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 PCB 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:
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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.
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 PCB design 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:
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 patternSome 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 is 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.
By default, Altium 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.
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 PCB 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 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 constraints, high-speed electrical requirements, and other physical requirements for your particular application. This automates a review process for validating your PCB layout.
Note that many design processes state that you should run PCB 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. When a design is finished and ready to be released to manufacturing, Altium makes it easy to collaborate and share your projects.
“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!”
The PCB assembly services comprise a vital step in the electronics manufacturing process, where cutting-edge technology is utilized to mount electrical components onto printed circuit boards. This operation is also referred to as printed wiring assembly, circuit card assembly, and electronics assembly.
Efficient and precise PCB assembly services are crucial for the development of the electronics industry. Additionally, they offer cost-saving advantages by reducing manufacturer labor expenses.
Electronics play a crucial role in our everyday routines, encompassing everything from our mobile devices to our vehicles, which are all powered by electronic components. At the core of these advanced technologies lies the printed circuit board, commonly referred to as a PCB.
These are commonly known as printed circuit boards, typically seen as small green chips covered in lines and copper parts at the core of disassembled electronic devices. They are typically constructed with fiberglass, copper lines, and other metal parts, and are held together by epoxy and insulated by a solder mask, which gives them their signature green hue.
Although often overlooked, the components solidly stuck on circuit boards serve a vital purpose and should not be dismissed as mere decorations. In order for a circuit board to function properly, it must have components mounted on it. This process is known as PCB assembly or PCBA for short. The copper traces on the bare board act as electrical pathways, connecting the various connectors and components and allowing the board to perform its intended functions. These functions can range from simple to complex, and remarkably, PCBs can be smaller than a thumbnail in size.
Learn more about how these devices are manufactured. The PCB assembly process involves both automated and manual steps. During each step, board manufacturers have a variety of options to choose from, including both manual and automated methods. To gain an understanding of the complete PCBA process, we have outlined each step in detail below.
PCB Design Basics
The initial stage of the PCBA process involves the fundamental element of the PCB: the base. Comprised of multiple layers, each one serves a crucial function in determining the overall functionality of the end product. These layers are interspersed with one another and have various responsibilities.
• Substrate: This is the base material of a PCB. It gives the PCB its rigidity.
• Copper: A thin layer of conductive copper foil is added to each functional side of the PCB — on one side if it's a single-sided PCB, and on both sides if it's a double-sided PCB. This is the layer of copper traces.
• Solder mask: The copper layer is topped with a solder mask in a distinct green color, which serves to insulate copper traces and prevent accidental contact with other conductive materials, avoiding any potential shorts. Solder is applied through holes in the solder mask to attach components to the PCB, ensuring everything stays in its proper place. The application of solder mask is crucial for a seamless PCBA manufacturing process, as it prevents solder from being applied to unwanted areas, preventing shorts.
• Silkscreen: The final layer of a PCB board is a white silkscreen that adds labels in the form of characters and symbols, indicating the function of each component.
Parts and materials used in PCBs typically remain consistent, except for the substrate which varies depending on the desired characteristics such as cost and flexibility, as determined by the designer.
The three primary PCB types include:
• Rigid PCB: Rigid PCB bases, which make up the majority of PCBAs, are typically made from a solid core that provides both rigidity and thickness. These inflexible bases are commonly constructed with materials such as fiberglass, known as "FR4", while less expensive options may use epoxies or phenolics, which are not as durable.
• Flexible PCB: Flexible PCBs are known for their increased pliability compared to rigid counterparts. They are typically made with a bendable, high-temperature plastic material such as Kapton.
• Metal Core PCB: These boards are a viable alternative to traditional FR4 boards. With a metal core, these boards are able to efficiently distribute heat and safeguard heat-sensitive board components. This results in better heat dissipation.
There are two types of mounting technologies are prevailing in the modern PCBA industry:
Surface Mount Technology: Sensitive components, some very small, such as resistors or diodes are placed automatically onto the surface of board. This is called SMD assembly, for surface mount device. Surface mount technology can be applied on small size components and integrated circuits (ICs). For example, PCBCart is capable of mounting package with min. size , which is even smaller than the size of a pencil point.
