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In the era of miniaturization and HDI PCBs, space optimization on the PCB might frequently affect proper component placement. Appropriate component placement on a PCB improves the signal quality and performance of the device.
From a PCB assembly point of view, proper placement of components reduces the cost of production and reduces project production lead times while maximizing the operational efficiency of the PCB assembly. This makes PCB design considerations for component placement a critical part of the design process.
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This article outlines the aspects of proper component placement from a performance and production aspect. Also, how such aspects can decrease the turnaround time for a high-quality PCB assembly. Here are some aspects of strategies for proper component placement:
The first step in PCB layout design is to have a floorplan. A floorplan is a rough sketch that allocates general areas where blocks of circuitry and connectors are to be placed on the board.
A PCB schematic is a fundamental two-dimensional circuit representation showing the functionality and connectivity between different active and passive components.
The floorplan links the schematic drawing and the layout. It provides a head start to the designer on component placement. A pre-placement floorplan will allow the designer to optimize the component placement for signal integrity goals. Also, the small components, like bypass capacitors and termination resistors will be placed at an early stage instead of getting squeezed into the design at the end. The functional blocks of circuitry like power conditioning, RF, digital, analog, etc., should be arranged as groups to reduce signal crossing.
A floorplan gives an insight into how the signals flow between functional blocks. For an efficient board, group the power conditioning together so that their signals do not have to cross through sensitive areas of RF circuitry. Plan your placement to maintain the connection lengths as short as possible. The components that are part of high-speed signal paths where multiple nets connect a series of components should be placed as close together as possible. The designer has to consider the routing channels while planning the component placement to ensure there is adequate space. Check out our tutorial on how to place components in KiCad.
The requirements of power and ground planes should also be considered when planning out the placement of functional blocks of circuitry. Always implement continuous power planes unless there is a scenario where the design demands a split power plane. When dealing with split power planes, be cautious while placing the connected components across the split. The high-speed transmission lines should not cross the splits in power planes since it will break up the return path for those signals. Finally, avoid placing components of a different functional group in the midst of another circuit. For example, separate analog circuits from digital circuits. The figure below shows an example of a PCB floorplan:
If your layout has repetitive design blocks, auto-group the components to speed up the routing process. Read our article on 6 design mistakes that lead to PCB assembly errors to know more about the prevalent design pitfalls that can cause incorrect component placement.
The placement routing topologies should be implemented in a way where the analog and digital GND do not mix up. The block diagram below shows the component placement. As we can observe in the first placement scheme, the analog and digital currents overlap each other. This degrades the analog signals due to crosstalk. The PCB designer must avoid such practices. In the second scheme, we can see the GND paths for analog and digital are isolated. Here, the digital return current does not interfere with the analog reference GND. The designer should adopt a placement strategy such that even if the GND is not isolated (same net), the currents of either section do not interfere with each other. To deal with crosstalks in HDI boards read how to avoid crosstalk in HDI substrates.
As a first step, the designer should read the schematic and breakdown into sub-sections depending on the circuit function. For instance: analog, digital, high-speed, high-current, power supply, etc.
Apart from the circuit function, the voltage and current levels should be analyzed. Circuits with similar VCC and GND should be grouped and placed together.
Some of the component placement considerations are discussed below.
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The component placement aspect of PCB design depends on the product housing, floor plan by the client, and the density & type of components used.
While starting placement a particular order should be followed:
Some of the important factors that should be considered while carrying out component placement are listed below:
Apart from the circuit function, voltage & current levels should be considered. Circuits with similar VCC and GND should be grouped and placed together as shown in figure no (4). The different functioning sections are prioritized and placed. The most critical are in order of priority
Placing components in close proximity could cause difficulty in the soldering and assembly process, thereby increasing assembly costs. For strategies to keep the assembly expense down, see how to reduce PCB assembly cost.
