Modern electronics use a sophisticated kind of electronic circuit popularly referred to as printed circuit boards. PCBs traverse different sectors in terms of application and importance and range from the military and aerospace industry to consumer electronics. Such significance in electronics has led to increased demand – which gets met through PCB production and electronic pcb assembly.
Production of PCBs entails design, fabrication, and electronic PCB assembly. All the stages of production require proficiency, technology, and in the case of fabrication and assembly, modern equipment as well. However, this article will focus on electronic PCB assembly, especially the methods and applicable technologies available.
The Advent of Electronic PCB Assembly
All printed circuit boards need electronic components such as resistors, transistors, diodes, etc., to perform their respective tasks. The targeted application primarily defines the type of components that forms the PCB assembly. The history of assembly components is synonymous with the invention of the first circuit boards and the first electronic component – the transistor.
Electronic PCB assembly by definition implies a process that connects electronic components or parts with conducting traces or wirings within a PCB. The copper traces often entail engraved conductive pathways on the laminated copper, with the copper laminate sandwiching a non-conductive substrate.
The general assembly process of printed circuit boards entails a series of stages. It includes solder paste application, components pick and place, soldering, inspection, besides testing. The process steps are fundamental in ensuring the assembly of high-quality PCBA products. Therefore, each process step needs to get monitored keenly. However, note that the printed circuit board assembly process steps depend on the PCBA technology deployed.
Electronic PCB assembly has two main types based on the technology used for the assembly. It can come as wither surface-mount tech approach or through-hole technology approach. While the demonstrated PCB assembly process steps above highlight the surface mount process for PCBs, the situation is slightly different for through-hole technology. It is also vital that some PCBA companies combine both technologies in the printed circuit board assembly approach.
So how do the two different PCB assembly approaches entail? How do they compare?
Types of Electronic PCB Assembly
The printed circuit assembly process can take either one of two forms – surface-mount technology or through-hole technology approaches. The two technological approaches deployed in the assembly of printed circuit boards include surface-mount and through-hole technologies.
Through-Hole Technology
It is a unique electronics PCB assembly approach. THT, also referred to as thru-hole, encompasses attaching components bearing leads to the PCB board through the pre-drilled holes. After placing the components in place, they get soldered to secure them in place. The process happens manually (by hand) or at times through automated machines.
The solder joint quality is always crucial for diverse reasons. Firstly, the solder connection or joint represents the real connection between the electronic components with the board. Its quality corresponds to the connection’s quality.
Features
- The through-hole assembly approach of mounting components always provides a sturdy mechanical bond than SMT. However, it can become costlier to produce than the surface-mount technology approach.
- THT also restricts the signal trace routing area, especially on layers located immediately below the upper or top layer (for multilayer boards). It arises because the holes have to penetrate the entire stack of layers to the other end. Because of this, through-hole technology primarily applies to bulkier components like electrolytic capacitors. Such components need the extra mounting strength or support strength – in this case, components like plug connectors.
Process
- Preparation of the pad and hole happens before the lead gets placed through or into the hole. Leads need a specific orientation to ensure the bend is above the soldered spot, diminish the solder requirements and heat.
- The second step involves the uniform application of heat to the pad or hole and the lead. Heating helps liquefy the solder, which ensures that it adheres to both surfaces (hole).
- The flow of molten solder creates mounds on the top and bottom sides of the hole. Because the solder execution happened at the top side of the panel, you need to inspect the bottom end to ensure it has sufficient solder. Another aspect you need to check entails the quality of the joint. However, some assemblies may need solder applications from either side to guarantee quality. Remember, sufficient hole clearance allows for the seamless flow of the solder. Conversely, a tight or restricted clearance may hamper the solder flow, leading to soldering on both ends.
Importance
- Straightforward prototyping: the PTHT or THT allows for rapid prototyping when compared to SMT. You can solder the pin THT components either by wave or hand soldering. It is also possible to breadboard the PCB design before making the actual board.
