10 Questions You Should to Know about multilayer pcb design tips

Whether you are moving at a high speed or you're designing a high-speed printed circuit board, good board design practices help ensure your design will work as intended and can be manufactured at volume. In this guide, we've compiled some of the essential PCB board design and layout guidelines that apply to most modern circuit boards. Specialty designs may need to follow additional board layout guidelines, but the PCB layout guidelines shown here are a good place to start for most board designs.

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The guidelines shown here are focused on a few key areas that will help you with routing, manufacturability, basic signal integrity, and assembly:

When starting a new printed circuit board design, it&#;s sometimes easy to forget about the important design rules that will govern your project. There are some simple clearances that, if determined early in the design, will eliminate a lot of component shifting and re-routing later. So where can you get this information?

The first place to start is to talk to your PCB design rules fabrication house. A good fabricator will usually post their capabilities online or will supply this information in a document. If it's not in an obvious location on their website, send them an and ask for their capabilities. It's best to do this first before you start placing components. While you're at it, make sure to submit your proposed stack-up for review, or look for their standard stack-up data and use that.

Once you've found their list of capabilities, you should compare these to whatever industry reliability standard you'll work with (Class 2 vs. Class 3, or a specialty standard). Once these points are determined, you should select the more conservative design layout limits needed to ensure manufacturability and reliability, and you can encode these into your board design rules.

As you proceed through the layout process, your board design rules will help you eliminate most design errors that will lead to fabrication and assembly problems. After setting the board design rules, you can start the placement process.

The component placement stage of your PCB layout design process is both an art and a science, requiring a strategic consideration of the prime real estate available on your board. The goal in component placement is to create a board that can easily be routed, ideally with as few-layer transitions as possible. In addition, the design has to comply with the design rules and satisfy must-have component placements. These points can be difficult to balance, but a simple process can help a board designer place components that meet these requirements:

1. Place must-have components first. There are often components that must be placed in specific locations, sometimes due to mechanical enclosure constraints or due to their size. It's best to place these components first and lock in their position before proceeding to the rest of the layout.
2. Place large processors and ICs. Components like high pin count ICs or processors generally need to make connections to multiple components in the design. Locating these components centrally makes trace routing easier in the PCB layout.
3. Try to avoid crossing nets. When components are placed in the PCB layout, the unrouted nets are normally visible. It's best to try and minimize the number of crossing nets. Each net intersection will require a layer transition through vias. If you can eliminate net crossings with creative component placement, it will be easier to implement the best routing guidelines for a PCB layout.
4. SMD PCB board design rules. It&#;s recommended to place all surface mount device (SMD) components on the same side of the board. The main reason for this arises during assembly; each side of the board will require its own pass down the SMD soldering line, so placing all SMDs on one side will help you avoid some extra assembly costs. 
5. Experiment with orientation. It's okay to rotate components to try and eliminate net intersections. Try to orient connected pads so that they face each other as this can help simplify routing.

If you follow points #1 and #2, it's much easier to lay out the rest of your board without too much crossover between routes. In addition, your board will have that modern look and feel to the layout, where a central processor supplies data to all the other components around the perimeter of a board.

The main processor in this PCB layout design is centrally located with traces routed out from the edges. This is the ideal placement of larger ICs and peripherals.

The main processor in this PCB layout design is centrally located with traces routed out from the edges. This is the ideal placement of larger ICs and peripherals.

With components placed, it&#;s now time to route power, ground, and signal traces to ensure signals have a clean and trouble-free path of travel. Here are some guidelines to keep in mind for this stage of your layout process:

It&#;s generally the case that power and ground are placed on two internal layers. For a 2-layer board, this might not be so easy, so you would want to place a large ground plane on one layer, and then route signals and power traces on the other layer. With 4-layer circuit board stack-ups and higher layer counts, you should use ground planes instead of trying to route ground traces. For components that need direct power connections, it&#;s recommended to use common rails for each supply if a power plane is not used; ensure you have wide enough traces (100 mils is fine for 5 to 10 A) and don't daisy chain power lines from part to part.