In high-speed PCBs, the termination resistors must be placed along with the rest of the components instead of squeezing them in the end. Since termination resistors are part of the circuit as a whole, their placement is critical for the accurate operation of the circuit.
Here are two commonly used schemes for placing termination resistors:
A termination resistor is placed on the end of the circuit nearest to the receiver while the other side is attached to the power or ground plane. Higher trace lengths from the load pin to the resistor will mean that the circuit will be more susceptible to signal reflection. Hence, the designer should place the resistor as close as possible to the load pin of the receiver to minimize the stub length of the connecting trace.
The series resistor is attached inline after the driver pin of the circuit within lumped distance. Since the resistor is an inline part, it doesn't have the stub length to be considered like in a parallel termination resistor. After figuring out the right strategy for placing the termination resistors, the designer can include it in the floorplan.
It's advisable to place all the surface mount devices (SMD) on the same side of the board. All through-hole components should be placed on the top side of the PCB to reduce the assembly steps.
In a high-speed circuit, any metal on the PCB should be considered as part of that circuit. The trace lengths, the via size, and the via depth should be taken into account for high-speed circuit calculation. The designer should understand that the via drill size impacts the size of the via. The smaller the vias, the better will be the performance of the circuit. Once the designer decides the vias sizes, the right placement of these vias close to their respective pads should be considered.
In high-speed design, the vias are placed closed to the pad to avoid parasitic capacitance. Sometimes they are placed partially on the pad or entirely within the pad. These adjustments require DRC adjustments.
To learn about pad design, read What is a Pad in PCB Design and Development?
It is good practice to minimize the use of sockets since they can introduce inductance. The designer should select the correct package, and accompanying component footprint while designing for high-speed. Components like op-amps are available in different packages. One type of op-amp may support shorter trace lengths in a circuit than the other. Finally, the component footprint shapes should be optimized for thermal considerations. The best way to dissipate heat is to place power pads right under the IC footprints that are connected to an internal plane. Even the smallest change in the pad shapes can tighten up the routing and decrease the connection lengths. This will result in a compact design.
Isolate the ground planes when required in the digital section. The figure presented below shows the necessity of isolation within the digital sections. The USB and RJ45 connectors communicate externally. The board GND needs to be isolated to avoid any noise or interference from external chassis/body GND.
To protect circuits from external voltage surges often surge suppressing diodes and inductors are provided in the circuits. Place these surge suppressors close to the incoming signal connectors and avoid vias connecting these surge suppressors since these vias have parasitic inductances.
Read about via design in our article PCB Via Design Using Altium Designer
The following factors directly affect the component placement speed during assembly.
Placement speed: the complete process of picking, inspecting, and placing the component on the board needs to follow standards to ensure optimal component placement speeds. By reducing the number of steps, one can increase the speed of placement. For example, if the components are oriented in a similar direction the components need not be rotated during placement, thus increasing the speed of assembly.
Part size: PCB components will vary in size. The manufacturer will have to consider the sizes of the components to decide the speed of the process accordingly. For every different part size, the pick and place tool will have to change.
Feeder capacity: The number of tape feeders that can be loaded into the machine at a time will directly affect the speed of the process. The size of the feeders will vary depending on the size of the component.
Align similar components in the same direction. This helps in effective routing in PCB design and ensures an error-free soldering process during assembly.
It is essential for manufacturers to oversee proper component placement in order to reduce downtime and turnaround time while streamlining the PCB assembly process. Component placement affects the performance of the device in terms of signal integrity and EMI and is taken care of during the PCB design. Similarly, the mechanical orientation of the component selection affects the assembly time and cost and therefore needs to be considered carefully. Executing the correct testing method proves to be beneficial in determining any wrong placement or faulty components.
To understand the fundamental things such as component selection, schematic checks, etc., required to produce a robust design read, mastering the art of PCB design basics.
Need help designing a cost-efficient circuit board? Post your questions and get them answered on our PCB forum, SierraConnect.
