance ±10%;

For medical device products, the thickness of the plate must be ≥1.6mm, A1 and A2 grades;

For instruments used in flammable and explosive occasions, the flame retardant grade should be marked on the PCB board.

1.3  Coating

PCB coating types are tin-plated (preferred), nickel-plated gold, tin-plated PCB is easy to oxidize when exposed to air for a long time,

Vacuum packaging should be used for plant storage.

1.4 Number of layers

The multi-sided PCB board has outstanding effects in electromagnetic compatibility protection.

At the same time, it has a higher cost of plate making,

Design compromises should be made according to signal requirements.

When fclk>5MHz, or tr<5ns (the rising or falling edge of the pulse), it is recommended to use a multi-layer board;

After the multi-layer board is determined, the number of wiring layers is determined according to the Pin density.

Pin density

If a double-layer board must be used, one side of the printed board must be used as a complete formation.

 

1.5 Design for manufacturability

When designing a PCB, the main consideration is to leave enough space and benchmarks for the production process to avoid technical hidden dangers in the production process.

During the assembly and welding process, the transmission side of the PCB is left with a blank width of ≥5~10mm, and no components or solder joints are placed as the process side.

If the PCB cannot be left with the craft side, the two transfer sides can be widened by 5-10mm as the craft side, and then broken off after welding.

The reference point (also known as the optical MARK point, or MARK point) used in the assembly process must be set on the PCB as the reference point when the device is mounted.

Please contact the process engineer of the assembly manufacturer to determine the pad form and symmetrical layout of the reference point.

Make a background area around the MARK point, and there can be no other pads, silk screen and solder mask in the background area.

The distance between the center of the datum point and the edge of the board is greater than 5mm; there are at least two datum points of each type, which are set at diagonal and asymmetrical positions.

Local reference points should be set for BGA and multi-pin packages. In order to prevent thermal errors during operation, or cumulative errors of PCBs, the placement of fine-pitch pin devices will be offset and lead to patch deviations.

For BGA packaged devices and other packaged devices with more than 100 pins, a pair of local reference points must be placed on the opposite corners;

The optical reference point should be placed within 5mm of the device periphery; no components can be placed within 3mm of the optical reference point.

2 Pads, vias

2.1 Pad

The quality of the pad directly affects the effect of soldering, so the design of the pad is very important.

When the pad aperture is 0.3 – 0.5mm wider than the device pin lead,

When the overall diameter of the pad is 2 – 2.5 times the through diameter of the pad,

It is an ideal condition for welding to achieve a good wetting angle.

The diameter of the tin-penetrating pad of the in-line resistors, capacitors, inductors, magnetic beads, diodes, and triodes is D = the outer diameter of the component pins d + (0.3 ~ 0.5mm).

The pads at both ends of the patch pad should be well symmetrical to ensure the balance of the surface tension of the molten solder and avoid suspension bridges and displacement during welding;

If this requirement is violated, it is easy to solder defects during reflow soldering.

When there are many pin pads in each row of plug-in components, the arrangement direction of the pads is parallel to the direction of the board;

It is recommended to set tin stealing pads to absorb excess solder, and avoid connecting 2-3 pads at the end of the wave crest parallel to the board entry direction.

2.2 Vias

Via Design Rules: Standard via dimensions are:

Aperture: plate thickness ≥ (1:6)

For high-speed signals, a via hole generates an inductance of 1 to 4nH and a capacitance of 0.3 to 0.8pF. When laying a high-speed signal line, the via hole should be the least; for high-speed signal parallel lines, such as address lines and data lines, if the layers of Changes are unavoidable, and it should be ensured that the number of vias for parallel signal lines is the same.

2.3 Mounting screw holes

The mounting screws on the PCB should have a forbidden area. The diameter of the forbidden area = (screw diameter * 2.2) + (2 ~ 3mm) to ensure sufficient electrical insulation space.

3  Layout rules

3.1 Device orientation

Inserts with multiple pins on the soldering surface, such as connectors, DIP packaged devices, and T220 packaged devices, should be laid out so that their axes are perpendicular to the direction of wave soldering.

Active components such as PLCC and QFP should be avoided by wave soldering.

The same type of plug-in components are placed in one direction in the X or Y direction;

For discrete components of the same type with polarity, it is best to keep the polarity direction consistent in the X or Y direction within a functional area.

