What is the process of SMT technology?
Surface-Mount Technology - Wikipedia
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Surface-mount components on a USB flash drive's circuit board. The small rectangular chips with numbers are resistors, while the unmarked small rectangular chips are capacitors. The capacitors and resistors pictured are 0603 (1608 metric) package sizes, along with a very slightly larger 0805 (2012 metric) ferrite bead.
A MOSFET transistor, placed upon a British postage stamp for size comparison.
Surface-mount technology (SMT), originally known as planar mounting, is a technique where electrical components are mounted directly onto the surface of printed circuit boards (PCBs). Such components are referred to as surface-mount devices (SMDs). This method has largely replaced the older through-hole technology mainly due to the increased automation potential and cost savings, along with improved quality. SMT also allows for denser packing of components on a given substrate area. Although through-hole technology is still used for components that can't be surface-mounted, such as large transformers or heat-sinked power semiconductors, both technologies can coexist on the same PCB.
SMT components are typically smaller than their through-hole counterparts, generally lacking leads or possessing smaller leads like short pins, flat contacts, solder balls (BGAs), or terminations on the component body.
Evolution of Surface-Mount Technology
The development of surface-mount technology began in the 1960s. By 1986, it accounted for about 10% of the market, rapidly gaining momentum and dominance in high-tech electronic assemblies by the late 1990s. IBM made significant contributions to the technology, initially implementing it in a small-scale computer in 1960 and later utilizing it in critical applications such as the Launch Vehicle Digital Computer for the Saturn IB and V vehicles. SMT led to smaller component designs and the feasibility of placing components on both sides of a PCB, allowing higher circuit densities and more compact boards.
Surface tension of the solder generally keeps parts attached to the board. Adhesives might be used to secure parts, especially when components are on the bottom side during reflow soldering or when wave soldering is involved. SMT's compatibility with automated assembly significantly reduces labor costs and boosts production speed. However, manual assembly of SMT components proves challenging, making through-hole components preferred for one-off prototypes or low-scale production. SMT components, though smaller and cheaper, may require advanced equipment for soldering them.
Frequent Abbreviations in SMT
Several terms are used to describe components, techniques, and machinery related to SMT:
- SMD: Surface-mount devices (including active, passive, and electromechanical components)
- SMT: Surface-mount technology (techniques for assembling and mounting components)
- SMA: Surface-mount assembly (modules assembled with SMT)
- SMC: Surface-mount components (components designed for SMT)
- SMP: Surface-mount packages (various case forms for SMDs)
- SME: Surface-mount equipment (machines used for SMT assembly)
Techniques for Assembling SMT
During the assembly process, components are mounted on printed circuit boards (PCBs) using various techniques. These include solder paste printing, component placement, and reflow soldering.
Solder Paste Printing
In this initial stage, solder paste—a sticky mix of flux and solder particles—is applied to all the PCB pads using a stainless steel or nickel stencil. This can be done through a screen-printing process or a jet-printing mechanism. Controlling the quality of solder paste application is crucial, as defects here can lead to issues later in the assembly process. Often, automatic inspection is available in solder paste printers to ensure process accuracy.
Component Placement
Once the solder paste is accurately applied, components are placed using automated pick-and-place machines. These machines take components from their packaging and precisely position them on the PCB. High-speed machines can place up to 80,000 components per hour. Inspection is vital here to detect any placement errors that could lead to costly rework if not addressed.
Reflow Soldering
The final step is reflow soldering, where the PCB is heated in an oven to melt the solder paste, forming solid solder joints that electrically and mechanically secure the components to the PCB. Control of the temperature profile is critical to avoid damage to components or inadequate soldering.
Advantages of Using SMT
- Allows for smaller, more compact components.
- Higher component and connection density on PCBs.
- Components can be mounted on both sides of a PCB.
- Improved mechanical performance under stress and vibration.
- Increased production speed and reduced labor costs through automation.
Drawbacks of SMT
- Not suitable for components that undergo frequent mechanical stress.
- Manual assembly and repair are challenging and often impractical for small-scale production.
- Requires specialized equipment and skilled operators.
- Voids in solder joints can compromise reliability and necessitate careful control.
Rework in SMT
Reworking defective components typically involves melting solder to remove the defective parts, cleaning the PCB, reapplying solder paste, and placing new components followed by reflow soldering. Non-contact methods like infrared or hot gas soldering are often used to avoid damage during reworking processes.
Infrared Rework
Uses electromagnetic radiation for heating solder joints. It allows direct control of component temperature and ensures uniform heating without needing compressed air.
Hot Gas Rework
Employs hot gas, either air or inert gas, to heat solder joints. It's efficient and suitable for large volume rework but requires precise nozzles to avoid overheating adjacent components.
Hybrid Rework Systems
Combine infrared radiation with hot gas to achieve efficient and uniform heating, minimizing the risks associated with each individual method.
Types of SMT Packages
SMT packages come in standardized shapes and sizes, which are smaller than their through-hole counterparts and designed for machine handling.
Component Identification
- Resistors
- Marked with a three-digit code indicating resistance value.
- Capacitors
- Typically unmarked; value determined by measurement.
- Inductors
- Smaller inductors resemble ferrite beads; larger ones are marked with value.
- Discrete Semiconductors
- Markings are abbreviated codes that correlate with traditional part numbers.
- Integrated Circuits
- Usually large enough to bear full part numbers and manufacturer logos.
What is the SMT process and why should OEMs care?
Original Equipment Manufacturers (OEMs) are organisations that manufacture or sell products to another company. This company then uses these components in part of a wider process to create products for their customers under their branding. A big part of creating products to sell on to other companies centres around the SMT process.
What is SMT?
Surface mount technology (SMT) is the process wherein components are mounted onto the surface of a printed circuit board. The components are designed specifically to be directly mounted, rather than hardwired, onto the circuit board for most electronics.
Surface mount technology has been widely used since the 1980s. This is because the SMT process is automated, saving time and costs for both manufacturers and customers. Previously, hand wiring through holes between boards was needed to bring components together, involving manual labor and the risk of human error. The SMT process is also more adaptable for technological advances than through-hole wiring methods.
Other advantages of SMT include higher component density, smaller component sizes, and better performance under pressure. SMT also allows components to be mounted on either side of the board.
What is the typical SMT process?
The SMT process generally involves three main steps: solder paste printing, component placement, and reflow soldering.
Solder Paste Printing
The solder paste printing process is carried out by a machine to ensure accuracy and speed. During this part of the assembly, a printer applies solder paste using a pre-made stencil of the PCB and squeegees. The solder paste, typically a mixture of flux and tin, is used to connect SMC and solder pads on the PCB. Accurate application is crucial as any defects can lead to issues later in the process. Automatic inspections help ensure quality.
Component Placement
Post inspection, the PCB moves to the component placement phase. Each component is removed from its packaging using a vacuum or gripper nozzle and placed in its programmed location by the machine. These machines are highly accurate and fast, with some capable of placing 80,000 components per hour. Inspection ensures correct placement; otherwise, rework may be needed.
Reflow Soldering
During reflow soldering, the PCB is placed in a reflow soldering machine where heat melts the solder paste, forming electrical connections. Accurate temperature control is crucial to avoid damage or inadequate soldering. The PCB passes through a cooling zone to solidify the connections. Final inspection verifies all solder joints.
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