Introduction:
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Surface-mount technology (SMT) is a method for producing electronic circuits in which the components are mounted or placed directly onto the surface of printed circuit boards (PCBs). An electronic device so made is called a surface-mount device (SMD). In industry, it has largely replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board. Both technologies can be used on the same board, with the through-hole technology used for components not suitable for surface mounting such as large transformers and heat-sinked power semiconductors. By employing SMT, the production process speeds up, but the risk of defects also increase due to component miniaturization and to the denser packing of boards. In those conditions, detection of failures has become critical for any SMT manufacturing process. An SMT component is usually smaller than its through-hole counterpart because it has either smaller leads or no leads at all. It may have short pins or leads of various styles, flat contacts, a matrix of solder balls (BGAs), or terminations on the body of the component.
Main Contents of SMT:
SMT is a very complex system engineering, whose basic composition includes surface assembly components, assembly substrate, assembly materials, assembly process, assembly design, detection technology, assembly and detection equipment, control and management and many other contents and technologies. See the figure below.
1. Assembly Components:
1) Design: Structural dimension, terminal form, welding resistance.
2) Manufacturing: The manufacturing technologies of various of components.
3) Packaging: Braided type, pipe type, tray, bulk, etc.
2. Substrates of PCB: Single side PCB, double layers PCB, aluminum substrate, copper substrate, ceramic substrate, etc.
3. Assembly Design: Electrical design, thermal insulation design, component layout design, substrate graphic layout design, etc.
4. Assembly Process:
1) Assembly Materials: Adhesive, solder, flux, cleaning agent, etc.
2) Assembly Technologies: Coating technology, paste technology, welding technology, cleaning technology, testing technology and so on.
3) Assembly Equipment: Coating equipment, bonding machine, welding machine, cleaning machine, testing equipment and so on.
4) Assembly system control and management: Assembly line or system composition, control and management, etc.
5. Detection Technologies:
1) Vision Inspection.
2) Automated Optical Inspection(AOI).
3) X-Ray Testing.
4) Ultrasonic detection.
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5) ICT & FCT.
Basic Contents of SMT Process:
The basic content of SMT process technology can be divided into four parts: assembly material selection, assembly process design, assembly technology and application of assembly equipment, as shown in the following figure:
SMT Process:
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The following aspects are able to fully explain the significance of PCB cleaning after surface mount soldering:
&#; PCB cleaning after surface mount soldering can stop electrical defects from occurring.
Among all the electrical defects, electrical leakage is the most protruding one, which is an essential element reducing long-term reliability of PCB boards. This type of defects is caused mainly by ionic contaminants, organic residues and other adhesion substances left on the surface of circuit boards.
&#; PCB cleaning after surface mount soldering can eliminate erosive substances. Erosion will damage circuits, leading components or devices to brittleness. Erosive substances can conduct electricity in humid environment, which will further arouse PCB boards to shorts or even failures. Erosive substances elimination is actually excluding negative elements hindering long-term reliability of PCB boards.
&#; PCB cleaning after surface mount soldering can make board appearance look clear.
PCB boards cleaned after surface mount soldering look clear in appearance, making some defects exposed, convenient for inspection and troubleshooting such as heat damage and lamination.
&#; Manual Cleaning Method
Acetone solution is used to soak PCB boards for about 10 minutes. Then, a banister brush is used to brush off contaminants at solder connection in ethanol solution. Next, board is taken out before deionized water is used to wash for 3 minutes. After that, absolute ethyl alcohol is leveraged for dehydration. Finally, nitrogen gas gun is used to dry the board surface until no water mark won't be seen.
During the process of manual cleaning, acetone with higher solubility is leveraged to soak circuit boards, which can effectively make contaminants dissolved into the solution. Then circuit board goes over physical brushing by placing circuit boards in ethyl alcohol with detailed flux contaminants brushed off. Next, organic solvent has to go through dehydration with the help of deionized water. Finally, nitrogen gas is used to dry the board surface to complete manual cleaning after surface mount soldering. This process can be summarized into the following figure.
&#; Ultrasonic Cleaning Method
Acetone solvent is used in ultrasonic cleaning method. First, PCB board is soaked into acetone solvent for ten minutes. Then, it is placed into quartz container specialized for absolute ethyl alcohol in which circuit board is soaked. Next, ultrasonic cleaning is implemented after placing quartz container into ultrasonic cleaning tank. Ultrasonic cleaning lasts for five minutes with ultrasonic power of 240W, after which quartz container is taken out with a basket after turning off ultrasonic cleaning switch. Then, deionized water is used to wash for 5 minutes and absolute ethyl alcohol is used to carry out dehydration on circuit board. Finally, nitrogen gas is used to make the surface dry.
During the process of ultrasonic cleaning, circuit board is put into ethyl alcohol agent, which is different from manual cleaning. Depending on the principle of ultrasonic vibration, ultrasonic cleaning aims to wash off flux residue and dehydration is carried out by deionized water. Finally comes drying stage with the help of nitrogen gas. The whole procedure of ultrasonic cleaning is demonstrated in the following figure.
&#; Gas Phase Cleaning Method
Equipment condensing system is turned on first to let it run for 5 to 10 minutes. Then, equipment heating system is turned on to heat cleaning agent to boiling temperature. circuit board is placed into cleaning basket that is then placed into boiling tank. Boiling time lasts for 3 to 5 minutes. Next, basket is then placed into steam zone for steam cleaning for 3 to 5 minutes. Then, circuit board surface is sprayed with cleaning agent for 10 to 20 seconds. After that, basket is slightly placed into rinsing tank for 1 to 2 minutes. Finally, devices are dried through condensation and basket is taken out after the complete volatilization of agent.
When PCB board is placed into the steam of hot agent, hot steam will be condensed on the surface of circuit board that is relatively cold in surface, leading grease dirt on PCB component surface to be dissolved. Dissolved grease dirt is dipped into boiling agent while steam is accumulated on condensation coil that is around boiling agent. Steam will return to separation cell in a liquid state that can be recycled with water separated and impurity filtered. Agent gas phase cleaning principle is indicated by the following figure.
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