What is SMT

SMT is the abbreviation of Surface Mounted Technology, which is the most popular technology and process in the current electronic assembly industry. It compresses traditional electronic components into devices with a volume of only a few tens of times smaller, thereby achieving high density, high reliability, miniaturization, low cost, and automated production of electronic product assembly. This miniaturized component is called: SMY device (or SMC, chip device). The process of assembling components onto a printed (or other substrate) is called the SMT process. The related assembly equipment is called SMT equipment.

At present, advanced electronic products, especially in computer and communication electronic products, have generally adopted SMT technology. The output of SMD devices in the world has increased year by year, while the output of traditional devices has decreased year by year. Therefore, with the passage of time, SMT technology will become more and more popular.

What are the characteristics of SMT:

1. High assembly density, small size and light weight of electronic products. The volume and weight of SMT components are only about 1/10 of traditional plug-in components. Generally, after using SMT, the volume of electronic products is reduced by 40%~60% and the weight is reduced by 60%~80%.

2. High reliability, strong vibration resistance and low solder joint defect rate.

3. Good high frequency characteristics. Reduced electromagnetic and radio frequency interference.

4. Easy to realize automation and improve production efficiency. Reduce costs by 30%~50%. Save materials, energy, equipment, manpower, time, etc.

Why use surface mount technology (SMT)?

1. Electronic products pursue miniaturization, and the perforated plug-in components used previously cannot be reduced in size

2. Electronic products have more complete functions, and the integrated circuits (ICs) used no longer have through-hole components, especially large-scale, highly integrated ICs, which have to use surface mount components.

3. Product batching and production automation. The factory should produce high-quality products at low cost and high output to meet customer needs and enhance market competitiveness.

4. Development of electronic components, development of integrated circuits (ICs), and diversified applications of semiconductor materials

5. The electronic technology revolution is imperative, and we must keep up with the international trend

1. SMT process flow---single-sided assembly process

Incoming material inspection--> Silk screen solder paste (dispensing SMD glue)--> SMD--> Drying (curing)--> Reflow soldering--> Cleaning--> Inspection--> Rework

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2. SMT process flow---single-sided mixed assembly process

Incoming material inspection-->Print solder paste on the A side of PCB (apply SMD glue)--> SMD-->Drying (curing)-->Reflow soldering-->Cleaning-->Plug-in-->Wave soldering-->Cleaning-->Inspection-->Rework

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3. SMT process flow---double-sided assembly process

A: Incoming material inspection --> Screen printing solder paste on side A of PCB (apply SMD glue) --> SMD --> Drying (curing) --> Reflow soldering on side A --> Cleaning --> Flip board --> Screen printing solder paste on side B of PCB (apply SMD glue) --> SMD --> Drying --> Reflow soldering (preferably only on side B --> Cleaning --> Inspection --> Rework) This process is suitable for when larger SMDs such as PLCC are mounted on both sides of the PCB.

B: Incoming material inspection --> Screen printing solder paste on PCB A side (applying SMD glue) --> SMD --> Drying (curing) --> Reflow soldering on A side --> Cleaning --> Turn over --> Apply SMD glue on PCB B side --> SMD --> Curing --> Wave soldering on B side --> Cleaning --> Inspection --> Rework) This process is suitable for reflow soldering on PCB A side and wave soldering on B side. This process is suitable for SMDs assembled on PCB B side when there are only SOT or SOIC (28) pins or less.

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4. SMT process flow------double-sided mixed assembly process

A: Incoming material inspection --> PCB B side SMD glue --> SMD --> Curing --> Turn over --> PCB A side plug-in --> Wave soldering --> Cleaning --> Inspection --> Rework. First paste and then plug-in. Suitable for the situation where there are more SMD components than discrete components.

B: Incoming material inspection --> PCB A side plug-in (pin bending) --> flip board --> PCB B side SMD glue --> SMD --> curing --> flip board --> wave soldering --> cleaning --> inspection --> rework

Insert first and then mount, suitable for situations where there are more discrete components than SMD components

C: Incoming material inspection--> PCB A side screen printing solder paste--> SMT--> Drying--> Reflow soldering--> Plug-in, pin bending--> Turn over--> PCB B side SMT glue--> SMT--> Curing--> Turn over--> Wave soldering--> Cleaning--> Inspection--> Rework

Mixed assembly on A side and mounting on B side.

