Here is the SMT process
Step 1: circuit design
Computer-aided circuit board design is nothing new. We've always been about continually improving the productivity of our designs through automation and process optimization. Meticulous analysis of all the important components of the product and elimination of errors before the design is complete, so spending more time beforehand and being well prepared can speed up time to market. New Product Introduction (NPI) is a structured and framed approach to product development, design and manufacturing that ensures effective organization, planning, communication and management. All documents that guide design for manufacturing (DFM) must include the following:
SMT and perforated component selection criteria; printed circuit board dimensional requirements; pad and metallization hole dimensional requirements; identifiers and naming conventions; component orientation; datums; locating holes; test pads; information about nesting and depaneling; ? Requirements for print lines; Requirements for through-holes; Requirements for testable designs; Industry standards, such as IPC-D-279, IPC-D-326, IPC-C-406, IPC-C-408, and IPC-7351. For more information on this topic, go to the Web site at www.ipc.org上查看. Related IPC specifications.When designing printed circuit boards with in-system programming (ISP) capabilities, some preliminary planning is required, and doing so can reduce the number of board design iterations. Engineers can optimize the printed circuit board for (ISP) programming on the production line in several ways. Engineers can identify programmable components on the board. Not all devices are
in-system programmable, such as parallel devices. The design engineer must first carefully read the programming specifications for each component and then lay out the pin connections, with access to the pins on the board. Another step is to determine how the programmable components are powered up during production, and to figure out which devices the manufacturer prefers to use for programming.
In addition, information traces should be considered, for example, data about configurations. Used correctly, board design and DFM can effectively ensure that products are manufactured and tested, shortening and reducing the time, cost, and risk of product development. Inaccurate board designs can jeopardize the quality and reliability of the final product, so design engineers must fully understand the importance of DFM.
Step 2: Process Control
Process control is the most effective means of preventing defects, and it can be tracked throughout the assembly line. With the trend toward globalization, more and more companies are establishing factories around the world, and they need to have effective control over production and, more importantly, effective management of the supply chain. The combination of smaller sized and more precise components, the use of lead-free, and highly reliable products makes process control more complex. Defects can be reduced by eliminating the potential for human error. Statistical Process Control (SPC) can be used to test processes and monitor variations due to general and specific causes. Several SPC tools need to be used to realize the benefits of process control. We should also use SPC to stabilize new processes and improve existing ones. Process control also achieves and maintains pre-process levels, stability and repeatability. It relies on statistical tools for testing, feedback and analysis.
The most basic elements of process control are:
Control item: the process or machine to be monitored; Monitoring parameter: the control item to be monitored; Inspection frequency: the number of intervals between inspections or the time of day; Inspection method: the tools and techniques; Reporting format: SPC charts; Data type: attributes or volatile data; Trigger point: the point at which an inspection occurs; Reporting format: SPC charts; Data types: attributes or volatile data; Trigger point: the point at which an inspection occurs. /p> Trigger point: the point at which a change will occur.With the advent of lead-free electronics comes a new requirement for process control: tracking materials. Lower and lower product prices and higher quality requirements require tighter control throughout the assembly process. In all areas, traceability is required. A key aspect is the tracking of materials. With material tracking systems, we can see at a glance the status of materials and their location in the workshop. Component tracking is also important in cases where alloys are mixed. Putting lead-free and tin-lead components together incorrectly can have very serious consequences.
Other elements of process control include:
calibration of equipment; use of good boards as a comparison to find defects; machine repeatability; open software interfaces between systems; manufacturing execution systems (MES); enterprise resource planning.Process engineers must study the development of complete and effective assembly processes and high-quality planning during the introduction of a new product (NPI). The development of machine software and data structures has to be done simultaneously, and the interfaces must be open so that engineers can design, control, and monitor the SMT process on multiple lines at the same time. Improving quality starts with a plan, a set of goals that differ from specific standards, a variety of test tools, and a way to make changes and communicate them to improve the quality of the final product.
