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Essential Elements of SMT Assembly

SMT, the short form of Surface Mount Technology, a type of PCB (Printed Circuit Board) assembly technology, refers to the technology of getting components directly soldered onto the surface of PCB to replace THT (Through-Hole Technology) that has to make use of drilling holes. This article will discuss the essential elements of SMT assembly so that readers are able to capture a sketch about SMT.


Introduction of SMT

When SMT assembly is used in electronics manufacturing, components with short or no leads (SMCs or SMDs) are placed on corresponding positions onto circuit board or base board. Then, reflow or wave soldering is applied to make components permanently fixed on board.


Attributes of SMT

a. Miniaturization


SMCs/SMDs feature light weight, small volume, high mounting accuracy so that the weight and volume of assembled PCBs going through SMT is about one tenth of those going through THT. As a result, the volume of final product with the application of SMT can be shrunk by 40% to 60% while the weight can be reduced by 60% to 80%.


b. High Performance


Component soldering in SMT assembly features low reject ratio and higher capability to resist vibration.


c. High Reliability


Electronic products using SMT assembly feature high frequency with EMI (Electromagnetic Interference) and RF interference reduced.


d. High Efficiency


SMT assembly makes automatic manufacturing so accessible as to improve manufacturing efficiency.


Comparison between SMT and THT

The reason why THT assembly is gradually replaced by SMT assembly lies in the fact that THT fails to meet the current electronic demand in terms of miniaturization. Therefore, SMT assembly has to be applied to make components “stuck to” board surface instead of penetrating through board.


The detailed procedure of SMT assembly is indicated below:

The procedure of SMT assembly can be simplified as the following four steps: solder paste printing, chip mounting, reflow soldering and inspection.



Material Used in SMT Assembly

The material used in SMT assembly includes solder paste, adhesive, flux, cleaning agent, heat transfer medium etc.


Solder paste


Solder paste in SMT assembly procedure plays a role of both solder material and adhesive to fix SMCs/SMDs onto PCB surface. The leading element of solder paste, Sn63/Pb37 and Sn62/Pb36/Ag2 features a comprehensive performance while Sn43/Pb43/Bi14 performs well in solder paste with low melting temperature. Sn-Pb IMC performs well in strength and wettability so it is regarded as the most suitable solder.


Flux


Flux in SMT assembly procedure plays a role in assisting soldering in being smoothly carried out. Flux is classified into acid flux and resin flux, playing a role in eliminating oxide and dirt from metal surface and getting metal surface wet.


Adhesive


Adhesive plays a role in fixing SMDs in SMT assembly to stop SMDs from being displaced and falling off.


Cleaning Agent


Cleaning agent is used to clean off the residues on board left by solder paste etc. Cleaning agent should chemically perform well and feature thermal stability. In addition, it should not be decomposed during storage and application, failing to have chemical reaction with other chemical substances. Plus, it shouldn’t corrode contact material with noninflammability and low toxicity. Cleaning should be implemented with safety and low loss during the procedure and it should be effectively done within set time and temperature.



SMC/SMD Mount Technology

SMC, short for Surface Mount Component, mainly includes rectangular chip component, cylindrical chip component, composite chip component and alien chip component. SMD, short for Surface Mount Device, is a type of component applied for SMT assembly.


Chip Mounter

Structure


Chip mounter is an automated equipment with high accuracy applied for SMT assembly is controlled by computer and integrating light, electricity and mechanism.


Mounting Efficiency


Mounting accuracy and speed should be carefully considered when selecting chip mounter. Thus, how to improve and guarantee mounting efficiency of chip mounter is top responsibility to effectively increase product quality and manufacturing efficiency and reduce cost.


Elements Affecting Chip Mounter


The elements affecting chip mounter moving include x-y axis structure, x-y axis moving error, x-y axis inspection, vacuum nozzle z axis moving effect on mounting error etc.


Visual System


To accurately make fine-pitch components stuck to board, pixel elements and optical magnifying times should be scientifically set on camera.


Software System


The software system of high-accuracy chip mounter depends on secondary computer control system, usually using DOS interface but sometimes using Windows interface or UNIX operation system. The system consists of central control software, automatic programming system, mounter control system and visual processing software.


Electrostatic Discharge (ESD)

Static Electricity Generated in Electronic Products


The unmovable flow of electricity is called static electricity generated by the gathering of positive charges and negative charges. As a type of electric energy, static electricity exists on the surface of objects, which is a phenomenon generated when partial imbalance occurs among positive and negative charges. Static electricity brings forward damage to electronic products with attributes of disguise, potentiality, randomness and complexity.


Particularity of ESD


Static electricity features some randomness so not all electronic products definitely suffer from its damage. The energy charged by static electricity is mostly low and therefore, the electronic product suffering from damage from static electricity won’t instantly perform badly. The damage even fails to be obviously seen when products leave warehouse.


ESD Protection Measures


a. ESD protection measures should be made in manufacturing shop;


b. Antistatic wrist band and gloves should be worn;


c. Grounding should be achieved;


d. Static inspections should be termly implemented.


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