- High tolerance for power: THT assemblies easily satisfy requirements for high power. It becomes a reality because THT components tend to run along with the board, allowing it to withstand high voltages besides mechanical and environmental stress.
- Incredible physical connections: the THT or PTH helps create sturdy mechanical bonds between the circuit board and the electronic components. Components can entail capacitors, transformers, semiconductors, connectors, etc. Such electronic components have to tolerate high power, high voltage, and mechanical stress.
- Component durability. THT technique is perfect for building sturdy physical bonds between the circuit board and the components. As a result, the components get firmly held in place by the solder from both ends, improving their durability.
Application
Most designers prefer through-hole assembly methods for prototyping purposes. It is simple to use with breadboard-based sockets. But with high-frequency or high-speed designs, the prototypes may need to get assembled using the surface-mount technology. It minimizes errant inductance besides capacitance in leads (wire) which can impair the circuit’s functionality. Additionally, ultra-compact designs can also dictate SMT assembly, including its prototype stage.
Surface-Mount Technology
SMT PCB assembly is among the most popular techniques of assembling components on printed circuit boards. As it is popularly referred to, SMT encompasses the mounting of components to the printed circuit board’s surface. The mounted components, in this case, get inferred as SMDs or surface mount devices.
The technique came about as a direct response to the need for efficiently utilizing the circuit’s board space while minimizing the manufacturing costs. SMT approach has empowered PCB designers to actualize the idea of complex circuits possessing smaller assemblies.
Surface-mount technology came to the fore in the 1960s but got widespread utilization in the 80s. However, by the 1990s, the technology had engrained itself and had application in many high-end printed circuit board assemblies. Orthodox components had to undergo a redesign to incorporate end caps or metal tabs, which could get attached to the board’s surface. It replaced wire leads that had to get passed through the tiny drilled holes,
Surface-mount technology has led to smaller electronic components. It has also enabled the frequent and efficient placement of components on either side of the board compared to the through-hole mounting technique. SMT allows for refined automation that minimizes labor costs besides expanding the production rates. All these factors result in an advanced design and PCB development.
Features of SMT
- Surface-mount technology allows the mounting of electrical or electronic components on the printed circuit board’s surface without drilling. Such components or parts have no leads or may possess smaller leads. Additionally, the SMT components come smaller, a pale comparison to through-hole components.
- Surface-mount components are also extra compact, and thus, ideal for higher routing densities. It arises because SMT components or parts do not need a lot of holes.
Process
The SMT manufacturing or surface-mount technology assembly process starts at the design phase and ends with the inspection and cleaning of the complete PCB assembly. But what are the process steps in electronic PCB assembly?
Preparation of the materials and inspection
The initial step in the process entails the preparation of the surface-mount components and the printed circuit board. It is prudent to ascertain that the PCB and SMC have no defects. The board, for instance, should come flat, with a tin-lead, gold, or silver-plated brazing.
Preparation of the template
The process utilizes a steel mesh instrumental for the printing of solder paste. It is placed in a specific fixed position when printing the solder paste. The manufacture of the template has to align with the pad’s position according to the PCB design.
Printing of solder paste
A solder paste printer becomes a key aspect of this stage. You have to install the machine, and it will become helpful in applying solder paste into the respective solder pad. It uses a scraper and a template to execute the printing of the solder paste. While popular in SMT for solder paste applications, other techniques other than the solder paste printer exist. You can use spry printing, a method that has rapidly gained traction, especially for subcontract departments. It is unnecessary to use templates for such departments, and modifications also seem easier in making the solder paste. In most instances, the tin mixture and flux connect the circuit board’s solder pads and surface-mount components.
