All electronic devices and equipment rely on printed circuit boards to function. However, PCBs without the necessary components cannot benefit electronic products. We refer to the process of soldering PCB components on the circuit board as PCB assembly. Sometimes, the process gets referred to as PCB components assembly or PCBA (printed circuit board assembly). While essential, it encompasses different processes, guidelines, technology and techniques to achieve. As a PCB designer, it is instrumental in understanding everything about PCB component assembly. So what does it entail?
Background of PCB Components Assembly
The Assembly of components on PCBs can trace back to the invention of the first PCB component, the transistor. It has since developed into a complex process involving multiple miniaturized components like transistors, transducers, resistors, diodes, etc. While important, printed circuit board assembly products often get confused with printed circuit boards. But a significant difference exists between the two. PCBs are circuit boards without any electronic components attached or assembled, while PCBA products encompass the printed circuit board complete with the assembled components.
The Assembly of printed circuit boards also differs from the manufacturing process of PCBs. The manufacturing process of printed circuit boards starts with the design and ends up with the development of the prototype. Immediately the PCB becomes ready, passive and active electronic components get soldered to the board before getting deployed to various electronic gadgets and equipment.
The Assembly of electronic components often depends on other factors such as the PCB board type, the electronic component types, and the circuit board’s purpose. However, the key ingredients include the PCB, the PCB assembly components, and soldering materials (solder paste, solder wire, and solder balls primarily for BGA). It also consists of the soldering flux and the soldering equipment (including the wave soldering equipment, soldering station, testing and inspection equipment, and SMT equipment.
PCB Assembly Components
You can equate components of a printed circuit board to vital organs, especially if you consider the PCB’s copper traces as the skeleton of the circuit board. Each electronic component performs a distinct role. It also provides the circuit with the distinctive qualities necessary for performing its function as designed. Based on the type of electronic equipment or device a bare PCB circuit board gets designed for, there is need for diverse components for various circuits. Such components can range widely, though the most common PCB assembly components include the following.
It is an instrumental component in a PCB and acts as the circuit’s voltage or current source. Without it, a printed circuit board cannot function unless connected to a direct current source.
It is a form of amplifier responsible for switching or controlling the board’s electronic signals. While transistors have a myriad of diverse variations, the typical one is the bipolar one. Bipolar transistors have three pins, including the collector, base, and emitter.
It resembles a capacitor in nature in the way it stores energy as a magnetic field upon the flow of current through them. Inductors are primarily utilized in blocking signals in a PCB board. Such signals can come as interference from other sources like electronic devices or equipment.
It ranks second after resistors when it comes to popularity owing to their commonality. It holds an electrical charge in the circuit board and releases it when there is need for extra power elsewhere. Capacitors accomplish this function by getting opposite charges from the two conductive layers (the insulating material separates them)
It is a miniature device within a circuit responsible for allowing the unidirectional flow of electrical current. Diodes get utilized in stopping or blocking the flow of current in an unwanted direction. A famous example of a diode includes the LED primarily utilized in emitting light.
It is an essential component deployed mainly to transfer electrical energy from one circuit to another by increasing or decreasing the voltage.
Sensors are critical electronic components responsible for detecting environmental changes. It detects and generates electrical signals corresponding to the detected change. Such signals get sent to other circuit components. In other words, sensors convert physical elements like air quality, light motion, or sound to electrical energy.
PCB Design and Assembly
It is always prudent to design printed circuit boards with PCB manufacturing & assembly in mind. However, this concept proves a commonly misunderstood concept. It is common knowledge that the Assembly of components can happen via one or both through-hole and surface-mount technology approaches. You have to identify the appropriate approach in the design phase to ensure that you approach the right PCB assembly partner for your needs.
PCB Assembly with Through-Hole Electronic Components
It is an assembly process that features electronic components that possess lead protrusions getting fixed through small holes within the PCB before soldering. The soldering process can either entail hand soldering or wave soldering. Wave soldering entails a printed circuit board assembly process where there is insertion of the solder (solder bar) in a high-temp bath. As it remains in the molten form inside the bath, it forms waves at high temperatures. However, the temperature ranges with the type of solder used.
The process entails the insertion of components, application of flux, preheating before wave soldering. It then undergoes cleaning and testing to identify any solder joint flaws or faults. Conversely, hand soldering happens through manufacturing units devoid of any excessive workloads or repairs. It happens by hand and mostly post- the wave soldering process.
PCB Assembly with Surface-Mount Electronic Components
It is an assembly approach that uses surface-mount technology in assembling SMD electronic components. Surface mount devices do not have legs or leads and thus get mounted on the circuit board’s surface. It uses a separate set of electronic components, equipment, and other solder materials in the assembly process from the THT approach.
DFA or Design for Assembly
It is a fundamental aspect of design that most PCB novices do not take into account. However, it is an aspect that needs consideration before embarking on the fabrication phase of your printed circuit board. It can encompass both HDI (high density interconnect), flex, or otherwise. After all, the fabricated bare panels require Assembly besides incorporating extra components (which includes the memory and the processors).
