Introduction
Surface mount technology (SMT) is an aspect of electronic assembly where electronic components, also called surface mount devices (SMD), are directly mounted onto the surface of a printed circuit board (PCB). Because of its cost and quality efficiency, SMT has become highly sought after in the industry.
Are you interested in learning more about Smt Technology Process? Contact us today to secure an expert consultation!
What is Surface Mount Technology (SMT)?
Surface mount technology (SMT) is an assembly and production method that applies electronic components directly onto the surface of a printed circuit board (PCB). This process allows for automated production to complete more of the required assembly to create a working board. Any electrical component mounted in this manner is referred to as a surface-mount device (SMD). Unlike conventional assembly, SMT does not require components to be inserted through holes, rather components are soldered onto the board directly through reflow soldering.
Initially called Planar Mounting, SMT was first developed and applied by IBM to build small-scale computers in the 1960s, thereby becoming a replacement for its predecessor, Through-Hole Technology. However, it did not take off until 1986 when surface-mounted components managed to reach 10% market popularity. By 1990, surface-mounted devices or SMDs could be found in the majority of all high-tech printed circuit assemblies (PCAs).[1]
SMT components were designed to have small tabs where solder could be applied to attach SMDs to the surface of the PCB. In the era of Through-Hole Technology, components were mounted through lead holes that were drilled into PCBs. The holes were sized to fit each component to hold each piece tightly, then the grip was soldered. With SMT, the hole drilling step is bypassed as SMDs are quickly sorted and attached to the top of the PCB with little to no hole leads; thereby significantly cutting the process of device assembly.
The SMT assembly process can be tedious and time-consuming if done manually, due to the precision it requires to create high-quality surface mount assembly (SMA). So for better efficiency, most SMT manufacturing is done through automated assembly machines, especially when production is large scale.[2]
SMT components are significantly smaller than through-hole components, which makes for the production of sleek and attractive electronic devices suitable for modern times. Therefore these days, SMT is used in almost every electronic device ranging from toys to kitchen appliances, to laptops and smartphones.[3]
SMT Manufacturing Process
The SMT manufacturing process is broadly grouped into 3 stages, namely: solder paste printing, components placement, and reflow soldering. However, due to the demands of the SMT production process, these stages are further analyzed in the following outline:
1. SMC and PCB Preparation
This is the preliminary stage where the SMCs are selected and PCB designed. The board usually contains flat, commonly silver, tin-lead, or gold plated copper pads devoid of holes, called solder pads. Solder pads support the pins of the components like transistors and chips.
Another vital tool is the stencil, which is used to provide a fixed position for the next phase of the process (solder paste printing), according to the predetermined positions of solder pads on the PCB. These materials, along with others that are to be used in the manufacturing process must be properly examined for flaws.
2. Solder Paste Printing
This is a critical phase in the SMT process. During this phase, a printer applies solder paste using the prepared stencil and squeegee (a tool for cleaning in printing) at an angle ranging from 45° to 60°. Solder paste is a putty-like mixture of powdered metal solder and sticky flux. The flux serves as a temporary glue to hold the surface mount components in place as well as cleansing the soldering surfaces of impurities and oxidation.
The solder paste, on the other hand, is used to connect the SMC and solder pads on the PCB. It is pertinent that each pad is coated in the correct quantity of paste. Otherwise, there will be no connection established when the solder is melted in the reflow oven. In the electronics manufacturing industry, a reflow oven is an electronic heating device used in surface mount technology (SMT) to place electronic components on printed circuit boards (PCBs).
3. Components Placement
Next, pick-and-place machines are used to mount components on the PCB. Each component is removed from its packaging using a vacuum or a gripper nozzle, and the placement machine places it in its designed location. The PCB is carried on a conveyor belt while the electronic components are placed on it by the quick and accurate machines, some of which can place 80,000 individual components per hour.
Accuracy is required in this process because any erroneous placement soldered into a position can be costly and time-consuming to rework.
4. Reflow Soldering
After SMCs are placed, the PCB is then conveyed into the reflow soldering oven, where it passes through the following zones to undergo the soldering process:
Preheat zone: this is the first zone in the oven, where the temperature of the board and all the attached components is raised simultaneously and gradually. Temperature is cranked up at the rate of 1.0℃-2.0℃ per second until it enters 140℃-160℃.
Soak zone: here, the board will be kept at a temperature between 140℃ and 160℃ for 60-90 seconds.
Reflow zone: the boards then enter a zone where the temperature is ramped up at 1.0℃-2.0℃ per second to the maximum of 210℃-230℃ to melt the tin in the solder paste, welding the component leads to the pads on the PCB. While this is going on, the components are kept in place by the surface tension of the molten solder.
Cooling zone: this is the final section that ensures solder freezes upon exiting the heating zone to avoid joint defects.
If the printed circuit board is double-sided then these processes may be repeated using either solder paste or glue to hold the SMCs in place.
