SMT technology: disassembly and assembly of sheet components with basic and automated installation as the core

Surface mount technology (SMT), which is based on light, small, and thin sheet components and focuses on automated installation , is creating a technological revolution in the entire electronics industry with a brand new attitude. The application of sheet components in electronic products has been further popularized, and its maintenance and welding technology has long been the focus of many amateur enthusiasts.

The welding of chip components is the key technology of SMT , which is the guarantee of product quality and reliability. Surface assembly technology is to align the welding terminals of chip components with the pads on the printed circuit board, use the viscosity of adhesive or solder paste to stick the chip components to the printed circuit board, and then realize welding through wave soldering or reflow soldering. The typical process flow of SMT is as follows: printed circuit board design ---- coating adhesive or printing solder paste ---- mounting chip components ---- wave soldering or reflow soldering ---- cleaning ---- testing. The corresponding SMT equipment includes glue dispenser or printer, chip mounter, wave soldering machine or infrared reflow soldering machine, rework bench, cleaning machine and testing equipment.

The above is the production method of professional factories. For amateurs and maintenance personnel, they usually only use electric soldering irons for manual operation, so the operator should have the basics of proficient use of electric soldering irons. Manual soldering has a positive remedial significance for product development, trial production and product maintenance in the absence of professional equipment.

1. Mounting method and pads of chip components

There are generally four ways to mount a surface: single-sided pure sheet component mounting, double-sided pure sheet component mounting, single-sided sheet component and plug-in component mixed mounting, double-sided sheet component and plug-in component mixed mounting. The pad shape of sheet components has an important influence on pad strength and reliability. The basic requirements are: (1) The center distance between adjacent pads of the same component should be equal to the center distance between the corresponding pins; (2) The pad width is equal to the pin or end solder head width plus or minus a constant, and the value can be adjusted in practice; 3) The pad length depends on the end solder head or pin height and depth. Generally speaking, the pad length is more important than the width.

2. Removal of chip components

The desoldering and removal of chip components is different from that of mounted components. The molten solder on the through-hole board of the plug-in component can be sucked away one by one by a tin sucker, or by taking advantage of the flexibility of the metal pins and removing each pin one by one to remove the component. However, all the pins of the chip components must be heated at the same time and can only be removed after the solder is completely melted, otherwise the pad will be damaged.

1. Removal with a soldering iron The soldering iron is relatively effective in removing rectangular chip components, diodes, transistors and small integrated circuits, but it is difficult or even impossible to remove large-sized, multi-lead chip components. To improve the removal quality, a special soldering iron tip can be equipped for an ordinary soldering iron. There are many ways to remove chip components with a soldering iron, but the purpose is the same, which is to melt all the solder joints of the removed component at the same time to successfully remove the component .

When disassembling, first use a small brush to apply flux on the solder joints to remove the oxide layer, and tin the specially shaped soldering iron tip so that it can be in close contact with the solder joints, which is conducive to heat conduction and accelerates the melting process. Then use the soldering iron tip to directly heat all the solder joints at the same time. Once all the solder joints are melted, immediately remove the components with tweezers or push them away from the pad area. Note that the heating time should not exceed 5 seconds. Finally, use a general soldering iron tip and a tin-absorbing belt or tin-absorbing device woven with fine copper wire to remove the remaining solder on the pad in preparation for the next soldering.

2. Use a handheld pocket hot air gun to remove. This method uses hot air to melt solder joints. Usually the temperature of a hot air gun is as high as 400 degrees. In order to accurately control and guide the hot air flow to the required pads and component pins, a special nozzle corresponding to the component needs to be added to the hot air outlet to avoid affecting other adjacent components. In addition to melting solder joints to remove components, hot air guns can also be used for hot air leveling of pads, reflow of solder paste, and completion of the placement of sheet components . This method is generally used by enterprises or professional repair stations.

The hot air gun is light and easy to use. It can disassemble and assemble large-sized, multi-lead, and arbitrarily switchable components. It can heat locally without contacting the workpiece. Compared with the electric soldering iron, it has a higher success rate, but it requires a complete set of nozzles that match different components, so the cost is high. Using an electric soldering iron is both economical and simple, but it is limited by the number of component pins and switches, and has high requirements for operation. It takes many practices and experiments to master it, otherwise the pads are easily damaged during disassembly. 3. Amateur soldering of sheet components

The welding of chip components is also different from that of plug-in components. The latter are inserted into through holes through leads, and will not shift during welding . The components and pads are on both sides of the printed circuit board, making welding easier. Chip components are easy to shift during welding. The pads and components are on the same side of the printed circuit board. The shapes of the welding terminals are different, the pads are small, and the welding requirements are high. Therefore, you must be careful and cautious when welding to improve accuracy.