Design for Assembly is a concept that aims to sort out the problem of incompatibility that arises post the fabrication of the bare board. It comes about because there is no leaving out any thought for configuring the best way to fit the PCB in its targeted application device. Such a lapse in judgement can lead to significant complications.
The design can seem perfect and acceptable if considered in isolation, though some specific design decisions can make the assembly process difficult further along the product pipeline. For instance, electronic components may prove too close, leading to a non-functioning electronic product or cause issues in performance.
Another crucial issue entails the availability of components, which influences the flow and ease of the printed circuit board assembly process. The PCB assembly manufacturer must have the necessary components upon receipt of the boards for Assembly. It allows for a timely PCB components assembly and thus the time-to-market period.
Tips on Approaching PCB Design with Assembly in Consideration
If you want to avoid pertinent issues during your PCB assembly process, it is prudent to consider the following.
Consider the part-to-part spacing.
Part-to-part spacing is often a challenging aspect most PCB novices grapple with. As a result, it is a typical question posed by upcoming PCB designers. The spacing of electronic components needs to adhere to the correct specification to avoid issues with the Assembly or performance of the PCB. For instance, spacing the electronic components too close to each other can result in issues that may need re-fabrication and redesign. Consequently, it leads to losses in times of money and time.
An excellent technique to deploy and avert this situation entails designing the PCB using a specific footprint. The footprint should allow for sufficient gaps between the boundaries of components. Such an approach alleviates any possible issues that can arise from closely spaced components. Always remember to adhere to the minimum component spacing regulations, especially in your design. Most part or component types have established placement and spacing dimensions.
Pick electronic components in the design stage or phase.
It is always recommended to pick the desired components at the onset of the design process. Prior selection of the components is an excellent way of averting conflicts between the design and components getting assembled. For instance, factoring in component sizes from the outset makes the size and spacing concern a non-issue during the PCB assembly process.
It is also prudent to have conversations between circuit designers to determine whether the component sizes need reducing or whether it is vital to create extra space on the circuit board. Remember, the smaller the component, the smaller the footprint on the circuit board.
Separate the lead-free from the non-led-free components
Always avoid mixing the lead-free parts or components with those lacking specification for a lead-free assembly. Whenever a component needs lead-free Assembly, but no alternative for the conventional leaded solder is available, try and assemble the whole board using a lead-free approach. Additionally, the components need a qualification for a lead-free assembly.
At times, the sole package available for certain devices is lead-free BGAs. But circuit boards with application in the military require Assembly with the standard leaded solder according to the government’s specifications.
Place the large electronic components evenly.
It is crucial to consistently distribute large components or parts across the circuit board during the layout. It helps to achieve optimal thermal distribution during the solder reflow process. Ensure that the assembly partner has a reflow oven’s thermal profile to suit your PCB assembly needs.
Avoid combining technologies
You need to avoid mixing the different technologies (SMT and THT) to get the best results. Additionally, using a single thru-hole cannot outweigh the extra money and time spent assembling the PCB. Instead, it can prove wiser and extra efficient in deploying multiple through-holes or avoid using any. Further, when utilizing through-hole technology in assembling PCB components, place them on one of the board’s sides to limit the smt manufacturing time.
Select the right package size
It is pivotal to initiate and carry out comprehensive talks at the onset of the PCB layout process, especially between the PCB designer and the engineer. Designers need to review the bill of materials besides carefully examining the components entailed in the PCB design. For instance, the designer can recommend more prominent components whenever there is sufficient space on the circuit board, deploying small components unnecessarily. It becomes crucial in averting complications during the assembly process.
Picking the correct package size also helps during the design phase. Pick a small size when the situation demands it, otherwise always opt for the larger or bigger alternatives. Sometimes, engineers pick small component packages that create issues later on in Assembly concerning assembly yield. It proves a difficult task trying to rework or touch up smaller components. Most of the time, it would prove wise to wholly rebuild the circuit board by removing and soldering new components.
Search for protracted lead-time components
Availability of components is crucial to avoid significant delays. As a result, you should get ahead of things by checking on the availability of all parts or components before starting the design process. Whenever there are components that need long lead times, it is important to order them early. You also need to consign the components to the printed circuit board assembly vendor. The PCB assembly partner can handle the electronic component procurement. It ensures that all the necessary parts become available and ready, especially when the boards are ready for Assembly.
Update your bill of materials while designing
A BOM or bill of materials is critical in the design and assembly process of printed circuit boards. Whenever issues exist in the bill of materials, the assembly partner will have to put the project on hold till the issues get resolved by an engineer. An excellent way of updating the BOM entails reviewing the BOM whenever something changes in the design. For instance, when you incorporate new components to the schematic during the layout process, update other aspects like descriptions, part numbers, etc.
It can sometimes happen that the engineer forgets to update the bill of materials when they make changes to components. Such changes arise due to long lead times, availability of or lack of components, size, etc., which may cause delays or other issues. The standard BOM format follows the following structure.