Step 3: Soldering Materials
For many years we have been using tin-lead solder in our production, but now products sold in the EU and China require a switch to lead-free solder alloys. While there are many lead-free solder options available, tin-silver-copper (SAC) solder alloys have become the lead-free solder of choice.
Solder comes in many types of products, including strips, nuggets, wire, powder, molded solder, balls and paste. The soldering process uses a variety of different fluxes, the most common of which are: rosin, lightly activated agents (RMA) and organic acid fluxes. There are basically two types of fluxes: those that need to be cleaned with water or a cleaning solvent, and no-clean fluxes.
The industry chose (SAC) because of the following considerations:
Low melting point: When heated, low melting point alloys go from solid to liquid without going through a "paste" stage. Initially, this was the reason why many industry organizations considered (SAC) to be the most suitable low melting point alloy. Subsequent work has shown that if the temperature of the (SAC) alloy deviates just a little from this low melting point, failures, such as the problem of one end of a passive discrete component standing on end, can be minimized considerably. The ideal alloy is (SAC305), of which 3.0% silver, 0.5% copper and the rest tin. Melting point: The melting point or liquidus line of a solder alloy varies depending on its metallurgical composition. the melting point of SAC305 or other near-low melting point lead-free solders is approximately 217°C. The melting point of a solder alloy is approximately 217°C. The melting point of a lead-free solder alloy is approximately 217°C. Alloy price: Because of the high price of silver, it is better to have less silver in the alloy. For solder pastes, this is not a big deal because the price of the solder paste manufacturing process is much higher than the price of the material. However, for wave soldering, the price of lead-free solder is higher. Tin whiskers: The lead contained in the lead-free surfaces on component pins can cause tin whiskers. Wetting characteristics: Lead-free solder alloys have poor wetting capabilities compared to tin-lead or traditional low melting point solder alloys.Auto-alignment: Since lead-free alloys have significantly less wetting ability than tin-lead alloys, they are also not auto-aligned. As a result, there is a lower chance of aligning the balls of solder in reflow soldering.
Rheology: The tack and surface tension of the solder is an issue that needs to be emphasized and should be evaluated first when selecting a new lead-free solder paste. Reliability: The reliability of solder joints is a pressing issue for lead-free technology. Lead-free solder joints are brittle and can be easily damaged by impact or dropping them to the ground. However, at lower pressures, the reliability of SAC is comparable to, if not better than, that of tin-lead alloys. Also, the long-term reliability of lead-free solder alloys is very questionable, as we don't have the same reliability data for these alloys as we do for tin-lead solder alloys. IPC Standards: J-ST D - 0 02/0 03, JSTD - 0 0 4 / 0 0 5/ 0 0 6, IPC-TP-1043/1044 (For detailed information on all IPC standards, go to the web site: www.ipc.org).Step 4: Printing
The solder paste printing process consists of a series of interrelated variables, but in order to achieve the desired print quality, the printer plays a decisive role. For an application, the best approach is to select a screen printer that meets specific requirements.
In a manual or semi-automatic press, solder paste is applied by hand with a squeegee to one end of the stencil/screen. An automatic printer will apply the solder paste automatically. In a contact printing process, the board and stencil remain in contact during the printing process and are not separated as the squeegee travels over the stencil.
In a non-contact printing process, the stencil peels off or detaches from the board after the squeegee passes over it, and returns to its original position after the solder paste has been applied. The distance of the screen from the board and squeegee pressure are two important equipment-related variables.
Driver wear, pressure and hardness determine print quality. Its edges should be sharp and straight. A low squeegee pressure can cause missed prints and rough edges, while a high squeegee pressure or a soft squeegee will blur the solder paste printed onto the pads and may damage the squeegee, stencil or screen.