1. The power of the electric soldering iron for manual soldering of general sheet components is 25W, and the maximum should not exceed 40W. The power and temperature should be adjustable. The soldering iron tip should be sharp, and a long-life soldering iron with an anti-oxidation layer is preferred. The soldering time should be controlled within 3 seconds, and the diameter of the solder wire should be 0.6-0.8mm. When soldering, first use tweezers to place the component on the corresponding position of the printed circuit board, and then use the electric soldering iron to solder. To prevent the component from shifting during soldering, a homemade fixture can be used to fix the component. The auxiliary fixture can use a slightly thicker iron sheet (thickness of 1.5-2.0mm) as the base, and an iron rod with a diameter of about 10mm as the bracket. The bottom of the iron rod is threaded, and two nuts fix it on the base. A through hole with a diameter of 1.2mm is drilled on the upper part of the iron rod so that it can be equipped with a steel wire with a diameter of (1.0-1.2)mm. The elasticity of the steel wire should be larger. A hole with an M3 thread tapped on the side of the small hole is opened, and the steel wire is fixed with a screw, and the steel wire can be formed. When using it, gently lift the wire with your fingers, then place the components to be welded and the printed arc under it, lower the wire to clamp the components so that the components do not shift, ensuring accurate welding. This method is particularly suitable for welding rectangular chip components and small triodes.

2. The welding of wing-shaped pin SOP can be done by soldering iron pull welding. Use a flat soldering iron tip with a width of 2.0--2.5mm, and the tin wire can be slightly thicker, 1.0mm. Check the pad before welding. If there is any contamination, wipe it with anhydrous ethanol, then check the device pins. If there is any deformation, use tweezers to carefully adjust it. To improve the solderability, you can also apply flux first, then place the device in the welding position, and first solder one or two pins to fix the device. When all pins are in the same position as the pad, pull welding can be performed. That is, dip the clean soldering iron tip in solder, hold the electric soldering iron in one hand to solder the pins from left to right, and hold the solder wire in the other hand to continuously add tin. When pulling welding, the soldering iron tip cannot touch the root of the device pin, otherwise it is easy to cause a short circuit. The pressure of the soldering iron tip on the device pin should not be too great. It should be in a state of "floating" on the pin. Use the solder tension to guide the molten solder bead to move slowly from left to right, only in one direction, and never back and forth. At the same time, visually observe the formation of each pin solder point and the uniformity of the tin amount. If a welding short circuit occurs, use a soldering iron to transfer the remaining tin on the short circuit point, or use a stainless steel needle to cut it from the middle of the molten solder point.

3. Tinning soldering can be replaced by a simple tin furnace instead of a wave soldering machine. The soldering temperature is 240-260 degrees. The tinning time should be less than 5 seconds. Before tinning, use environmental resin glue to stick the components to the corresponding positions on the printed circuit board. After the glue point size and position are cured, apply flux, pick it up with stainless steel tweezers, and send it to the tin furnace for tinning.

After welding, clean it in time and check the quality of the solder joints with the help of a magnifying glass. a and b are ideal solder joints, c and e are bridging phenomena. Regardless of whether they are connected in electrical performance, bridging phenomena should not occur. They can be repaired with a soldering iron. Otherwise, due to different stresses, cracks in components are likely to occur, resulting in reduced reliability. d is excessive solder, but the impact is relatively small.

IV. Notes

1. Before soldering, first pay attention to whether the components have special requirements, such as soldering temperature conditions, assembly methods, etc. Some components cannot be soldered by immersion tinning and can only be soldered with an electric soldering iron, such as chip potentiometers and aluminum electrolytic capacitors.

2. The electric soldering iron and soldering furnace used should have a good grounding device to prevent static electricity from damaging components.

3. Try to reduce the number of times components are disassembled and installed during maintenance. Repeated disassembly and installation will lead to the complete scrapping of the printed circuit board. For mixed printed circuit boards, if there are plug-in components that hinder the disassembly and installation of sheet components , they can be removed first.

4. For tinning soldering, it is best to immerse only once. Multiple tinning will cause the printed circuit board to bend and the components to crack.

5. The printed circuit board should be selected with small thermal deformation and large copper foil covering force. Since the copper foil traces of surface assembly are narrow and the pads are small, if the anti-peeling ability is insufficient, the pads are easy to peel off, so epoxy fiberglass substrate is generally selected.

6. For rectangular chip capacitors, it is easier to weld with larger ones, such as 1206 type, but cracks and other thermal damage are prone to occur due to uneven welding temperature; smaller ones, such as 0805 type, are more difficult to weld, but are less likely to crack and thermal damage, and have higher reliability.

The amateur welding of chip components is not very complicated, but it directly reflects the quality of the product and affects the reliability of the product. Therefore, you should be careful and meticulous when welding, observe carefully, and constantly explore and summarize experience in practice to adapt to the development of chip components .