D: Incoming material inspection--> PCB B side SMD glue--> SMD--> Curing--> Flip board--> PCB A side silk screen solder paste--> SMD--> A side reflow soldering--> Plug-in--> B side wave soldering--> Cleaning--> Inspection--> Rework

Mixed assembly on side A, mounting on side B. First mount both sides of SMD, reflow soldering, then insert, wave soldering

E: Incoming material inspection --> PCB B-side screen printing solder paste (applying SMD glue) --> SMD --> Drying (curing) --> Reflow soldering --> Flip board --> PCB A-side screen printing solder paste --> SMD --> Drying --> Reflow soldering 1 (local soldering can be used) --> Plug-in --> Wave soldering 2 (if there are few plug-in components, manual soldering can be used) --> Cleaning --> Inspection --> Rework

SMT basic process composition:

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Basic process components:

Screen printing (or dispensing) --> Mounting --> (curing) --> Reflow soldering --> Cleaning --> Inspection --> Rework

Silk screen printing: Its function is to print solder paste or patch glue onto the pads of the PCB to prepare for the soldering of components. The equipment used is a silk screen printer (screen printer), which is located at the forefront of the SMT production line.

Glue dispensing: It is to drip glue onto the fixed position of PCB. Its main function is to fix the components to the PCB. The equipment used is a glue dispensing machine, which is located at the front end of the SMT production line or behind the testing equipment.

Mounting: Its function is to accurately install surface mounted components on the fixed position of the PCB. The equipment used is a placement machine, which is located behind the screen printer in the SMT production line.

Curing: Its function is to melt the patch adhesive so that the surface mounted components and the PCB board are firmly bonded together. The equipment used is a curing oven, which is located behind the patch machine in the SMT production line.

Reflow soldering: Its function is to melt the solder paste so that the surface mounted components and the PCB board are firmly bonded together. The equipment used is a reflow oven, which is located behind the placement machine in the SMT production line.

Cleaning: Its function is to remove the soldering residues such as flux that are harmful to the human body on the assembled PCB board. The equipment used is a cleaning machine, which can be located in a non-fixed position and can be online or offline.

Inspection: Its function is to inspect the welding quality and assembly quality of the assembled PCB board. The equipment used includes magnifying glass, microscope, online tester (ICT), flying probe tester, automatic optical inspection (AOI), X-RAY inspection system, functional tester, etc. The location can be configured at a suitable place on the production line according to the needs of the inspection.

Rework: Its function is to rework the PCB board that has been detected to be faulty. The tools used are soldering iron, rework workstation, etc. It is configured at any position in the production line.

SMT related technical components

1. Design and manufacturing technology of electronic components and integrated circuits

2. Circuit design technology for electronic products

3. Circuit board manufacturing technology

4. Design and manufacturing technology of automatic placement equipment

5. Circuit assembly manufacturing technology

6. Development and production technology of auxiliary materials used in assembly manufacturing

SMT Components Introduction

SMC: Surface Mounted commponents

There are mainly rectangular chip components, cylindrical chip components, composite chip components, and special-shaped chip components.

SMD: Surface Mounted Devices

There are mainly chip transistors and integrated circuits. Integrated circuits include SOP, SOJ, PLCC, LCCC, QFP, BGA, CSP, FC, MCM, etc. Examples are as follows:

1. Interconnect: Provides mechanical and electrical connection/disconnection, consisting of connecting plugs and sockets, connecting cables, brackets, chassis or other PCBs to PCBs; however, the actual connection to the board must be through surface mount contacts.

2. Active electronic components (Active): In analog or digital circuits, they can control voltage and current to produce gain or switching effects, that is, they respond to applied signals and can change their basic characteristics. Passive electronic components (Inactive): They do not change their own characteristics when an electrical signal is applied, that is, they provide a simple, repeatable response.

3. Odd-form electronic components: Their geometric shape factors are peculiar, but not necessarily unique. Therefore, they must be mounted manually, and the shape of their housing (in contrast to their basic function) is non-standard.

For example: many transformers, hybrid circuit structures, fans, mechanical switch blocks, etc.

Chip resistors, capacitors, etc., size specifications: 0201, 0402, 0603, 0805, 1206, 1210, 2010, etc. Tantalum capacitors, size specifications:

TANA, TANB, TANC, TANDSOT

Transistor, SOT23, SOT143, SOT89, etc.

melf cylindrical components, diodes, resistors, etc.

SOIC integrated circuit, size specifications: SOIC08, 14, 16, 18, 20, 24, 28, 32

QFP Fine Pitch IC PLCC IC, PLCC20, 28, 32, 44, 52, 68, 84

BGA ball grid array packaging integrated circuit, array pitch specifications: 1.27, 1.00, 0.80

CSP integrated circuit, component side length does not exceed 1.2 times the side length of the chip inside, array pitch <0.50 microBGA

SMT Glossary

Bridge: Solder that connects two conductors that should be electrically connected, causing a short circuit.

Buried via: A conductive connection between two or more inner layers of a PCB (i.e., not visible from the outer layers).

C

CAD/CAM system (computer-aided design and manufacturing system): Computer-aided design is the use of specialized software tools to design printed circuit structures; computer-aided manufacturing converts this design into a physical product. These systems include large-scale memory for data processing and storage, input for design creation, and output devices that convert stored information into graphics and reports.

