Q&A about SMT Assembly

Q1: What is SMT assembly?

A1: SMT, short for Surface Mount Technology, refers to a type of assembly technology to stick components (SMCs, Surface Mount Components, or SMDs, Surface Mount Devices) to bare PCBs (Printed Circuit Boards) through the applications of a series of SMT assembly equipment.


Q2: What equipment is applied in SMT assembly?

A2: Generally speaking, the following equipment is applied for SMT assembly: solder paste printer, chip mounter, reflow soldering oven, AOI (Automated Optical Inspection) instrument, magnifying lens or microscope etc.


Q3: What are attributes of SMT assembly?

A3: Compared with conventional assembly technology, that is, THT (Through-Hole Technology), SMT assembly leads to higher assembly density, lower volume, lighter weight of products, higher reliability, higher shock resistance, lower defect rate, higher frequency, lower EMI (Electromagnetic Interference) and RF (Radio Frequency) interference, higher throughput, more access to automation, lower cost etc.


Q4: How is SMT assembly different from THT assembly?

A4: SMT assembly is different from THT assembly in the following aspects:


a. Components applied for THT assembly feature longer leads than those for SMT assembly;


b. THT assembly calls for drilling holes that should be drilled on bare circuit board while SMT assembly doesn’t since SMCs or SMDs are directly mounted on PCB;


c. Wave soldering is mostly applied in THT assembly while reflow soldering is mostly applied in SMT assembly;


d. Automation can be expected in SMT assembly while THT assembly just depends on manual operation;


e. Components for THT assembly are heavy in weight, tall in height and large in volume while SMCs can help to reduce more space.


Q5: Why is SMT assembly widely applied in electronics manufacturing?

A5: First, current electronic products have been striving for miniaturization and light weight, which is difficult for THT assembly to reach;


Second, to lead electronic products to be functionally integrated, IC (Integrated Circuit) components are leveraged to such a high extent to be compatible with large scale and high integrity requirement which is just what SMT assembly enables to do.


Third, SMT assembly adapts itself to volume production, automation and cost reduction all of which meet the demands of electronics market;


Fourth, SMT assembly is applied to better promote the development of electronics technology, ICs and multiple applications of semiconductor materials;


Fifth, SMT assembly conforms to international electronics manufacturing standard.


Q6: In which field of products is SMT assembly used?

A6: Currently, SMT assembly has been applied in advanced electronics products, especially those belonging to computer category and telecommunication. Furthermore, SMT assembly has been used in the products from all fields, including medical care, automotive, telecommunication, industrial control, military, aerospace etc.


Q7: What is the ordinary manufacturing procedure of SMT assembly?

A7: SMT assembly procedure usually consists of solder paste printing, chip mounting, reflow soldering, AOI, X-ray inspection and rework. Visual inspection is carried out after each step of the procedure.


Q8: What is solder paste printing and its role in SMT assembly?

A8: Solder paste printing refers to the process in which solder paste is printed onto the pad on PCB so that SMCs or SMDs can be stuck to board through the left solder paste on pad. Solder paste printing is implemented through the application of stencil on which there are a number of openings through which solder paste will be kept on pad.


Q9: What is chip mounting and its role in SMT assembly?

A9: Chip mounting contributes to the core meaning of SMT assembly and it refers to the process in which SMCs or SMDs are quickly placed onto the solder paste left on PCB pad. As a result, components are temporarily stuck to board surface based on the adhesion of solder paste.


Q10: Why type of soldering is used in SMT assembly procedure?

A10: Reflow soldering is used in SMT assembly to permanently fix components on PCB and is carried out inside reflow soldering oven containing temperature zones. In the process of reflow soldering, solder paste is first melted in the first and second phase under the high temperature. As temperature goes down, solder paste will become hardened so that components will be fixed on corresponding pads on PCB.


Q11: Do PCBs have to be cleaned after SMT assembly?

A11: PCBs after SMT assembly have to be cleaned before leaving workshop since the surface of assembled PCB may be covered by dust, residues after reflow soldering, flux for example, all of which will to some extent decrease the reliability of products. Therefore, assembled PCBs have to be cleaned off before leaving workshop.


Q12: What type of inspection is used for SMT assembly?

A12: To guarantee the quality and performance of assembled PCB, inspections are of great necessity during the whole process of SMT assembly. Numerous types of inspections have to be leveraged to expose manufacturing defects that will reduce the reliability of end products. Visual inspections are the most commonly-used in SMT assembly. As a direct inspection method, visual inspection can be used to indicate some obvious physical errors, such as component displacement, missing components, or component irregularity. Visual inspection doesn’t apply to inspection with naked eyes and some tools can be used as well, such as magnifying lens or microscope. To further indicate the defects occurring to solder balls, AOI and X-ray inspection can be leveraged as soldering is completed.


Q13: What requirement has to be met in terms of SMT assembly workshop?

A13: The fundamental requirement an SMT workshop has to meet is demonstrated as below:


Room temperature: 25±3℃ (if it can’t be obtained, temperature control equipment is needed);


Room internal height: 3 meters;


Room RH (Relative Humidity): 45% to 75% (if it can’t be obtained, humidity control equipment is needed);


Electrostatic requirement: 150KR±10% (static grounding is needed).


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