3.2 Device Layout

Recommended minimum spacing between components:

1. The spacing between the edges of the pads of small RC components is >0.3~0.7mm;

2. Between other chip components, between SOT, between SOIC and chip components is 1 ~ 1.25mm;

3. Between SOIC and between SOIC and QFP is 1.5-2.0mm;

4. The distance between PLCC and chip components, between SOIC and QFP is 2-2.5mm;

5. 3~4mm between PLCC.

6. The distance between the outside of the plug-in component pad and the outside of the chip component pad is greater than 1.5 to 2 mm;

7. The edge spacing of the pads between the plug-in components that have been wave soldered is greater than 1 to 2 mm;

8. The distance between BGA and adjacent components is greater than 3-5mm.

SMT components are not arranged within 3mm of the board surface around the adjustable and pluggable components to prevent the components from being damaged by plugging stress;

The plug connector remote from the mounting post is preferably a flat plug connector.

 

Under natural cooling conditions, temperature-sensitive components, such as ceramic capacitors, electrolytic capacitors, thermistors, and temperature-sensitive ICs, are kept away from heating components.

If it cannot be far away, the temperature must be tested to ensure that the temperature rise of the temperature sensitive device is within the derating range.

 

Keep temperature sensitive devices away from heat sources

Device Layout Rules Matching Soldering Processes:

1 In order to reduce the number of PCB heating, the device layout method must be selected in the following order: single-side insertion – single-side mounting – single-side mixed installation – A side mixed B side insertion – A side mixed B Surface mount – double-sided mixed;

2 If there are a small number of components, such as buzzers, light-emitting diodes, sockets, etc., they must be inserted on the bottom surface, and manual repair welding can be used.

If the thermal power density of the device is greater than 0.4W/cm3, a radiator should be added, or it should be installed on a metal frame or case to improve the overheating capability of the device.

High-power resistor without radiator, with a height of 3~5mm to dissipate heat.

4. Wiring rules

4.1 PCB wiring coating

The relationship between PCB copper foil line width/thickness/carrying current is determined according to (Figure 4.1):

The temperature rise of the printed wire caused by the current is related to the width of the wire when the thickness of the copper foil is constant.

If the design needs to increase the wire current and the wire cannot be widened, the temperature rise of the wire should be fully estimated.

According to the standard SJ/Z1675-81, when the thickness of the wire is constant, the relationship between the load current and the temperature rise of the wire of different widths is shown in the figure.


4.2 wiring rules

The gold fingers protruding from the edge of the board, the two ends of the board side where the gold fingers are located (rounded corners of R1.0 to R1.5), and the insertion end face of the gold fingers (the chamfer size refers to the thickness of the PCB, below R0.5, which is easy to insert Pull out), the above position should be chamfered;

There is no pad at the inner end of the gold finger, and the pad and components are far away from the gold finger.

A/D signals, input and output signals, large and small currents that differ by two orders of magnitude, high and low voltages, and high and low frequency signals must be kept away from each other.

Any printed line width with line spacing ≥3 times, that is, ≥3W, can avoid 70% of the electric field between the two lines from crosstalk.

The ground wire layer on the PCB protrudes 20H from the edge of the edge power supply layer and signal layer (the thickness of the power supply and the ground or the signal layer to the ground is 1H), limiting 70% of the electric field radiation to the edge of the ground layer.

There should be a 3mm forbidden area around the BGA device, and a 5mm forbidden area is the best.

Signal lines should be avoided under crystal oscillators, radiators, relays, optocouplers, power modules, transformers, and voltage regulators.

The wiring width of the same grid is kept the same. The variation of the line width will lead to uneven line impedance characteristics. When the transmission speed of the signal is high, reflection will occur. Try to avoid this situation in PCB wiring. When it is unavoidable to change the line width, try to reduce the effective length of the inconsistent part in the middle, such as the lead lines of BGA packages and the lead lines of connectors.

The signal line and its loop form a closed-loop loop, and floating wiring is not allowed on this closed-loop line; pay special attention to the possibility of self-loops at the same level or at different levels, resulting in unnecessary radiation and reception interference.

The pads on the large-area copper foil are connected to the pads through thermal insulation tapes, that is, thermal insulation pads, and the pads and the copper foil are connected in a “meter” or “cross” shape. Thermal pads cannot be used for pads that pass large currents.