Thru-Hole Technology: works well on components with leads or wires that have to be mounted on board by plugging them through holes on board. The extra lead part has to be soldered on the other side of the board. This technology is applied on PCB assemblies containing large components such as capacitors, coils to be assembled.
The assembly process for THT and SMT differs due to their distinctions. This article will cover additional considerations for materials and design in the PCB assembly process, specifically for THT, SMT, and mixed technologies.
Before the Assembly Process
Before initiating the PCBA process, it is necessary to complete a few preparatory steps. These steps allow PCB manufacturers to evaluate the functionality of a PCB design, and typically involve a DFM check.
In order to begin the PCB assembly process, it is important for companies to provide the PCB design file, design notes, and specific requirements. This allows the assembly company to thoroughly review the file for any potential issues that could impact the functionality or manufacturability of the PCB. This is commonly referred to as a Design for Manufacturability check, or DFM check for short.
The DFM check analyzes all PCB design specifications for any missing, redundant, or problematic features that may greatly impact the final project's functionality. A common flaw to be mindful of is insufficient spacing between components, which can lead to shorts and other malfunctions.
Our DFM checks are a crucial step in the manufacturing process, helping to prevent costly issues and reduce the number of scrapped boards. As part of our dedication to providing quality at an affordable price, every PCBCart project order includes FREE DFM and DFA checks. These checks are powered by the automatic system Valor DFM/DFA, known for its quick speeds and precise results.
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Step 1: Solder Paste Stenciling
During the PCB assembly, a solder paste is applied to the board by using a stencil made of stainless steel. This is similar to the process of screen-printing a shirt, but instead of a mask, the stencil is placed on top of the PCB. This technique enables assemblers to accurately apply solder paste to specific areas of the board where components will be placed in the final product.
The solder paste is composed of 96.5% tin, 3% silver, and 0.5% copper, and is made up of tiny metal balls. It must be accurately applied to the circuit board using a flux, which aids in the melting and bonding process. The paste itself has a grey appearance and should be applied with precision in specific locations and quantities.
A mechanical fixture is used in a professional PCBA line to securely hold the PCB and solder stencil. Once in place, an applicator precisely applies solder paste to designated areas. The machine then evenly distributes the paste across the stencil, ensuring complete coverage. Once the stencil is removed, the solder paste remains in the designated locations.
Step 2: Pick and Place
Following the application of solder paste to the PCB board, the PCBA process proceeds to the pick and place machine, where a robotic device precisely positions surface mount components, also known as SMDs, on the prepared board. SMDs are the most commonly used non-connector components on modern PCBs, and are then soldered onto the surface of the board in the following step of the PCBA process.
Originally, assembling components was a manual task using tweezers. However, advancements in PCB manufacturing have allowed for automated pick and place processes. This change is due to the precision and consistency of machines, which outperform humans in this task. Despite being able to work quickly, humans often experience fatigue and eyestrain after a few hours of handling such delicate components. In contrast, machines can work continuously without these limitations.
The process begins with the device picking up a PCB board using a vacuum grip and transferring it to the pick and place station. Once at the station, the robot aligns and applies the SMTs to the PCB surface according to preprogrammed coordinates, on top of the soldering paste.
Step 3: Reflow Soldering
To ensure successful PCB assembly, it is crucial to maintain the position of both the solder paste and surface mount components until the paste solidifies and effectively bonds the components to the board. This is accomplished through the essential process of "reflow".
Upon completion of the pick and place process, the PCB board is transferred to a conveyor belt. This belt guides the board through a reflow oven, which is comparable to a commercial pizza oven, gradually heating it using a sequence of heaters. The board reaches temperatures as high as approximately 250 degrees Celsius or 480 degrees Fahrenheit, resulting in the melting of solder in the solder paste.