Some recommendations state that plane layer placement must be symmetrical, but this is not strictly required for manufacturing. In large boards, this might be needed to reduce the chances of warping, but this is not a concern in smaller boards. Focus on access to power and ground, as well as ensuring all traces have strong return path coupling to the nearest ground plane first, then worry about perfect symmetry in the PCB design stack-up.

Next up, connect your signal traces to match the nets in your schematic. PCB layout best practices recommend that you always place short, direct traces between components when possible, although this may not always be practical on larger boards. If your component placement forces horizontal trace routing on one side of the board, then always route traces vertically on the opposite side. This is one of many important 2-layer PCB board design rules.

Printed circuit board design rules and PCB layout guidelines become more complex as the number of layers in your stack-up increases. Your routing strategy will require alternating horizontal and vertical traces in alternating layers unless you separate each signal layer with a reference plane. In very complex boards for specialized applications, many of the commonly-touted PCB best practices may no longer apply, and you'll need to follow PCB board design guidelines that are particular to your application.

PCB layout designs use traces to connect components, but how wide should these traces be? The required trace width for different nets depends on three possible factors:

1. Manufacturability. Traces cannot be too thin, otherwise, they can't be reliably manufactured. In the majority of cases, you'll be working with trace widths that are much larger than the minimum value your fabricator can produce.
2. Current. The current carried in a trace will determine the minimum required width to prevent the trace from overheating. When the current is higher, the trace will need to be wider.
3. Impedance. High-speed digital signals or RF signals will need to have a specific trace width to hit a required impedance value. This doesn't apply to all signals or nets, so you don't need to enforce impedance control on every net in your board design rules.

For traces that don't need specific impedance or high current, a 10 mil trace width is fine for the vast majority of low-current analog and digital signals. Printed circuit board traces that carry more than 0.3 A may need to be wider. To check this, you can use the IPC- nomograph to determine your PCB design trace width for a required current capacity and temperature rise limit.

Preferred routing (arrows indicate component shifting direction)

 

Non-preferred routing (arrows indicate component shifting direction)

The ground plane can act as a large heat sink that then transports heat evenly throughout the board. Therefore, if a particular via is connected to a ground plane, omitting the thermal relief pads on that via will allow heat to conduct to the ground plane. This is preferable to keeping heat trapped near the surface. However, this can create a problem if through-hole components are assembled on the board using wave soldering as you need to keep heat trapped near the surface.

Thermal reliefs are one PCB layout design feature that might be needed to ensure a board will be manufacturable in a wave soldering process, or in other words, for through-hole components connected directly to planes. Because it can be difficult to maintain process temperatures when a through-hole is a solder point directly to a plane, it&#;s recommended thermal reliefs be used to ensure the soldering temperature can be maintained. The idea behind thermal relief is simple: it slows the rate at which heat is dissipated into the plane during soldering, which will help prevent cold joints.

 

The typical thermal relief pattern

Some designers will tell you to use a thermal relief pattern for any via or hole that is connected to an internal ground or power plane, even if it is just a small polygon. This advice is often overgeneralized. The need for a thermal via on any through-hole component will depend on the size of the copper plane or polygon that will make a connection on the internal layer, and it is something you should request your fabricator review before you place your board into production.

Through-hole pads on copper pour could require the same thermal pad application as planes. When the pour is very large, it starts to look a lot like a plane, and so a thermal pad should naturally be applied if a through-hole pin will be soldered into that connection.

For SMD parts, this is not always the case. Whether the thermal gets applied to the pour region depends on how the PCB will be assembled. A reflow soldering process will heat up the board uniformly as the device passes through the reflow oven, so the potential form tombstoning is much lower for those SMD pads, regardless of the presence of a thermal connection.

If the design will be assembled by hand, such as with solder paste and a heat gun, then the PCB layout might need thermals to trap enough heat near the pad and prevent tombstoning. When soldering by hand, it can be difficult to maintain consistent heating across the component leads, and a thermal connection can help prevent a tombstoning defect.