Double-thickness stencils can add the proper amount of paste to both micropitch component pads and standard solder surface mount component pads. This is done by forcing the solder paste into the small holes in the stencil with a rubber squeegee. The use of a metal squeegee prevents variations in solder paste volume, but requires modifications to the design of the holes in the stencil to avoid applying too much paste to the micropitch pads. A width-to-thickness ratio of 1:1.5 for the stencil holes is preferred to prevent clogging.
Chemical etching of stencils: Chemical etching can be used to etch both sides of metal stencils and flexible metal stencils. In this process, the etching is done in a defined direction (longitudinal and transverse). The walls of these templates may not be flat and require electrolytic polishing.
LASER CUTTING STENCILS: This chipping process generates a template which uses a g e r b e r file to generate the laser directly. We can adjust the data in the file to change the size of the template.
Electroformed template: this is additional process which deposits nickel onto a copper substrate to form small holes. A photosensitive dry film is formed on the copper foil. After developing, a negative is obtained. Only the small holes in the template are covered by the photoresist. The nickel plating layer around the photoresist is increased until a template is formed. After a predetermined thickness is reached, the photoresist is then removed from the small holes, and the electroformed nickel foil is separated from the copper substrate, and then the copper substrate is taken away.
The proper combination of the right solder paste materials, tools and processes are needed to get the most optimal printing results. The best solder paste, equipment and usage are not enough to guarantee optimal printing results. The user must also control changes in equipment.
Step 5: Adhesives/Epoxies and Dispensing Technology
Epoxide adhesives offer good spreadability, consistent dot shape and size, high wetting and curing strength, fast cure, flexibility, and impact resistance. They are also suitable for high-speed application of very small dots, and the electrical characteristics of the board are good after curing. Bond strength is the most important parameter in adhesive performance. The degree of bonding between the component and the printed circuit board, the shape and size of the adhesive dots, and the degree of curing are the factors that will determine the bond strength.
Rheology affects the formation of the epoxy dot, as well as its shape and size. In order to ensure that the dot has the required shape, the adhesive must be thixotropic, meaning that the adhesive will become thinner when agitated and thicker when at rest. One of the most important things to consider when building a reusable adhesive dispensing system is how to combine the right rheological properties.
Adhesives are categorized by their electrical, chemical or curing properties, as well as their physical properties. Conductive and non-conductive adhesives are used for surface mounting.
Automated application systems are used for a wide range of applications, from the simple application of glue to the application of demanding materials, such as the application of solder pastes, surface mount adhesives (SMAs), sealants and underfill adhesives.
Injection dispensers can be controlled manually or pneumatically. Developed from injection technology, they are accurate, repeatable and stable. There are several different types of valves suitable for injection dispensers, including snap-tube dispensing pens, as well as diaphragm, spray, needle, slide and rotary valves. Needles are also an important component in benchtop dispensing equipment. Precise application requires the use of metal application needles.
Needles are available in diameters ranging from 0.1mm to 1.6mm and, of course, in other sizes. Spray coating technology is ideally suited to applications where speed and accuracy are important or where control of material placement is required. Its main areas of application include, chip scale packaging (CSP), flip chip, non-flowing and pre-coated underfill adhesives, as well as traditional conductive adhesives and surface mount adhesives. Spray technology uses mechanical, piezoelectric or resistive components to force material out of a nozzle.
Material application makes or breaks the final product. Fully understanding and selecting the optimal combination of material, dispenser and movement is critical to the success or failure of the product.
Step 6: Component Placement
Discrete components are getting smaller and smaller, so component placement is becoming more and more difficult. It's difficult to place components accurately, but also reliably and repeatably. 0201 components are becoming more and more common; however, we'll soon see 01005 components on boards. Component sizes are getting smaller and boards are getting more complex, requiring a wide variety of components to be placed on the board, and more of them.