As the solder is melted, the PCB progresses through the oven and encounters a sequence of cooler heaters. This controlled cooling process causes the molten solder to solidify, resulting in reliable and permanent solder joints to connect the SMDs to the PCB.
For optimal results, it is important to carefully consider the reflow process for PCBAs, particularly for two-sided PCB assembly. To ensure proper placement and reflow, each side of the board must be stenciled and processed individually. The first step is to stencil and reflow the side with fewer and smaller components, followed by the second side.
Step 4: Inspection and Quality Control
Upon completion of the reflow process and soldering of surface mount components, the assembled board must undergo functional testing. Movement during the reflow process may lead to subpar connection quality or a lack of connection, as well as the possibility of shorts caused by misplaced components connecting unintended portions of the circuit. It is vital to test for functionality to avoid these potential issues.
Possible inspection methods for identifying errors and misalignments include various techniques. Among the most frequently used techniques are those commonly referred to as inspection methods.:
• Manual Checks: While automated and smart manufacturing are growing in popularity, manual checks remain a crucial step in the PCB assembly process. For smaller batches, designers can ensure the quality of a PCB after the reflow process through an in-person visual inspection. However, as the number of inspected boards increases, this method becomes less practical and accurate due to optical fatigue potentially leading to less accurate inspections.
• Automatic Optical Inspection: The merchant recommends utilizing automatic optical inspection for larger batches of PCBAs. This method, also known as an AOI machine, employs multiple high-powered cameras to visually inspect PCBs. The cameras are strategically positioned to view solder connections from various angles, enabling the AOI to identify weaker solder joints based on light reflections. With its high processing speed, the AOI is capable of examining a large volume of PCBs in a short period of time.
• X-ray Inspection:One alternative for inspection is utilizing X-rays, which is typically reserved for intricate or multi-layered PCBs. It permits observation of inner layers and detection of any possible concealed issues.
If a board malfunctions, the PCBA company's standards will determine whether it is cleared and reworked or scrapped.
In order to ensure proper functionality, the next step after an inspection is to conduct a thorough testing of the part. This includes a quality check of the PCB connections. In the case of boards that need programming or calibration, additional steps are necessary to ensure their proper functioning.
Inspections are often conducted after the reflow process to detect any potential issues. These routine assessments are crucial in identifying and addressing errors promptly, resulting in time and resource savings for both the manufacturer and designer.
Step 5: Through-Hole Component Insertion
The PCBA board may contain additional components besides standard SMDs, depending on its type. Such components may include plated through-hole (PTH) components.
A plated through-hole allows for the transfer of signals from one side of the PCB to the other. Soldering paste is ineffective in this situation, as it will simply pass through the hole without adhering.
To complete the PCB assembly, PTH components require a specific soldering method rather than soldering paste. This ensures a successful and efficient process.:
• Manual Soldering: The process of manual through-hole insertion is quite simple. A single person is usually responsible for inserting one component at a designated PTH station. This person then hands off the board to the next station, where another individual inserts a different component. This cycle repeats for each PTH that needs to be outfitted, making the process longer depending on the number of PTH components that need to be inserted during a single PCBA cycle. Although many companies try to avoid using PTH components in their designs, they are still commonly used in PCB designs.
• Wave Soldering: Wave soldering is the automated version of manual soldering, but involves a very different process. Once the PTH component is put in place, the board is put on yet another conveyor belt. This time, the conveyor belt runs through a specialized oven where a wave of molten solder washes over the bottom of the board. This solders all of the pins on the bottom of the board at once.This kind of soldering is nearly impossible for double-sided PCBs, as soldering the entire PCB side would render any delicate electronic components useless.
Upon completion of the soldering process, the PCB can proceed to final inspection. It may also revisit previous steps in the event that additional parts need to be added or another side needs to be assembled.
Step 6: Final Inspection and Functional Test
After the soldering step of the PCBA process is complete, the PCB undergoes final inspection to determine its functionality. This testing procedure, known as a "functional test", replicates normal operational conditions to thoroughly evaluate the PCB's performance. Power and simulated signals are run through the PCB during this test, with testers closely monitoring its electrical characteristics.