Thermal connection on a polygon in Altium Designer.

By default, Altium Designer will maintain thermal connections onto polygons when you create a new project. This is configured using the Polygon Connect rule in the PCB design rules editor. You can change this setting to apply based on specific footprints, layers, component classes, nets/net classes, or any other conditions using the query language in Altium Designer.

There are some routing guidelines for PCB design rules about how to group and separate components and traces so that you ensure easy routing while preventing electrical interference. These grouping guidelines can also help with thermal management as you might need to separate high-power components.

Some components are best placed in the PCB layout design by grouping them in one area. The reason is that they might be part of a circuit and they may only connect to each other, so there would be no need to place the components on different sides or areas of the board. PCB layout then becomes an exercise in designing and laying out individual groups of circuitry so that they can be easily connected together with traces.

In many layouts, you'll have some analog and some digital components, and you should prevent the digital components from interfering with the analog components. The way this was done decades ago was to split up ground and power planes into different regions, but this is not a valid design choice in modern board designs. Unfortunately, this is still communicated in many board layout guidelines and it leads to many bad routing practices that create EMI.

Instead, use a complete ground plane below your components, and do not physically break the ground plane up into sections. Keep the analog components with other analog components operating at the same frequency. Also, keep the digital components with other digital components. You can visualize this as having each type of component occupying a different region above the ground plane in the PCB layout design, but the ground plane should stay uniform in the majority of board designs.

Example of digital & analog sections in a PCB.

It is also appropriate to separate components that will dissipate a lot of heat on the board into different areas. The idea behind separating these high-power components is to equalize the temperature around the PCB layout, rather than to create large hotspots in the layout where high-temperature components are grouped. This can be accomplished by first finding the &#;thermal resistance&#; ratings in your component&#;s datasheet and calculating the temperature rise from the estimated heat dissipation. Heatsinks and cooling fans can be added to keep component temperatures down. You may have to carefully balance the placement of these components against keeping trace lengths short as you devise a routing strategy, which can be challenging.

It&#;s easy to get overwhelmed toward the end of your design project as you scramble to fit your remaining pieces together for manufacturing. Double and triple-checking your work for any errors at this stage can mean the difference between a manufacturing success or failure.

To help with this quality control process, it&#;s always recommended to start with your Electrical Rules Check (ERC) and Design Rules Check (DRC) to verify you&#;ve met all of your established constraints. With these two systems, you can easily define gap widths, trace widths, common manufacturing requirements, high-speed electrical requirements, and other physical requirements for your particular application. This automates PCB design layout review guidelines for validating your layout.

Note that many design processes state that you should run design rule checks at the end of the board design phase while preparing for manufacturing. If you use the right design software, you can run checks throughout the design process, which allows you to identify design potential problems early and correct them quickly. When your final ERC and DRC have produced error-free results, it&#;s then recommended to check the routing of every signal and confirm that you haven&#;t missed anything by running through your schematic one wire at a time.

There you have it - our top PCB layout guidelines that apply to most circuit board designs! Although the list of recommendations is short, this guideline can help you get well on your way toward designing a functional, manufacturable board in no time. These PCB board design guidelines only scratch the surface, but they form a foundation for building upon and solidifying a practice of continual improvement in all your design practices.

If you want to get started with the best PCB board design software with a built-in rules-driven design engine that helps you stay accurate, use the advanced design tools in Altium Designer®. When a design is finished and ready to be released to manufacturing, the Altium 365&#; platform makes it easy to collaborate and share your projects.

We have only scratched the surface of what&#;s possible with Altium Designer on Altium 365. Start your free trial of Altium Designer + Altium 365 today.

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&#;I had not been exposed to much 3D PCB design capability in the past. As a relatively new ALTIUM user I have to say that designing flexible and rigid PCBs in 3D using Altium is a lot easier than I thought it would be. Trading 3D PCB layout design files with my mechanical cohorts for review has never been easier!&#;

Kelly Dack, CID+ CIT
PCB Designer / IPC Instructor

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Q1: How to choose PCB (Printed Circuit Board) material?