Designing with PCB assembly in mind is one of the most important and frequently misunderstood elements. This series is dedicated to helping you become a designing pro'someone whose board design has been optimized on the first try, ensuring a smooth quick-turn production process. Follow these tips when you design for assembly.
Before going any further, let's discuss what board assembly entails. After a board design is submitted and approved'be it flex, high-density interconnect (HDI), or otherwise'it is fabricated. Once that is complete, the bare boards will need to be assembled with additional components, including processors and memory.
Simple enough. The problem that often emerges'at least, among those who are not yet PCB Masters'is that the initial circuit board design does not fully take assembly into account. Instead, they focus exclusively on the board itself without a broader context of how the board will be used within the product or application.
Overlooking board assembly can lead to significant complications. A circuit board design may seem perfectly acceptable when viewed in isolation, but certain design decisions may make assembly difficult later on. For example, components may be situated too close to one another, which could lead to a non-functioning product or cause performance issues. We'll discuss this in-depth later in the article.
Then there's also the question of component availability. For the circuit board assembly process to flow smoothly'and, therefore, for the board production as a whole to progress effectively'the assembly vendor needs to have the requisite components on hand as soon as the boards arrive. If they aren't available, the whole process will be delayed, undermining the value of a quick-turn approach to board production.
Unlock the secrets of a successful PCBA with our infographic: 5 critical DFA norms every PCB designer should know.
Those are just some of the issues that will arise if you aren't approaching design with circuit board assembly in mind. The question is, how do PCB masters avoid making these mistakes? Here are a few key tips.
For more information, please visit Pcb Component Placement Machine.
One of the frequently asked questions posed by designers who are not yet board masters is regarding component-to-component spacing. Placing a component too close to another component can create various issues which may require re-design and re-fabrication, which results in a loss of time and money.
Board masters use several techniques to avoid part-to-part spacing issues during assembly. They design their footprint in such a way that there is always enough gap between component boundaries. This alleviates any potential issues that could arise from components being too close during placement.
Designers must carefully place components so the place-bound component shapes do not overlap each other. In example one, you can see putting components based on the place-bound shape will automatically keep them 50 mils apart. If the board does not have enough space, PCB Masters can disregard the place-bound shape and move components closer to one another, keeping the minimum spacing rules in mind. Component-to-component spacing is also important to optimize your design for testing.
Ensure component rules, requirements, and proximity regulations have been established in your design software. Board masters have specific component-to-component spacing guidelines for various part types. For example, the minimum spacing between discrete components like caps and resistors should always be at least 10 mils, with 30 mils being the preferred spacing. This simple action will help avoid many of the proximity issues. These issues can otherwise lead to assembly delays or other complications down the line. For additional spacing rules, refer to Table 1.1.
Table 1.1: Spacing Component-to-Component
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PCB Masters choose components early in the design process, realizing that this is the best way to ensure there are no conflicts between the actual design and the components being assembled. If you factor in component sizes from the very beginning, component space, and sizes are no longer a concern, and the circuit board assembly process can proceed without barriers. This also leads to the next tip.
The board designer should talk to the circuit designer and engineers to see if the component size can be reduced, creating additional space on the board. After all, a smaller part will mean a smaller footprint on the board.
To know more about board design basics, read mastering the art of PCB design basics.
Never mix lead-free components with components that are not specified for lead-free assembly. If any component requires lead-free assembly and no substitute for conventional leaded solder is available, then the entire board must be assembled lead-free, and all components must be qualified for lead-free assembly.
Sometimes the only package available for a particular device is a lead-free BGA. However, boards that will be used for military projects typically must be assembled with conventional leaded solder, as per government requirements. The designer must either obtain a waiver from the customer to allow lead-free assembly, modify the design to use a device that is available in a package for assembly with conventional leaded solder, or have the BGA rebelled for leaded solder (an expensive procedure that can damage parts).