Mounting a component is as simple as picking it up from a conveyor belt, rack, or tray and putting it right on the board. Component placement is categorized as manual, semi-automatic and fully automatic. Manual placement is ideal for rework, but it is poorly accurate and not fast enough for current component technology and production line requirements. Semi-automatic placement uses a vacuum to pick up the component and place it on the board. This method is much faster than manual placement, but, because it requires human intervention, there is still the potential for error. Fully automated placement is very common in high volume assembly. High-speed component placement uses probably such machines, with placement speeds ranging from three thousand to eighty thousand components per hour.
The types of mounters are categorized into three types: rotating rack type mounters, gantry mounters and flexible mounters. Gantry mounters are faster, smaller, and less expensive, and they are more programmable and easy to use with tape-loaded components, so gantry mounters will be used in all future SMT lines. The ability of this machine to quickly place large components and micro-pitch components is its strength.
Different production environments require the use of different types of mounters. The size of the production is the first thing to consider. Whether the machine is suitable for production depends on what components need to be placed on the board, how many components need to be placed, and what the specific production environment is. There are several types of mounters available, and a manufacturer may not be able to fulfill all of a user's requirements with just one machine. When purchasing a new mounter, you first need to clarify the following questions:
What size boards can it produce? How many different components will be used? What types/ sizes of components will be used? How many variations will occur? What is the average number of components mounted per panel? How many boards can be produced per hour? What level of return on investment can be achieved? What are the costs?Successful component placement is often associated with a variety of equipment. Understanding the various aspects of the overall process makes it easier to make the most favorable decision based on the pros and cons of different mounters.
Step 7: Soldering
Pb-free affects all aspects of manufacturing to a greater or lesser degree, but no aspect is comparable to reflow soldering. Due to the higher melting point temperature, lead-free solder alloy reflow temperature profile changes, so in the reflow management needs to make some adjustments. Reflow process parameters to consider include, peak temperature, time at liquid line (TAL), and temperature rise and fall rates. In addition, there are cooling requirements, leaving board temperature and flux control to consider.
The most common problems with lead-free reflow soldering are air bubbles, board distortion, and component damage that result when the reflow process exceeds the limits set by the specifications. Some components, such as aluminum electrolytic capacitors and some other plastic connectors, require lower temperatures to prevent damage from excessive temperatures, but larger components such as sockets require more heat to get a good solder joint, so developing a reflow temperature profile can be a challenging problem when there are these different types of components on the board. Backward compatibility (lead-free BGA components mounted on tin-lead circuit boards) also complicates matters.
In convection soldering, reflow temperatures are higher, which means that flux is required to not burn easily. For reflow furnaces, the flux collection system not only has to operate at higher temperatures, but also has to hold more flux.
Nitrogen (N2) prevents oxidation of metal surfaces during the heating process and ensures proper flux activation. However, it is worth noting that when using lead-free SAC305 alloy, nitrogen is not useful in a reflow oven. Price-sensitive industries may not want to use nitrogen in lead-free.
In the case of perforated or surface-mounted discrete components, when switching to lead-free wave soldering, the soldering furnace needs to be able to resist corrosion due to the higher percentage of tin in the lead-free solder and the higher furnace temperature. In lead-free solder, the highest content of tin, the required temperature is also higher, will promote the formation of residue.
Lead-free soldering furnaces require a high level of preventive maintenance and servicing to keep the machine functioning properly. Alloys like tin-silver-copper can erode materials used on older wave soldering machines.
The vapor-phase reflow soldering process has been successful with lead-free alloys, which
avoids the changes that occur with high-temperature processing. This process has good heat transfer characteristics.
Laser soldering is good for improving this automated process and is well suited for components that are more sensitive to temperature. This method is slower, but it meets lead-free requirements. Most of the points made about bulk soldering with lead-free alloys also apply to manual soldering for rework.
Flux selection is key when using the no-clean process. A no-clean flux with good curing ability will reduce soldering defects, but it will leave more flux on the board that is visible to the naked eye.