A1: PCB material has to be selected totally based on the balance between design demand, volume production and cost. Design demand involves electrical elements that should be taken into serious consideration during high-speed PCB design. In addition, dielectric constant and dielectric loss should be considered whether they go with the frequency.

Q2: How to avoid high-frequency interference?

A2: The leading principle to overcome high-frequency interference is to reduce crosstalk as much as possible, which can be achieved by enlarging the distance between high-speed signals and analog signals or equipping ground guard or shunt traces beside analog signals. In addition, the noise interference caused by digital ground on analog ground should be carefully considered.

Q3: How to arrange traces carrying differential signals?

A3: Two points should be focused in terms of traces carrying differential signals design. On one hand, the length of two lines should be the same; on the other hand, the spacing between two lines should maintain parallel.

Q4: How to arrange traces carrying differential signals when there&#;s only one clock signal line at output terminal?

A4: The premise of traces carrying differential signals arrangement is that both signal sources and receiving end should be differential signals. Therefore, differential routing can never work on clock signals containing only one output end.

Q5: Can matched resistance be added between differential pairs at receiving end?

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A5: Matched resistance is usually added between differential pairs at receiving end and its value is equal to that of differential impedance. As a result, signal quality will be better.

Q6: Why should differential pair traces be close to each other and parallel?

A6: Differential pair traces should be properly close and parallel. The distance between differential pair traces is determined by differential impedance that is a key reference parameter in terms of differential pair design.

Q7: How to resolve the conflicts between manual routing and auto-routing on high-speed signals?

A7: Now most automatic routers are able to control wire running method and number of through holes by setting constraint conditions. All EDA companies differ a lot from each other in terms of wire running methods and constraint condition setting. The difficulty of automatic routing is closely related with wire running capability. Therefore, this problem can be resolved by picking up a router with high capability of wire running.

Q8: In high-speed PCB design, the blank area of signal layers can be coated with copper. How should copper be distributed on multiple signal layers on grounding and powering?

A8: Generally, copper coating is mostly connected with the ground in blank area. The distance between copper coating and signal lines should be strictly designed because coated copper will reduce characteristic impedance a little. Meanwhile, characteristic impedance of other layers should not be influenced.

Q9: Can characteristic impedance on power plane be figured out by micro strip line model? Can micro strip line model be used on signals between power plane and ground plane?

A9: Yes. During the procedure of characteristic impedance calculation, both power plane and ground plane can be regarded as a reference plane.

Q10: Can the test points generated through automation on high-density PCB meet the testing demands of massive production?

A10: It all depends on the case whether regulations on test points are compatible with the requirement laid by test machines. In addition, if routing is run too densely and regulations on test points are very strict, there may be no ways to put test points on each segment of line. Of course, manual methods can be used to complement test points.

Q11: Can test point adding influence the quality of high-speed signals?

A11: It all depends on the case whether test point adding method and the running speed of signals. Basically, adding test points are obtained through adding them to lines or pulling a segment out. Both methods can more or less affect high-speed signals and the effect extent is related with frequency speed and edge rate of signals.

Q12: When a couple of PCBs are connected into a system, how should ground lines of each PCB be connected?

A12: Based on Kirchoff current law, when power or signals are sent from Board A to Board B, equivalent amount of current will be returned to Board A from ground plane and the current on ground plane will flow back at the path where the impedance is the lowest. Therefore, the number of pins contributed to the ground plane should never be too small at each interface of power or signal interconnection so that both impedance and noise on the ground can be reduced. Additionally, the whole current loop should be analyzed, especially the portion where current is the largest and connection of ground plane or ground lines should be adjusted to control the current running and decrease the influence on other sensitive signals.

Q13: Can ground lines be added to the middle of differential signal lines?

A13: Basically, ground lines cannot be added among differential signal lines because the biggest significance of differential signal line principle lies in the advantage led by mutual coupling between differential signal lines, such as flux cancellation, noise immunity etc. Coupling effect will be destroyed if ground lines are added among them.