6. Cleaning and Inspection
After soldering, the board is cleaned and checked for flaws. If any is found, the defects are repaired and then the product is stored. Common ways used for SMT inspection include the use of magnifying lenses, AOI (Automated Optical Inspection), flying probe tester, X-ray inspection, etc. Instead of the naked eye, machines are used for quick and accurate results.
Soldering of PCB with electronic components
SMT: Pros and Cons
SMT has proven beneficial for PCB assembly (PCBA), PCB manufacturing, and electronics production in many ways including these:
Allows for smaller components
SMT process encourages increased automation
Maximum flexibility in building PCBs
Improved reliability and performance
Reduced manual intervention for component placement
Smaller, lighter boards
Ease of PCB assembly, using both sides of the board without the hole limitations that exist in the conventional method
Can co-exist with through-hole components, even on the same board
Increased density i.e more SMD components in the same space, or the same number of components in a much smaller frame
Low Cost of materials
Simplifies the production process and reduces the production cost. [4]
Conversely, disadvantages of SMT to electronic manufacturing include:
Small volume
Easily broken due to fragility
High requirements for soldering technology
Components can be easily dropped or damaged when installed.
It is not easy to use visual inspection, which is difficult to test.
Contact us to discuss your requirements of Smt Unloader. Our experienced sales team can help you identify the options that best suit your needs.
Miniaturization and numerous solder joint types complicate the process and inspection.
Large investment in equipment such as the SMT machine
Technical complexity requires high training and learning costs.
Rapid development requires continuous follow-up. [5]
SMT vs SMD
SMT and SMD are frequently misunderstood and used simultaneously. Indeed, any technology and its actual components can be deeply entwined, creating confusion. Such is the case of SMT and SMD. This is why knowing the difference between an SMT assembly and individual SMD components is important.
In a simpler vein, SMT is the process in the technology, while SMD is the device involved in the technology. SMT is the technology that uses the method of directly placing and soldering electronic components on a PCB. These components are also sometimes called surface mount devices or SMDs. They are designed to be mounted on a printed circuit board (PCB).
SMDs make for devices produced faster, with more flexibility and less cost, without sacrificing functionality. They promise more functionality because smaller components allow for more circuits on small board space. This miniaturization is the major feature of SMD.[6]
Both SMT and SMD work together to provide users with faster, more energy-efficient, and more dependable PCBs.
Closing Thoughts
Smaller size, quicker production, and reduced weight are the major allures of SMT, leading to much easier electronic circuitry design and production, especially crucial in complex circuits. This higher level of automation has saved time and resources throughout the electronics manufacturing industry. As such, even though there is always a chance of developing new technology, SMT has undoubtedly secured its relevance.
References
SCRIBD. Surface-Mount Technology: History. 2019.[cited 2022 Jun 8]. Available from: https://www.scribd.com/document/106437192/SMD
Das S. SMT Machine and SMT Machine Manufacturers. 2021 [cited 2022 Jun 8]. Available from: http://www.electronicsandyou.com/smt-machine-and-smt-machine-manufacturers.html
Geospace Technology. SMT ASSEMBLY TECHNOLOGY: ALL YOU NEED TO KNOW. 2021.[cited 2022 Jun 9]. Available from: https://geospacemfg.com/blog/smt-assembly-technology/
Blog Industry News. Top Benefits of Surface-mount Technology (SMT) PCB Assembly. 2020. [cited 2022 Jun 9]. Available from: https://www.pcbnet.com/blog/benefits-surface-mount-technology-smt-pcb-assembly/
PCBWay. Advantages and Disadvantages of Surface Mounting Technology. 2019.[cited 2022 Jun 9]. Available from: https://www.pcbway.com/blog/PCB_Assembly/Advantages_and_Disadvantages_of_Surface_Mounting_Technology.html
6. History Computer (HC). Surface-Mount Technology. 2022. [cited 2022 Jun 9]. Available from: https://history-computer.com/smt-surface-mount-technology/
SMT (Surface Mount Technology) assembly has become a leading electronics manufacturing technology as far as performance and efficiency are concerned by electronics products. With high reliability guaranteed, low cost can be definitely regarded as the secondary key element that has to be considered by OEMs (Original Equipment Manufacturers).
SMT assembly procedure consists of so many steps each of which contributes to the quality of final products. Furthermore, any modification occurring to each manufacturing step may possibly cause a huge cost fluctuation. Therefore, it’s substantially positive to have a full understanding of SMT assembly procedure, which is also a shortcut to cost reduction without performance being sacrificed.
Generally speaking, SMT assembly procedure mainly contains the following steps: solder paste printing, solder paste inspection (SPI), chip mounting, visual inspection, reflow soldering, AOI, visual inspection, ICT (In-Circuit Test), function test, depanelization etc. And, a full understanding of the whole procedure helps you to reduce production cost.