Distribute large components across a board as evenly as possible during layout to achieve the best possible thermal distribution during solder reflow. Make sure the assembly contractor tailors a thermal profile for the reflow oven unique to each assembly job.
Check Sierra Circuits' PCB assembly capabilities to learn more.
Whenever possible, avoid mixing technologies. The payoff of a single through-hole, for example, doesn't outweigh the additional time and money spent. It would be more efficient to either use multiple through-holes or none. If you use through-hole technologies, placing all through-holes on one side of the board will reduce manufacturing time.
Communication between the electrical engineer and board designer should begin during the early stages of layout. The designer should review the BOM and carefully examine the parts being used in the design. The designer may recommend larger components if there is space on the board and the current design uses unnecessarily smaller components. This will help avoid complications during assembly. For example, it is beneficial to use a size capacitor/resistor rather than a whenever possible.
A PCB Master would choose a capacitor over a cap'if the can fulfill all the necessary requirements. This will free up significant space on the board.
They are also manufactured by various vendors. Choosing these components will avoid delays during the assembly process. It also gives both the electrical engineer and the designer the option of choosing alternate parts without changing the schematic or layout, a timely solution to components not in stock.
Finally, it is important to choose the appropriate package size while in the board design phase. A PCB Master only chooses smaller packages when there is a good reason to; otherwise, lean toward bigger options. In too many cases, electrical engineers choose unnecessarily small component packages. This can create issues in terms of assembly yield, as it is more difficult to touch up and rework smaller components. Depending on the amount of rework needed, it may be more cost-effective to completely rebuild the board the remove and solder on new components.
After you've confirmed an ideal package size, you can begin choosing components in the same density category.
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As mentioned above, component availability can potentially cause significant delays. However, PCB masters avoid this issue by checking the availability of all components before beginning the design. If there are parts that require long lead times, they can be ordered early and consigned to the board assembly vendor. The assembly vendor can also handle the procurement of the more widely available parts, so every component will be ready and waiting when your boards are ready to be assembled.
Reducing the component lead time can help curd down assembly costs. For more such tips, see how to reduce PCB assembly cost.
A bill of material (BOM) is a key aspect of both design and assembly. If there are any issues in your BOM, the assembly house will place the project on hold until the issues have been resolved with an engineer. One way to make sure your BOM is updated is to review your BOM any time there is a change in your design. When you add new components to your schematic while the layout is in process, make sure you've also updated the BOM with the correct part number, description, and component values. During the design process, the engineer might change a component due to long lead times, size, or availability and forget to update the BOM with the new part number. This can lead to various assembly issues and cause delays.
Format every bill of materials according to the below example to precisely identify all of the components to be assembled on each board. The properly formatted BOM has columns indicating:
It may also include:
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Component footprints are another major aspect of layout design. A PCB Master will make sure their footprints are created accurately as per the land pattern recommended in the datasheet. It is very important to use the numbering key correctly from the datasheet to identify the correct part and its land pattern. Reading the datasheet incorrectly will result in an incorrect footprint, which could require a complete redesign and re-fabrication of the boards. To learn more, see our case study, eliminating BOM and footprint errors in PCB assembly.
The image below shows an example of the numbering key and various types of footprints.
Refer to our post on board footprint creation in Allegro, Altium, and KiCad to understand how to design a footprint.
The leading show-stopper in assembly is missing pin 1 indicators or component polarity/orientation indicators on the silkscreen. Nearly 75% of the assembly orders my facility receives fail to identify the location of pin 1 for each IC, or they misrepresent or neglect to indicate the polarity of some capacitors, diodes, or LEDs.
The best way to avoid assembly problems: Confer with your manufacturer before design even begins.
Obey the convention for marking the polarity of diodes, including LEDs: Put a K on the silkscreen layer at the cathode end. Alternatively, use the electrical symbol for diodes in the correct orientation to guide assembly. Never indicate diode polarity based on the anode pad. Use a K to designate the cathode or line up the diode symbol in the correct position. Don't substitute any other marks or your contractor will misinterpret what you intend.