Q14: What is the principle of picking up suitable PCB and cover grounding point?

A14: The principle is to take advantage of chassis ground to provide a path with low impedance to returning current and to control the path of this returning current. For example, screw can be normally used near high-frequency component or clock generator to connect the ground plane of PCB with chassis ground to reduce the whole current loop area as much as possible, that is, to reduce electromagnetic interference.

Q15: Where should PCB debug start?

A15: As far as digital circuit is concerned, the following things should be done in order. First, all power values should be confirmed to averagely achieve design requirement. Second, all the clock signal frequencies should be confirmed to normally work and there&#;s no non-monotonic problem on the edge. Third, reset signals should be confirmed to achieve standard requirement. If the above things have been confirmed, chip should send signals in the first cycle. Then, debug will be carried out based on system running protocol and bus protocol.

Q16: What is the best way to the design of high-speed and high-density PCB with board area fixed?

A16: In the process of high-speed and high-density PCB design, crosstalk interference should be especially focused since it greatly affects timing and signal integrity. Some design solutions are given. First, the continuity and matching of the routing characteristic impedance should be controlled. Second, spacing should be noticed and spacing is normally twice line width. Third, proper termination methods should be picked up. Fourth, routing in adjacent layers should be implemented in different directions. Fifth, blind/buried vias can be used to increase routing area. In addition, differential termination and common-mode termination should be maintained so as to reduce the influence on timing and signal integrity.

Q17: LC circuit is usually applied to filter wave at analog power. Why does LC sometimes perform better then RC?

A17: The comparison between LC and RC should be based on the assumption whether frequency band and inductance are suitably selected. Because reactance of inductance is correlated with inductance and frequency, if the noise frequency of power is too low and inductance isn&#;t high enough, LC performs worse than RC. However, one of the disadvantages of RC lies in the fact that resistor itself will consume energy with low efficiency.

Q18: What is the optimal way to achieve EMC requirement without cost pressure?

A18: PCB board suffers from higher cost due to EMC usually because layer count goes up to strengthen shielding stress and some components are prepared such as ferrite bead or choke that are used to stop high-frequency harmonic wave components. Besides, other shielding structure on other systems should be used to meet the demands of EMC. First, components with low slew rate should be applied as many as possible so as to decrease high-frequency portions generated by signals. Second, high-frequency components should never be placed too near exterior connectors. Third, impedance matching, routing layer and return current path of high-speed signals should be carefully designed to cut down high-frequency reflection and radiation. Fourth, sufficient decoupling capacitors should be placed at power pins in order to reduce the noise at power plane and ground plane. Fifth, the ground near exterior connector can be cut away from ground plane and connector ground should be near chassis ground.

Q19: When a PCB board features multiple digital/analog modules, the ordinary solution is to divide digital/analog modules. Why?

A19: The reason for dividing digital and analog modules is that noise is usually generated at power and ground at the switching of high and low potential and the extent of noise is related with signal speed and current amount. If analog and digital modules are not divided and the noise generated by digital module is larger and circuit at analog area is similar, even if analog and digital signals don&#;t come across, analog signals will still be affected by noise.

Q20: When it comes to high-speed PCB design, how should impedance matching be implemented?

A20: As far as high-speed PCB design is concerned, impedance matching is one of the leading considerations. Impedance features absolute relationship with routing. For example, characteristic impedance is determined by a couple of elements including spacing between microstrip or stripline/double stripline layer and reference layer, routing width, PCB material etc. Differently speaking, characteristic impedance can never be determined until routing. The essential solution to this problem is to stop impedance discontinuity from occurring as much as possible.

Q21: In the process of high-speed PCB design, which measures should be taken in consideration of EMC/EMI?

A21: Generally speaking, EMI/EMC design should be considered from both radiated and conducted aspects. The former belongs to the portion whose frequency is higher (more than 30MHz) while the latter to the portion whose frequency is lower (less than 30MHz). Therefore, both high-frequency portion and low-frequency portion should be noticed. A good EMI/EMC design should start from components&#; placement, PCB stack up, routing, component selection etc. Once those aspects leave unconsidered, cost will possibly rise. For example, clock generator should not be close to exterior connector as much as possible. Additionally, connecting points should be properly selected between PCB and chassis.