Capillary action: A natural phenomenon in which molten solder flows against gravity on closely spaced solid surfaces.

Chip on board (COB): A hybrid technology that uses face-up glued chip components, traditionally connected exclusively to the substrate layer of the circuit board via flying wires.

Circuit tester: A method of testing PCBs during mass production. Includes: bed of nails, component footprints, guide probes, internal traces, loaded boards, empty boards, and component testing.

Cladding: A thin layer of metal foil bonded to the board layer to form the conductive wiring of the PCB.

Coefficient of the thermal expansion: The measured amount of expansion per degree of temperature when the surface temperature of a material increases.

Cold cleaning: An organic dissolution process in which liquid contact is used to remove residue after welding.

Cold solder joint: A solder joint that reflects insufficient wetting and is characterized by a gray, porous appearance due to insufficient heating or improper cleaning.

Component density: The number of components on a PCB divided by the area of ​​the board.

Conductive epoxy: A polymer material that allows the passage of electric current by the addition of metal particles, usually silver.

Conductive ink: An adhesive used on thick film materials to form a conductive wiring diagram for a PCB.

Conformal coating: A thin protective coating applied to a PCB to conform to the assembly shape.

Copper foil: A cathodic electrolytic material, a thin, continuous metal foil deposited on the substrate of the circuit board, which serves as the conductor of the PCB. It easily adheres to the insulating layer, accepts the printed protective layer, and forms a circuit pattern after corrosion.

Copper mirror test: A flux corrosion test using a vacuum deposited film on a glass plate.

Cure: Change in the physical properties of a material, either through chemical reaction or reaction to heat with or without pressure.

Cycle rate: A component placement term used to measure the machine speed from picking up, positioning on the board and returning, also called test speed.

D

Data recorder: A device that measures and collects temperature from a thermocouple attached to a PCB at specific time intervals.

Defect: A component or circuit unit that deviates from normally accepted characteristics.

Delamination: The separation of board layers and the separation between board layers and conductive covering layers.

Desoldering: The removal of soldered components for repair or replacement. Methods include: desoldering with wicks, vacuum (solder straw) and hot pulling.

Dewetting: The process by which molten solder first covers and then retracts, leaving irregular residues.

DFM (Design for Manufacturing): An approach to producing a product in the most efficient way, taking into account time, cost, and available resources.

Dispersant: A chemical added to water to increase its ability to remove particles.

Documentation: Information about an assembly that explains basic design concepts, types and quantities of components and materials, specialized manufacturing instructions, and latest versions. Three types are used: prototypes and low-volume runs, standard production lines and/or production quantities, and those government contracts that specify actual graphics.

Downtime: The time when equipment is not producing product due to maintenance or failure.

Durometer: Measures the hardness of the rubber or plastic of the scraper blade.

AND

Environmental test: One or a series of tests used to determine the total external effects on the structural, mechanical, and functional integrity of a given component package or assembly.

Eutectic solders: An alloy of two or more metals with the lowest melting point. When heated, the eutectic alloy changes directly from solid to liquid without going through a plastic stage.

F

Fabrication(): The process of manufacturing a bare board after design and before assembly. Individual processes include stacking, metal addition/subtraction, drilling, electroplating, routing, and cleaning.

Fiducial: A special mark integrated with a circuit wiring diagram and used by machine vision to find the orientation and location of the wiring diagram.

Fillet: The connection formed by solder between the pad and the component pin, i.e. solder joint.

Fine-pitch technology (FPT): The center spacing of the pins of surface mount component packaging is 0.025" (0.635mm) or less.

Fixture: A device that connects a PCB to the center of a processing machine.

Flip chip: A pinless structure that generally contains circuit elements. It is designed to be electrically and mechanically connected to a circuit through an appropriate number of solder balls (covered by a conductive adhesive) located on its surface.

Full liquidus temperature: The temperature level at which solder reaches its maximum liquid state, which is optimal for good wetting.

Functional test: Testing of the entire assembled appliance by simulating its expected operating environment.

G

Golden boy: A component or circuit assembly that has been tested and known to function to specifications and is used to test other units by comparison.

H

Halides: Compounds containing fluorine, chlorine, bromine, iodine or astatine. They are the catalyst part of the flux and must be removed due to their corrosiveness.

Hard water: Water containing calcium carbonate and other ions that may accumulate on the inner surfaces of cleaning equipment and cause blockages.

Hardener: A chemical added to a resin to cure it prematurely, i.e. a curing agent.

I

In-circuit test: A component-by-component test to verify component placement and orientation.

J

Just-in-time (JIT): Minimize inventory by supplying materials and components to the production line directly before production begins.

L

Lead configuration: A conductor extending from a component that serves as both a mechanical and electrical connection point.

