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Surface Mount Technology Basics

View: 11008/29/2019  

Surface mount technology, which is SMT (short for Surface Mounted Technology), is a popular technology and process in the electronics assembly industry.




definition


Surface Mount Technology


Synonym: Surface Mount Technology; Surface Mount Technology


It is not necessary to insert the holes into the printed board (1), and directly attach and solder the surface-assembled components to the specified position on the surface of the printed board.


Note:


1) The circuit substrate used in the usual surface mount technology is not limited to a printed board.


2) "Welding" or "welding" as used in the text of this standard generally refers to the use of soldering methods to achieve the mechanical and electrical connection between the soldering of components or the pads of printed boards and boards; this standard The terms "solder" and "flux" as used in the text refer to "solder" and "solder", respectively.


Characteristics


The assembly density is high, the size of the electronic product is small, and the weight is light. The volume and weight of the chip components are only about 1/10 of that of the conventional plug-in components. After the SMT is generally used, the volume of the electronic product is reduced by 40% to 60%, and the weight is reduced by 60%. 80%.


High reliability and strong anti-vibration ability. The solder joint defect rate is low.


High frequency characteristics are good. Reduced electromagnetic and radio frequency interference.


Easy to automate and increase productivity. Reduce costs by 30% to 50%. Save materials, energy, equipment, manpower, time, etc.


positioning accuracy


(Placement Accuracy) - Deviation between actual patch position and set patch position X, Y, repeatability Replacability - describes the ability of the placement machine to repeatedly return the position of the patch, the patch precision is usually replaced by the middle point Resolution (Resolution) - refers to the minimum equivalent of the mechanical displacement of the placement machine, which depends on the resolution of the linear encoder on the servo motor and the shaft drive mechanism. The patch accuracy/patch accuracy in actual production. In addition to repeatability, chip accuracy should include PCB/pad positioning error, pad size error, PCB trace error (CAD), and chip component manufacturing error placement machine process capability index Cp/CpkCp=T/B =(Tu-Tl)/6q=(Tu-Tl)/6S T is the tolerance range; the points of the upper and lower limits are the center of the tolerance Tm, the distribution point u, the tolerance point and the distribution point coincide with u=Tm, the process is not Offset; offset occurs when not coincident, and the calculation of the process capability index should be corrected. The corrected process capability index is recorded as Cpk, Cpk=(1-k)Cp is a one-way deviation for the placement machine, Cpk=Zmin/3q1.33Cpk capacity factor is lacking.


Process flow


Printing (or dispensing) --> Mounting --> (Curing) --> Reflow soldering --> Cleaning --> Inspection --> Rework printing: its role is to print solder paste or patch glue to the PCB On the pad, prepare for the soldering of the components. The equipment used is a printing press (solder paste press) at the forefront of the SMT line.


Dispensing: Since most of the boards used today are double-sided patches, in order to prevent the components on the input surface from being peeled off due to the solder paste melting again, the dispenser is equipped with a dispenser, which drops the glue to the surface. In the fixed position of the PCB, its main function is to fix the components to the PCB. The equipment used is a dispenser located at the forefront of the SMT line or behind the inspection equipment. Sometimes the customer needs to dispense the surface, but now many small factories do not use the dispenser, if the input surface components are large, manual dispensing.


Mounting: Its function is to accurately mount surface-mounted components to a fixed position on the PCB. The equipment used is a placement machine located behind the press in the SMT line.


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


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


Cleaning: The function is to remove the welding residue (such as flux) which is harmful to the human body on the assembled PCB. The equipment used is a washing machine, the position can be fixed, it can be online or not.


Inspection: Its function is to test the quality and assembly quality of the assembled PCB. The equipment used includes a magnifying glass, a microscope, an on-line tester (ICT), a flying probe tester, an automatic optical inspection (AOI), an X-RAY inspection system, and a function tester. The position can be configured in a suitable place on the production line according to the needs of the inspection.


Rework: Its role is to rework PCB boards that have failed. The tools used are soldering irons, rework stations, and the like. Configure anywhere in the production line.


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