4.3 Socket pin wiring

There should be at least one ground pin for each power source on the socket lead.

5. Identification

5.1  Identification type

The following areas and contents on the PCB should be marked with silkscreen, PCB version number, pin symbol, device polarity, device orientation, safety label, hazard label, parameter values ​​of key points (such as power supply voltage beside the power socket, Vcc1 3.3 V;

Mark the power supply voltage and parameters on the voltage pins of key ICs, key voltage test points, and other functional parameters such as waveform, frequency, etc.).

Use all uppercase letters for cautionary identifiers and lowercase letters beginning with an uppercase letter for descriptive text.

 

Identification requirements

The fuse label should include the fuse serial number, fusing characteristics, rated current value, explosion-proof characteristics, rated voltage value, English warning labels, etc. If there is no space arrangement label on the PCB, the warning content can be explained in the product instruction manual.

The five safety signs (certification mark, manufacturer, manufacturer model, certification file number, flame retardant grade) of the PCB board are complete.

5.2 Identification requirements

The silkscreen labels are placed next to the corresponding components, and will not be covered by the component body after installation, or blocked by the body or other components. the same direction.

There should be no silk screen on pads, tin tracks, vias, and test pads.

The representative pin number should be clearly marked on the silk screen, which is convenient for identifying other pins and checking the direction of the connector.

The spacing between PCB screen printing characters is >5mil, and the distance between the pad edge is >5mil. The width of the screen printing character line is 6mil~12mil, and 8~10mil is recommended. The height of the screen printing character is 50mil~60mil; it can be reset when the PCB board area is small.

6. Design for Testability

The ICT test point on the PCB should be on the soldering surface; the size of the test point pad is ≥24mil (0.6mm), and the minimum distance between two separate test points is 60mils (1.5mm);

For a single board that needs to be tested for ICT, two 125mil non-metallized holes should be designed on the opposite corners of the PCB for ICT test positioning;

Surface mount component pads do not double as test pads.

When the machine is powered on, a digital multimeter can be used to detect the total power supply voltage of the single board, the power supply voltage of each functional circuit, the voltage of key components, and the waveform of key parts.

When designing the PCB, it is necessary to set these test pads and mark them with TP1, TP2… serial numbers, and indicate the voltage value and test waveform in the operation guide.

 

7. Cable

Unshielded signal wire splicing guidelines:

1. Unshielded signal cables, incompatible signal cables (in/out, A/D signals, high-level/low-level signals, high-frequency/low-frequency signals) should not be twisted together;

2. The outgoing signal and the return signal of the signal line are twisted with each other.

 

The outgoing signal and the return signal are spliced ​​with each other

On shielded cables, the magnetic ring should be installed on the inner side of the shielding layer.

The magnetic ring on the cable in the chassis should be fixed on the circuit board or the chassis wall with hot melt adhesive.

 

8. Board-level grounding measures

Signals above 10MHz should be grounded at multiple points nearby, and the ground layer should be covered with copper in a large area.

High and low frequency mixed circuit grounding rules:

≤1MHz, then ground at one point (but pay attention to common impedance interference);

≥10MHz, then ground at multiple points nearby;

Between 1 and 10MHz, if the length of the ground wire is less than λ/20, one point is grounded, and if the length of the ground wire is greater than λ/20, more points are grounded nearby; λ is the wavelength.

It is recommended that low-frequency digital ground (below 10MHz), analog ground, working power ground (relay, stepper motor), safety ground, anti-surge ground, shield ground, etc. be grounded separately first.

Then connect the ground to the ground by means of direct connection, small resistance, magnetic beads, capacitance, large resistance, etc.

The specific connection method depends on the purpose of grounding.

The purpose of grounding is as follows:

1. If only ground-ground equipotentiality is required, direct connection;

2. If there is a pulse current on the digital ground, but the equipotentiality of the two grounds is required, connect it with a small resistance to slow down the impact of the ground current;

3. If the interference on the ground is relatively high-frequency, use magnetic beads to consume the high-frequency pulse current;

If you want to discharge static charge, but want to isolate DC, use capacitor connection.

 

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About Author

Aidan Taylor
Aidan Taylor

I am Aidan Taylor and I have over 10 years of experience in the field of PCB Reverse Engineering, PCB design and IC Unlock.

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