In the traditional assembly process of electronic devices, the wave soldering technology is generally used for the welding of PCB components with PTH. There are many shortcomings in wave soldering: 1. High-density, fine-spaced chip-mounted components cannot be spread all over the welding surface; 2. More bridging and false welding; 3, need to spray welding flux; Pcb is subject to great thermal shock expansion and contraction deformation.
At present, the relative density of power circuit assembly is getting higher and higher, and it is difficult to avoid that there may be high-density and fine-spaced chip-mounted components all over the welding surface. The traditional wave soldering process has long been at a loss for this matter. Generally, only the chip-mounted components on the welding surface are reflowed independently first, and then the software spot welding is made by hand. However, there is a problem of poor consistency of spot welding quality. Four new mixed welding technologies are emerging. 01 Selective welding. In selective welding, only a part of special areas touch solder wire wave. Because PCB itself is a poor heat-conducting substance, it is not easy to heat and melt the spot welding of adjacent electronic devices and PCB areas during welding. Before soldering, the soldering flux must be applied in advance. The soldering flux is only applied to the position to be soldered under the PCB, but it can be selectively soldered and is not suitable for soldering SMD components.
02. Immersion welding technology is applied to select welding technology, which can weld spot welding of 0.7 mm ~ 11 mm. The welding technology of short leg position and small-sized welding layer is more stable, and the bridging probability is also small. The distance between adjacent spot welding edges, components and welding nozzles should exceed mm The following basic parameters can be applied to the selection of immersion welding process: ① solder wire temperature is 275℃ ~ 300℃ ② The infiltration rate is 22 mm/s ~ 25 mm/s. ③ Infiltration time is 1s ~ 3s. ④ The soaking rate is 3mm/s. ⑤ The speed of shock pump is determined according to the total number of welding nozzles. .
03. Through-hole reflow (THR) of Through-holeReflow,THR furnace is the application of reflow soldering technology to assemble through-hole components and special-shaped components. Because commodities pay more and more attention to practicality, promotion and relative density of components, many single-sided and double-sided circuit boards are dominated by surface-mounted components. However, due to the original compressive strength, credibility and universality, in some cases, through-hole components are still superior to SMC, and they are very RF connectors on the edge of PCB. The disadvantage of applying through-hole components to circuit boards dominated by surface mounted components is that it costs a lot to spot weld alone. For this kind of assembly line, the key point is to show the same reflow oven for through-holes and surface mounted components in the whole process of a single comprehensive process.
Compared with the general surface mount process, the amount of solder paste used in the through-hole reflow process is about 30 times more than that of the general SMT. At present, two kinds of solder paste coating techniques are selected for the through-hole reflow process, including solder paste packaging and printing and automatic solder paste dispensing. 1. Solder paste packaging and printing. Steel screen packaging printing is the preferred way to deposit solder paste on PCB. The thickness of steel mesh is an important factor, which will endanger the amount of solder paste on PCB. Stepped steel mesh can be selected, and the thicker area is specially designed for through-hole components. The design scheme of this kind of screen plate can consider different requirements of solder paste amount.
2. Automatic solder paste dispensing. Automatic dispensing of solder paste has successfully accumulated proper volume of solder paste for through holes and special-shaped parts, which shows the coordination and working ability of many solder paste accumulations that steel mesh packaging and printing will not be able to accomplish. When dispensing solder paste for the exposed PlatedThroughHole (PTH), it is suggested to use a nozzle with a slightly larger diameter than PTH. In that way, when the solder paste is ordered, the solder paste is forced to abut against the hole edge of PTH, and the raw materials are slightly squeezed from the bottom of PTH, and then the components are inserted from the reverse direction of the solder paste ordering. If the nozzle with smaller diameter than PTH is used, the solder paste will be discharged from the hole and cause serious damage to the solder paste.
Through-hole reflow soldering furnace can replace wave soldering machine in many aspects to complete the welding of plug-in components, especially in solving the welding of software spot welding with high-density patch components (or SMD with fine intervals) on the welding surface, which greatly improves the welding quality, which can make up for the shortage of expensive machinery and equipment. The appearance of through-hole reflow oven is of great help to enrich the welding methods, improve the relative density of pcb assembly, improve the welding quality and reduce the process steps.
04. Application of shielded die wave soldering technology. Because the traditional wave soldering technology can’t solve the welding of the chip-mounted components with fine spacing and high density on the welding surface, a new method comes into being: shielding the chip-mounted components with shielded die is used to complete the wave soldering of the inserted wires on the welding surface. 1. the technical advantages of wave soldering of the mold are shielded by application. 1) the production of wave soldering machine for two-sided mixed PCB can greatly improve the productivity of two-sided mixed PCB and prevent the problem of poor quality consistency in manual welding. 2) Reduce the advance preparation time for sticking solder resist, improve productivity and reduce product cost. 3) The production capacity is equal to the traditional wave soldering machine.
2. Shielding off the raw materials of the mold 1) The mold must be antistatic. The common raw materials are: aluminum alloy profile, synthetic stone and plywood. In order to prevent the controller of wave soldering machine from non-magnetic induction when using synthetic stone, it is suggested that it is unnecessary to use gray-black synthetic stone. 2) Make the die plate thin and thick. According to the thickness of the components on the back of the machine plate, select a plate with a thickness of mm ~ 8mm to make a mold.
3. Mould process specifications 1) Size of the mold: the length and width of the mold are equal to the length and width of the PCB plus the total width of the moving carrier edge of 60mm, and the total width of the mold must be ≦350mm. When the total width of PCB is less than 14b250m, it can be considered to place two additional PCBs in a mold for welding. 2) The edge of the process is 8mm away from the edge. In addition, the electric board with a width of 11mm and a height of 11mm is modified near the edge on both sides to improve the compressive strength of the die and reduce the deformation of the die. 3) Every lifting gear must be fixed with screws, and the distance between screws is below 150mm. 4) After the mold is made, it is necessary to install the press button (fixing the PCB on the mold) around and within 100 mm, and pay attention to the following contents: (1) Do not touch the parts after rotating once; ② No harm to DIP software; ③ the PCB can be fastened to the mold. 5) The four corners of the mold should be rounded with R5. 6) When the PCBA on the mold passes through the tin furnace, some parts will float high due to the impact of tin waves, so some parts that are very easy to float high are treated by pressing parts. The key methods selected at this stage: (1) metal material wood pressure pieces; (2) installing a pressing fastener on the mold; (3) Make the fixture of anti-floating pressure parts. Summary The selection of which welding technology depends on the characteristics of the products: if the products are in large quantities and small in variety, the technology of optional drag welding can be taken into account, and it is not necessary to make professional molds, but the investment in machinery and equipment projects is very large. If the product variety is single, and the batch is large, and it wants to adapt to the traditional wave soldering process, it can be considered to select and apply the wave soldering process technology of shielding the mold, but the project investment must be made to make a professional mold. These two welding techniques are easy to operate, so Shengde, the assembly and manufacturing of electronic devices, is widely used at this stage. Through-hole reflow welding is a very promising welding method, because the difficulty coefficient of process manipulation is very high, and the first two are less compared, but it is of great help to improve welding quality, enrich welding methods and reduce process steps.