- The manufacturer’s component or part number
- Item number
- Manufacturer’s name
- Quantity for each board
- An entire parts description
- Reference designators where there is use of commas to separate them
- Distributor’s parts number
- Distributor’s name
Careful inspection of the footprint development
Another vital layout design aspect entails the component’s footprint. It is prudent to ensure that your electronic component footprint is accurate based on the recommended land pattern in the datasheet. Proper utilization of the numbering is vital in identifying the correct component or part besides its land pattern. Therefore, you must ascertain this. Further, any incorrect interpretation of the datasheet can lead to the need for an entire re-fabrication or redesign of the circuit board.
Ensure the presence of every necessary indicator
A missing component polarity or missing pin-1 indicator on the circuit’s silkscreen can prove disastrous. In most instances, almost three-quarters of the orders received lack pin-1 identifiers or misrepresented polarity of components like LEDs, capacitors, diodes, etc. Always adhere to the rules for polarity marking. It is instrumental, for instance, to inscribe a K for the cathode end or utilizing the appropriate diode electrical symbol to guide the assembly process. Additionally, always avoid using symbols that are not universal and thus simple for the assembler to misinterpret.
Always ensure to fill the via-in-pads
It is crucial to fill the via-in-pads unless they are in the thermal pads. The pad medium or matrix upon which the BGA gets installed can include blind vias and through vias. However, both of them have to get planarized and filled to avoid compromising the solder joints. Add vias located in the thermal pads, especially under QFNs, to assist the flow of solder to the conductive planes. Vias often secure solder joints besides preventing the solder from detaching the package when in Assembly. Package floating during Assembly can hinder the formation of decent solder joints.
However, most assembly houses compensate for the absence of thermal pad’s through-vias by incorporating windowpane-shaped openings in the stencil (solder paste) on top of the pad. It relieves outgassing and solder pooling during Assembly. But such a fix becomes less effective in the presence of a via.
A singular pad for each connection
All the connections to components need to have their individual and autonomous pads. The pads should also correspond to the sizes of their mates. Whenever two components or parts share a single pad like capacitor or resistor, aligning them properly during Assembly becomes impossible. Further, when a pad proves larger than the mate for components, component tombstoning may result, primarily from a solder deposition imbalance.
It is prudent to ensure a mask-defined pad with the right size when the plane functions as the contact point. Whenever a device has non-solder mask pads for connection purposes on top of soldermask, it is crucial to avoid editing the apertures of the solder mask pads.
Simplify the CAM setup
The early identification of issues with the potential of impeding PCB components assembly can prove a game-changer. When the circuit board has to undergo fabrication and Assembly within the same PCB facility, always verify if you can present the design information in the ODB++ format. It helps speed up the identification of likely manufacturing issues besides easing the CAM setup. Additionally, outputting your design in a similar format captures Assembly, fabrication, and testing data within an integrated structure. It thus supports the automated analysis besides avoiding data conversions that may prove time-consuming at this phase (CAM). Remember, all EDA platforms can output design information or data in ODB++.
Address the aspect of component delivery
The aspect technically does not cover the design for the assembly element but is equally vital to the assembly partner. Whenever your plan entails the Assembly of components based on consignments, it is prudent to supply all or some components to the assembly partner. The assembler, in this instance, does not procure the components. You, therefore, have to supply the components in carefully organized kits that match the bill of materials (BOM). Supply all the surface-mount components in reels. Alternatively, you can also supply them in tapes that measure at least six inches in length. You can also supply the SMT components in trays or tubes.
It is also important to supply more components than specified in the bill of materials to cover for attrition when assembling. For instance, most assembly houses will require at least 20% extra resistors than the BOM specifies.
Extra PCB assembly guidelines
Some of the other crucial details to consider for your PCB assembly process includes the wash and heat aspects. You have to know the maximum limit of the heat levels your components can withstand. Further, check whether you can wash the components and what assembly type they the components require. Orientation of capacitors and components that radiate heat are some of the other aspects to consider.
PCB Automated Assembly Components
A lot of modern printed circuit board assembly products rely on automated Assembly. Here, the PCB automated assembly components undergo a machine-operated placement process besides the soldering process. However, it often still requires the input of manual assembly technicians.
Automated Assembly of PCB components results in a high level of consistency besides guaranteeing speed. As a result, it helps assembly companies to stay competitive. But in unique circumstances where the PCBA has a particular design or requires specific components, manual Assembly comes into play.
Automated Assembly of PCB components has obvious benefits. It includes speed of product-to-market deliveries, high precision assembly, capacity to assemble components in mass numbers, etc. However, manual Assembly of components also has its advantages, especially when assembling unique PCB products.
PCB components assembly is vital for producing functional printed circuit boards, besides the ready deployment for diverse applications. Understanding the various PCB components, the assembly processes, the technologies involved is vital for quality PCB assemblies. Further, guidelines on how to go about the assembly process are also instrumental in designing printed circuit boards that will prove assembly-ready upon fabrication.