In the lead-free soldering need to consider the following aspects: soldering methods, soldering equipment, solder alloys, flux, thermocouples, nitrogen, soldering furnace, but also to address the transition phase in the same board both tin-lead solder and lead-free solder.
Step 8: Cleaning
Cleaning a printed circuit board is a very important and value-adding process that removes contamination caused by different manufacturing processes and treatments. Without proper cleaning, surface contaminants can cause defects in the manufacturing process. Lead-free increases the importance of the cleaning process. Lead-free soldering processes typically require the use of more flux and higher-activity fluxes than tin-lead processes, so cleaning is often required to remove the de-fluxing residue.
In selecting the appropriate cleaning media and equipment, the following factors are considered: the system must be environmentally friendly and cost-effective; localized emissions of volatile organic compounds (VOCs) and wastewater regulations (COD/BOD/pH) may affect the choice of solutions and equipment; the cleaning agent must also be adapted to the requirements of the assembly materials and washing equipment.
The most common cleaning methods used in SMT assembly are in-line spray systems or batch spray systems. Ultrasonic and steam degreasing methods are among the other batch cleaning methods. Batch cleaning methods are best suited for low volume, high variety production. In-line spraying is for production with high volumes and a single variety, or for production with many varieties.
Aqueous Wash Cleaning - This cleaning method uses water or water containing a detergent (the detergent content is typically between 2-30%). The water soluble material usually consists of a liquid alcohol or VOC solution that can be used for spraying. This approach enables the cleaning of low residue fluxes using rosin in surface mounting or piercing techniques. Aqueous cleaning is typically used in high-pressure in-line cleaning equipment.
Semi-Wet Cleaning - This is a solvent cleaning/water rinse process. Some of the chemicals used in this technique include non-linear alcohols and synthetic alcohol compounds. Non-linear alcohols combine less active and moderately active materials, which can clean harder-to-remove fluxes, such as high-temperature resins and synthetic resins, as well as water-soluble fluxes and no-clean fluxes.
We use three common test methods to determine the cleanliness of SMT manufacturing operations: visual inspection, surface insulation resistance (SIR) and solution extraction. In visual inspection, we manually examine the board through a microscope. The solution extraction method involves immersing the board in isopropyl alcohol and deionized (DI) water to determine ionic conductivity.SIR testing involves the use of specialized test boards at the process design stage and at the mass production stage, and then these test boards are evaluated in an SIR chamber where energized test circuits are exposed to different environmental conditions.
Cleaning is a very important part of the assembly process. Lead-free solder alloys place several demands on board surface cleaning: the use of higher grade and more reactive fluxes requires higher reflow temperatures. Such high temperatures can cause flux residues to smear off, making removal more difficult, especially if traditional chemical cleaning techniques are used.
Step 9: Test and Inspection
Shorter time-to-market, smaller component sizes, and the move to lead-free production have necessitated the use of more test methods and inspections. Requirements for defect levels (defects created during the manufacturing process) and the effectiveness of test and inspection are driving the test industry forward. The best test strategy is often limited by the characteristics of the board. Several important factors to consider include the complexity of the board, the planned scale of production, whether the board is single-sided or double-sided, power-on versus visual inspection, and component-specific issues.
The industry's current approach to testing is:
In-circuit testing (ICT) is performed after reflow soldering, this is, where components are individually energized and tested to verify that there are no problems with the printed circuit board. Traditional ICT systems use pin-bed test equipment to access multiple test points on the underside of the printed circuit board.
The flying probe is a type of ICT test that uses a single probe to perform the test while energized, with no need for a pin-bed interface between the test equipment and the printed circuit board. It examines the printed circuit board with a large number of needles traveling around.
Boundary-scan testing can make up for the lack of power-on inspection. Boundary scan uses an edge connector or a limited bed-of-pins device, which allows testing of components under test and circuit nodes that are out of reach of ICT and flying needles.