Inspection of the solder paste
Most presses (solder paste) have an alternative of including automatic detection. However, the process, depending on the printed circuit board size, can prove time-consuming. It, therefore, becomes essential to at times pick a different machine other than the solder paste presser. The solder paste printer’s internal detection system deploys the use of two-dimensional technology. On the other hand, the dedicated SP equipment deploys the three-dimensional technology to ensure a detailed detection process. It includes detecting the volume of solder pads on each pad and not only the print region or area.
Electronic components placement in their correct locations
It comes as the next step and includes the component placement. Immediately post the confirmation of the right amount of solder applications, placement of components begins. A clamping or vacuum nozzle removes each electronic component from its package. It gets checked through a visual system before placement at high speeds in the programmed position or location.
FAI or first piece inspection
A significant challenge that subcontractors face regarding electronic PCB assembly entails FAI or the first piece of inspection or first assembly. The process verifies the customer information, which ordinarily proves time-consuming. It is an important step (first assembly) to ensure no error gets carried along. Mistakes, when not identified and corrected early, can lead to lots of rework or revision.
Reflow soldering
Upon the completion of the component position checks, the following process encompasses the reflow soldering. Here, the printed circuit board assembly gets transferred to a reflow welder. The subjection of the PCBA to heat (enough temperature), the electric welding joints get formed between the PCB and the electronic components. While seemingly the least complex part of the assembly procedure, it also needs the right reflux profile to ensure satisfactory solder joint quality. The joints should not overheat and impair other components or the PCB assembly.
Cleaning and inspection
After completing the welding process, it is prudent to clean the circuit board and check probable defects. Always repair or rework the defects spotted before storing the PCBA products. Some of the standard equipment instrumental in the inspection phase include old masters (for the automatic optical checks) and X-ray machines. It also consists of the flying needle tester and other optical inspection equipment. Such equipment should have provision for connecting them to the machine for the component’s positional adjustment. Besides this, the SPI needs to get connected to printing equipment to adjust the board’s alignment template.
Importance
The PCB SMT assembly approach has plenty of advantages when compared to the THT approach. Some of the surface-mount technology benefits include the following.
Surface-mount technology allows for a smaller printed circuit board design. It accomplishes this by allowing closer placement of more components on the circuit board. As a result, the design becomes more compact and lightweight.
The production setup process for SMT is quicker than the through-hole technology. SMDs do not require drill holes to get assembled, which also contributes to lowering the costs.
Surface-mount technology also supports enhanced circuit speeds as printed circuit boards developed through the SMT process are extra compact.
In the PCB SMT assembly process, you can place components on either side of the electronic circuit board. Additionally, you can incorporate a higher density of components, complete with more connection per component.
It is easier to realize EMC or electromagnetic compatibility because of the package compactness and the low lead inductance that predominantly features in SMT assembly.
Surface-mount technology also enables a lower inductance and resistance at the solder joint or connection. It thus mitigates the undesired influence of the RF signals by providing enhanced high-frequency performance.
If you consider the design aspects, the SMT approach also benefits reduced weight, optimal utilization of circuit board space, and a considerable reduction of electrical noise.
The reduced costs of boards, minimal costs for material handling, and a meticulous manufacturing process comprise some of the manufacturing-allied benefits.
Moments to Use SMT or Surface-Mount Technology
A lot of the recent electronic PCB products have deployed the surface-mount technology approach to assembling PCB components. However, SMT cannot apply to every situation. Some of the situations that call for SMT include the following.
- If you have to house a high component density
- Whenever you need a compact yet small electronic PCB product
- If you desire a light and sleek ultimate electronic product that has a high component density
- The need for producing large PCB quantities using an automated technology also calls for SMT
- If you need the electronic product to produce little to no noise when deployed
Application
Surface-mount technology has a wide application in the assembly of high-production and low-cost consumer electronics. It is necessitated through the replacement of THT component assembly with placing them directly on the circuit board’s surface. It is the preferred assembly method adopted by many printed circuit boards. Such popularity arises from the ability to condense the components within a limited and small circuit board space.