To orient tantalum capacitors, tag the positive side with a plus symbol on the silkscreen. Remember, tantalum capacitors can ignite if mounted with swapped polarity. Short of reverse-engineering a schematic, there's no way for a contractor to figure out part polarity unless it's clearly displayed. Silkscreens must not interfere with pads, and symbols should not be printed beneath the body of any component.
Our article on 6 design mistakes that lead to PCB assembly errors will give you insights into how significant PCBA errors can be avoided by addressing the common design issues.
Every connection to every component must have its own independent pad. Each pad must be commensurate in size with its mates. If two components share a pad'let's say, a resistor and a capacitor'neither can be properly aligned during assembly. And if one pad is substantially larger than its mate for a component, component tombstoning can result from an imbalance of solder deposition.
If a pour or plane will be a point of contact, there must be a mask-defined pad of appropriate size. If a device involves non-solder mask defined pads for connections as well as solder mask'defined pads a BGA matrix on a loose pitch in which some adjacent outer balls are common to a ground pour, for example'stipulate in a design note that the board fabricator shall not edit the solder mask apertures for those solder mask-defined pads.
To know about pad design, read What is a Pad in PCB Design and Development?
Needless to say, it's far better to catch issues that might impede assembly before the boards are fabricated. If the board will be fabricated and assembled by the same facility (thereby coordinating operations), verify whether to provide the design data in ODB++ format to speed identifying potential manufacturing issues and ease CAM setup. Outputting a design in ODB++ captures the data for fabrication, assembly, and testing in a unified structure that supports automated analysis and avoids time-consuming data conversion at the CAM stage. Practically all major EDA platforms can output design data in the ODB++ format.
Technically, the 10th tip does not address design but component delivery to the contractor. If assembly will be performed on a consignment basis'some or all of the components will be supplied by the assembly customer, instead of being procured by the assembler'the parts must be provided in a carefully organized kit matching the BOM. All SMT components must be supplied in reels or on continuous tapes at least 6' in length or in tubes or trays.
Extra components are required for every part number listed on the BOM to cover attrition during assembly. For example, an assembly shop may require a minimum of 100 or 20% more 1k ohm resistors than called for on the BOM. The parts for each line item on the BOM must be sent in a clearly marked bag separate from the other parts. All ICs must be shipped in their original, unopened protective packages that include desiccant, or else they must be baked for eight hours or so to remove moisture before assembly, which could set back assembly for a day.
In other words, eight strips of eight pieces of a -size 12-pF capacitor do not fulfill a BOM requirement for 64 pieces of that part. The strips are too short for loading the pick-and-place feeders, and under the best circumstances, not all the parts will wind up on boards. Check and double-check your components before shipping using our kitting guidelines.
We recommend the best way to avoid assembly problems is to consult your manufacturer before the design even begins.
Additional assembly details to keep in mind during the design stage include heat and wash. Make sure you know what the maximum heat levels of your components are, whether they can be washed, and the type of assembly it requires. If it will be hand-soldered, the board design must include the space for a soldering tool.
Make sure your capacitors all face the same direction. This saves the pick and place machine time during component placement. Pay attention to components that emit heat. The space around these components must be carefully reviewed. Make sure heat does not negatively affect any components or traces around the component.
To ensure an error-free PCB assembly, consider our PCB Design for Assembly Checklist: 9 Important Checks for Layout Engineers.
To continue on the path toward becoming a PCB master, keep an eye open for the next piece in the series. If you have any further questions about the board design tips for assembly, please let us know in the comments section. Check out our DFM design guide to learn about design for manufacturing aspects.
10 Chapters - 40 Pages - 45 Minute Read
If you want to learn more, please visit our website Surface Mount Component Identification.