Line certification: Confirm that the production line sequence is controlled and reliable PCBs can be produced as required.

M

Machine vision: One or more cameras used to help find the center of a component or improve the accuracy of the component placement of the system.

Mean time between failure (MTBF): The average statistical time interval between expected possible failures of an operating unit, usually calculated per hour. The result should indicate actual, expected or calculated.

N

Nonwetting: A condition where solder does not adhere to a metal surface. Nonwetting is characterized by visible exposure of the base metal due to contamination of the surface to be soldered.

THE

Omegameter: A meter used to measure the amount of ionic residue on the surface of a PCB by immersing the assembly in a mixture of alcohol and water of known high resistivity, after which the resistivity drop caused by the ionic residue is measured and recorded. Open: Two electrical connection points (pins and pads) become separated, either due to insufficient solder or poor coplanarity of the connection point pins.

Organic activated (OA): A flux system with organic acid as the activator, water-soluble.

P

Packaging density: The number of components (active/passive components, connectors, etc.) placed on a PCB; expressed as low, medium or high.

Photoploter: Basic wiring diagram processing equipment used to produce original PCB wiring diagrams (usually at actual size) on photographic film.

Pick-and-place: A programmable machine with a robotic arm that picks up components from an automatic feeder, moves them to a fixed point on the PCB, and places them in the correct position in the correct orientation.

Placement equipment: A machine that places components on a PCB with a combination of high speed and accurate positioning. It is divided into three types: bulk transfer of SMDs, X/Y positioning, and in-line transfer systems, which can be combined to adapt components to the circuit board design.

R

Reflow soldering: The process of placing surface mount components in solder paste to achieve permanent connection through various stages including: preheating, stabilization/drying, reflow peak and cooling.

Repair: The action of restoring the functionality of a defective assembly.

Repeatability: The ability of a process to accurately return to characteristic targets. A metric that evaluates processing equipment and its continuity.

Rework: A repetitive process of bringing an incorrect assembly back into compliance with specifications or contract requirements.

Rheology: Describes the flow of a liquid, or its viscosity and surface tension characteristics, such as solder paste.

S

Saponifier: An aqueous solution of organic or inorganic ingredients and additives used to facilitate the removal of rosin and water-soluble fluxes such as dispersible cleaners.

Schematic: A diagram that uses symbols to represent the layout of a circuit, including electrical connections, components, and functions.

Semi-aqueous cleaning: A technique involving solvent cleaning, hot water flushing, and drying cycles.

Shadowing: In infrared reflow soldering, the component body blocks energy from certain areas, resulting in insufficient temperature to completely melt the solder paste.

Silver chromate test: A qualitative, halide ion test for the presence of RMA flux. (RMA Reliability, Maintainability and Availability)

Slump: The diffusion of materials such as solder paste and adhesive after screen printing but before curing.

Solder bump: A spherical solder material adheres to the contact area of ​​a passive or active component to connect to the circuit pad.

Solderability: The ability of a conductor (pin, pad, or trace) to wet (become solderable) in order to form a strong connection.

Soldermask: A printed circuit board processing technology in which all surfaces except the connection points to be soldered are covered with a plastic coating.

Solids: The weight percentage of rosin in the flux formula (solid content)

Solidus: The temperature at which the solder alloy of some components begins to melt (liquefy).

Statistical process control (SPC): Use statistical techniques to analyze process outputs and use the results to guide actions to adjust and/or maintain quality control status.

Storage life: The time that an adhesive is stored and remains useful.

Subtractive process: Circuit wiring is obtained by removing selected portions of the conductive metal foil or covering layer.

Surfactant: A chemical added to water to reduce surface tension and improve wetting.

Syringe: A container of glue that drips out through its narrow opening.

T

Tape-and-reel: Component packaging for SMD. Components are placed in pits on a continuous strip, and the pits are covered with plastic tape so that they can be rolled onto the reel for use by the component placement machine.

Thermocouple: A sensor made of two dissimilar metals that, when heated, produces a small DC voltage in the temperature measurement.

Type I, II, III assembly: PCB with surface mount components on one or both sides of the board (I); PCB with pin components mounted on the main surface and SMD components mounted on one or both sides

Hybrid technology (II); Hybrid technology (III) characterized by passive SMD components mounted on the second side and lead (through-hole) components mounted on the main side.

Tombstoning: A welding defect in which a sheet component is pulled into a vertical position, leaving the other end unwelded.

IN

Ultra-fine-pitch: The center-to-center distance of the pins and the conductor spacing are 0.010" (0.25mm) or less.

V

Vapor degreaser: A cleaning system in which the object is suspended in a chamber and heated solvent vapors condense on the surface.

Void: A cavity inside a solder joint, formed by gas release during reflow or by flux residue trapped before solidification.

AND

Yield: The ratio of components used at the end of the manufacturing process to the number of components submitted for production.