Differences between Surface-Mount technology and Through-Hole Technology
- Both surface-mount and through-hole technologies are essential in the electronic PCB assembly process. Additionally, both lead to the assembly of components on printed circuit boards. However, the techniques differ in approach. So what makes up the key differences between the two electronic PCB assembly technologies?
- Surface-mount technology frees the board space’s limitation imposed by through-hole technology of mounting components.
- The Surface-mount technology approach features lesser manufacturing or assembly costs compared to through-hole assembly. It arises because SMT components have a lesser manufacturing cost than that of THT.
- Surface-mount components have no leads and thus get mounted on printed circuit board surfaces directly. On the other hand, THT components need lead wires, which go into the drilled holes before getting soldered.
- SMT components require advanced production and design skills to pull off compared to THT components.
- Surface-mount technology components can possess a higher count when it comes to pins than THT components.
- SMT permits for the assembly automation that is pivotal for higher production volumes while also proving low cost. It is the opposite of the through-hole approach of the printed circuit board assembly.
- Surface-mount technology requires a higher capital investment (for instance, machinery) than through-hole technology. However, the production costs are lower, which mitigates the cost-effectiveness in the long run.
- While through-hole mounting proves better-suited for bulky and large components production with targeted applications on stress and high power or voltage areas, SMT primarily applies and is more suitable for smaller components.
- Surface-mount technology can realize advanced circuit speeds as a result of its small size. It owes this to fewer holes, a reduced errant inductance, and capacitance.
Solutions to SMT Defects
- Raise the preheating temperatures but in line with the suggested guidelines. It should happen in a way that allows a meager difference in temperature between terminals during reflux.
- You should always pick PCB boards and components that have consistent solder-able pads and leads.
- It is vital to measure the solder paste deposition height to ensure it corresponds to the height of solder paste deposition between the solder pads. The measurement process occurs through a visual system.
- It is also valuable to avoid inefficient and viscous forces by avoiding both extremes in environmental conditions.
- You have to minimize or limit the assembly movement during the reflow process.
- It is essential to enhance the forces of assembly placement to ensure the right type of contact between the solder paste and assembly terminals. Remember, it should not prove too much to avoid forming a bridge.
If you take in these solutions but still experience challenges, RayMing PCB and Assembly can always come to your rescue. You will get not only proper technical advice but also expert services on your electronic PCB assembly needs.
SMDs or Surface Mount Device Packages
The electronic PCB assembly process that utilizes the surface-mount technology needs SMD packages. Surface mount device packages can come in a wide range of sizes and shapes. It can include the following.
Typical passive and discrete components
It primarily consists of capacitors and resistors. Such components form part of many present-day electronic devices. Below, some of the surfaces mount device packages and their corresponding details?
Transistors
It has two common package types in SOT-23 that have dimensions of 3 x 1.75 x 1.3mm. SOT represents a “small outline transistor.” The other package device type includes the SOT-223 that measures 6.7 x 3.7 x 1.8mm.
IC packages
It has a wide-ranging package, including the SOIC, QFP, and BGA, among other packages. SOIC, standing for a small outline integrated circuit, contains SOP and TSOP. The QFP stands for a quad flat pack, and they come flat and square. BGA stands for ball grid array and is one of the most popular IC packages. It includes a solder ball arrangement under the chip instead of pins. Ball spacing commonly prove 0.35mm, 0.4mm, 0.5mm, 0.8mm, and 1.27mm. PLCC or plastic leaded chip carrier comes as an enclosed chip (plastic mold casing). It can either prove rectangular or square.
Final Remarks
Electronic PCB assembly is an essential aspect of the printed circuit board production process. The preference for small electronic devices and increased sophistication demands a lot. It all starts from production, where the fusion of technology and craftsmanship in printed circuit boards results in high-quality PCBAs. Hopefully, the insights provided helps you with the design for assembly for your PCBA project.