Step#1: Solder Paste Printing
SMT assembly starts from solder paste printing that aims to place a proper amount of solder paste onto pad on which components will be soldered. The quality of solder paste printing is mainly determined by three elements: solder paste condition, scraping angle and scraping speed.
High quality can never be obtained by SMT assembled PCBs unless solder paste is properly stored and applied. Solder paste has to be stored in a fridge to be kept in a low temperature and its temperature should be recovered to room temperature prior to its application on SMT manufacturing line. Moreover, the uncovered solder paste has to be used up in two hours. Apart from solder paste status, solder paste printer parameters should be properly set, which is especially true for scraping angle and speed since both parties are closely related with the specific amount left on pad.
Step#2: Solder Paste Inspection (SPI)
Solder paste inspection in itself is an optional way to reduce cost because it’s better to reduce solder defects right now than to catch them later. SPI is not a must-be step in SMT assembly procedure but with it applied it’s beneficial to decrease your manufacturing cost and to improve the quality of products. After all, most defects in SMT assembly derive from solder paste printing and if they can be found out and dealt in the early phase, the threats possibly leading to defects in later stages of manufacturing will be decreased or even eliminated. SPI machine features two types: 2D and 3D. PCBCart owns a 3D SPI machine in the workhouse to provide better inspection service to customers.
Step#3: Chip Mounting
Chip mounting plays a core role in SMT assembly process. Chip mounting is completed by chip mounters that differ from each other primarily in terms of speed and mounting capability. Some small components are normally placed by high-speed chip mounters that are capable of placing them quickly to make those components quickly adhered to the solder paste on pad.
However, big components such as BGAs, ICs, connectors, etc. are usually placed by multi-function chip mounters that run at a relatively low speed. As far as those components are concerned, alignment does matter. It takes more time to achieve alignment prior to chip mounting, which is why the speed of multi-function chip mounter is much lower than that of high-speed chip mounter. Moreover, some of the components used in multi-function chip mounter don’t rely on tape on reel while some on tray or tube due to the limitations of size.
Step#4: Visual Inspection + Components Placement by Hand
After chip mounting, it’s necessary to carry out visual inspection so that reflow soldering can be largely ensured to be without defects. The leading issues to be found out in this step include misplacement, missing parts, etc. The defects are extremely difficult to be dealt with once reflow soldering is done because they will be solidly fixed onto PCB. As a result, products’ reliability will go down and production cost will rise as well.
On the other hand, some components can be placed directly by hand in this step, including some large components, DIP components or those that can’t be placed through chip mounter due to some reasons.
Step#5: Reflow Soldering
In the process of reflow soldering, solder paste is melted to generate IMC (Intermetallic Compound) to connect component pins and board. The temperature profile followed in the process of reflow soldering process covers preheating, temperature rise, reflow and cooling. Take lead-free solder paste SAC305 as an example, its melting point is approximately 217℃ so the solder paste can’t be remelted unless the temperature of reflow soldering oven is higher than 217℃. Furthermore, the highest temperature of reflow soldering oven shouldn’t be higher than 250℃, or a lot of components can’t be melted due to their incapability to withstand such a high temperature.
As a matter of fact, temperature profile setting determines reflow soldering quality and helps reducing production cost. Thus, it’s better to find out experienced SMT assembler as a CM (Contract Manufacturer) who is fully aware of elements influencing SMT soldering quality and improvement measures. All the elements will lead production cost to be cut.
Step#6: AOI (Automated Optical Inspection)
Up to now, components have been fixed onto PCB after reflow soldering, which means that the essential part of task concerning SMT assembly has been completed. However, the assembled boards can never be directly used in final products unless sufficient test and inspection have been made. Performance of solder joints can be inspected through the application of AOI that is capable of exposing some defects like tombstone, edge on, missing components, misplacement, orientation, bridging, empty solder etc.
Step#7: AXI (Automated X-ray Inspection)
X-ray inspection is a supplement to AOI since it is capable of indicating some defects more clearly and directly. It’s not a must-be measure after reflow soldering. However, as long as the SMT assembler cares more about products’ quality and reliability, X-ray inspection machine will be definitely applied to meet the rigorous demands of some OEMs for the achievemtn of higher efficiency.
Step#8: ICT or Function Test
The aim of ICT is to test whether opens and shorts are available in the circuit and to expose some defects of some components by measuring resistance, capacitance and inductance. As a result, components are tested to ensure their high performance after reflow soldering.
Function test is a supplement of ICT since ICT can only test the opens and shorts on bare board but the functions of assembled PCBs fail to be tested. Therefore, the functions of assembled PCBs should be tested through function test in order to maintain the high reliability of final products.
Now Up to $800 Discount Waiting for YOU
For more Smt Wholesaleinformation, please contact us. We will provide professional answers.
Comments
Please Join Us to post.
0