The final step in verifying that a printed circuit board is qualified is a functional test before the board is sent away. These test devices use edge connectors and/or test points to connect the printed circuit board. The test equipment simulates the final electrical environment and verifies that the board is functionally compliant.
Checking is different from testing, in that checking is verifying that the board is good or bad without powering it up. We can check as early as possible in the assembly process to achieve process monitoring and control. There are several inspection methods:
Manual inspection. This is the inspector with visual methods to check the printed circuit board to see if there are no defects. This method is the least reliable, and even more so for boards that use 0201 components and micro-pitch lead-free components. Also, manual inspection is very costly.
X-ray inspection. This method is mainly used to check components after reflow soldering, which are not accessible or cannot be tested with ICT or seen clearly with the naked eye. We can operate these systems manually, test samples, or test samples on the production line in a fully automated way (AXI).
Automated Optical Inspection (AOI). This method utilizes camera imaging technology to inspect printed circuit boards.AOI can quickly check for a wide variety of defects and can be performed on the production line, after each placement process is completed. AOI inspection after placement improves the accuracy of the placement process and allows you to check that components are placed on the printed circuit board. It can also be used to check the position and placement of components. Performing AOI inspection after reflow soldering can also reveal some defects that may be caused by reflow soldering.
Controlling defects and finding them throughout the assembly process will be directly related to quality control and cost. Manufacturers need to determine which tests and inspections best meet the requirements of the production line through a thorough testing and inspection process.
Step 10: Rework and Repair
Rework and repair are essential. All of the previous steps have one goal, to improve the accuracy and reliability of the process, but it is still inevitable that components will be removed and need to be replaced. Rework process consists of the following four steps:
1, find out the failure of the component, the possible causes of failure;
2, take down the failed component;
3, complete the preparation of the printed circuit board placement position;
4, mount the component, and then reflow soldering.
Lead-free production requires higher temperatures, which can create new challenges in the rework process. Because the board is at a higher temperature, it may damage the components and the board. The narrower window of the reflow process for lead-free soldering requires precise temperature control for components that are susceptible to temperature, such as BGAs and CSPs. When these larger packages are near their maximum temperatures, nearby smaller components can overheat due to their lower heat capacity and the higher temperatures of the reflow process. Larger sized multilayer printed circuit boards with arrays of packaged components used on them are the biggest challenge for the reflow process.
When encountering a damaged component, the rework technician must first determine if the component can be reworked by hand or if it must be removed and replaced. A functional test of the printed circuit board is also required.
Usually, only hand-operated chrome irons are used in rework. During hand soldering, the already hot chrome iron tip touches the pins and pads of the component, transferring heat to the pins and pads and raising the temperature above the melting point of lead-free solder (usually 217°C). The flux-containing soldering wire comes into contact with the heated part, melting the wire, wetting the surface, and forming an electrically and mechanically connected solder joint as it solidifies. The soldering iron must not touch the component directly to prevent possible thermal shock and breakage. Manual soldering stations are relatively inexpensive, but require a skilled operator.
Other rework work may require the use of a hand-operated hot-air pen, which uses forced convection to direct a small stream of hot air onto pins and pads to complete the soldering. Despite this approach
the use of a hot air pen is usually recommended. Rework of arrayed packages, such as BGAs and CSPs, requires the use of a rework station. These rework tables typically include a removable X/Y bracket (to mount and support the printed circuit board), a hot air nozzle, and a mechanism for optical alignment up/down. After alignment, the nozzle picks up the component and places it on the board. The nozzle then performs reflow soldering on this component. Some rework tables also use infrared light for heat or use lasers.
Moving to lead-free solder will make the rework process more difficult. While the basic steps are the same, the operator responsible for the rework must be aware of the narrower process window for lead-free, as well as the dangers that a rise in process temperature can bring to the printed circuit board and components.