Q22: What is routing topology?

A22: Routing topology, also called routing order, refers to the order of routing in terms of network with multiple terminators.

Q23: How should routing topology be adjusted to increase signal integrity?

A23: This type of network signals is so complex that topology is different based on different directions, different levels, different kinds of signals. Therefore, it&#;s difficult to judge which type of signals is beneficial to signal quality.

Q24: What&#;s the reason for copper coating?

A24: There are usually a couple of reasons for copper coating. First, massive ground or power copper coating will have shielding effect and some special ground, PGND for example, can have a role of protection. Second, to ensure high performance of electroplating or stop lamination from being deformed, copper should be coated on PCB board with less routing. Third, copper coating derives from the requirement on signal integrity. A complete returning path should be provided to high-frequency digital signals and DC network routing should be reduced. In addition, thermal dissipation should be taken into consideration as well.

Q25: What is return current?

A25: As high-speed digital signals are running, signals flow from drivers to carrier along PCB transmission line and then return to driver terminal through the shortest path along ground or power. The returning signals at ground or power are called return current.

Q26: How many types of terminals are there?

A26: Terminal, also called matching, is usually classified into source matching and terminal matching. The former refers to series resistor matching while the latter refers to parallel matching. A lot of methods are available, including pull-up resistor, pull-down resistor, Davenan matching, AC matching, Schottky diode matching etc.

Q27: What elements can determine matching types?

A27: Matching type is usually determined by BUFFER characteristics, topology, level classifications and judgment type. Besides, signal duty cycle and system energy consumption have to be considered as well.

Q28: What inspection should be carried out on PCB before it is released by manufacturing factory?

A28: Most PCB manufacturers implement on-off test on PCBs before they leave factory in order to make sure all the circuits are correctly connected. Up to now, some advanced manufacturers carry out X-ray inspection to find out some obstacles on etching or lamination. When it comes to the products going through SMT assembly, ICT is usually applied, which calls for ICT test points setting during PCB design phase. As soon as problems occur, a special type of X-ray inspection can be used as well.

Q29: For a circuit composed by a couple of PCB boards, should they share the same ground?

A29: A circuit composed by a couple of PCB boards should normally share the same ground because it&#;s impractical to apply a couple of powers in a single circuit. Of course, if your conditions allow, different powers can be used as well. After all, that&#;ll help reduce interference.

Q30: How should ESD be considered by a system containing DSP and PLD?

A30: As far as ordinary systems are concerned, the portions should be first considered with direct contact with human and proper protections should be done on circuit and structures. The extent of influence ESD bring towards system is usually determined according to different situations. In dry environment, ESD will become worse, especially on the system that is more sensitive. Even though larger system features unobvious effect on ESD, more attention should also be paid.

Q31: When it comes to a 4-layer PCB design, what side should receive copper coating on both sides?

A31: The following aspects should be taken into consideration for copper coating: shielding, thermal dissipation, reinforcement and PCB manufacturing demand. Therefore, the main reason should be considered. For example, in terms of high-speed PCB design, shielding should be most considered. Surface grounding is beneficial to EMC and copper coating should be completely done in case of lonely island. Generally speaking, if components on the surface receive too much routing, it&#;ll be difficult to keep copper foil complete. Therefore, it&#;s suggested that boards with many surface components or much routing aren&#;t coated with copper.

Q32: In the process of clock routing, is it necessary to add ground shielding on both sides?

A32: It depends on crosstalk or EMI of board. If shielding ground lines are not properly processed, it&#;ll bring forward bad effects on the contrary.

Q33: What is the strategy of clock routing for signals at different frequencies?

A33: In terms of routing for clock lines, signal integrity analysis should be first carried out and routing principles should be manipulated. Then it&#;s time to implement routing based on the principles.