The PCB may be recycled or scrapped in accordance with company standards if any of its characteristics, such as voltage, current, or signal output, exhibit excessive fluctuations or exceed a predetermined range during testing.
The process of PCB assembly is incomplete without thorough testing, which plays a critical role in determining its success or failure. That's why it's crucial to conduct continuous testing and inspection throughout the entire process.
After PCBA
When completing the PCB assembly process, it is important to note that soldering paste may leave behind flux and human handling can transfer oils and dirt onto the PCB surface. These factors can lead to a less than ideal appearance and also pose practical concerns.
Washing the product after completing the soldering process is crucial due to the development of a sticky and smelly residue on PCBs over time. This residue is not only acidic and harmful to solder joints, but it can also negatively impact customer satisfaction when new PCB shipments are covered in residue and fingerprints. Follow this important step to maintain the quality of your product.
A high-pressure washing apparatus made of stainless steel and using deionized water is the most effective method for removing residue from PCBs. PCBs can be safely washed in deionized water without any risk to the device. This is due to the fact that it is the ions found in regular water that cause damage to circuits, not the water itself. As a result, deionized water is harmless to PCBs during the wash cycle.
Following thorough cleaning, utilizing a rapid drying process involving compressed air efficiently prepares the PCBs for packaging and transportation.
Differences between PCBAs: THT Assembly, SMT Assembly and Mixed Technology
Thru-Hole Technology (THT) Assembly Process
The thru-hole mounting process, a traditional method for PCB assembly, relies on a combination of manual and automatic procedures to complete..
• Step 1: The placement of components is a crucial step in the PCB manufacturing process, requiring skilled engineers to manually arrange them according to the client's design specifications. In order to maintain high quality standards, the engineers must adhere to regulations and operation standards for thru-hole mounting, such as ensuring proper polarity and orientation of components and wearing anti-static wristbands when handling sensitive components like ICs. These meticulous measures guarantee a top-notch end product that is compatible with industry standards..
• Step 2: Once component placement is finished, the board is put into a corresponding transport frame for automatic inspection to ensure accurate placement. If any issues are found, they can be rectified immediately before the soldering process begins. This step is crucial for ensuring high-quality results in the PCBA process..
• Step 3: In the process of wave soldering, the circuit board is slowly moved over a high temperature liquid solder so that THT components can be accurately and securely attached. This method allows for successful connections with all leads and wires.
Mixed Technology
As modern science and technology advances, electronic products are becoming more intricate and requiring smaller, integrated PCB boards. This makes it nearly impossible for single-component PCBAs to be utilized.
Consider incorporating both thru-hole technology and surface mount technology when assembling a board. It's important to carefully arrange the sequence of components to ensure successful soldering, as this process can be affected by many factors.
PCBA with application of mixed technologies should be carried out in the following situations:
• Single Side Mixed Assembly: The manufacturing procedure for this product includes mixed assembly on one side. However, for small quantities of through-hole components, hand soldering can be used instead of wave soldering.
• One Side SMT One Side THT: Please note that using adhesives during the PCB assembly process is not recommended as it can increase the total cost of PCBA and potentially cause soldering issues.
• Double Side Mixed Assembly:When it comes to double side mixed assembly methods, there are two options available: applying adhesives during PCBA or not. Unfortunately, the use of adhesives can significantly raise the cost of PCB assembly. Additionally, this process will require the PCB to be heated three times, which can result in lower efficiency.
In regards to the previously mentioned mixed assembly procedures, it can be determined that hand soldering is suitable for PCB assembly with a large number of components on both sides, particularly SMD components. As such, when faced with a scenario where only a few THT components are needed, wave soldering is recommended.
The complex and technical process of PCB assembly requires careful consideration of numerous elements. Any small modifications can greatly impact cost and product quality. This article focuses on the standard procedures and technologies involved in the PCBA process. The practical manufacturing process is heavily influenced by design files and customer requirements.
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