Correct definition of the hottest selective weldin

2022-08-24
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Correct definition of selective welding process

1 Introduction

selective welding process can not only replace wave soldering, but also provide a new welding method for the innovation of electronic product design. Selective welding technology can weld various forms of special-shaped devices such as displays and flat cables, and can also weld several printed boards (PCBs) to another printed board

excellent selective welding technology breaks through the technical and market barriers of materials and devices, and high-quality solder joints are widely accepted in the automotive electronics manufacturing industry. The selective welding equipment has a mechanical arm system that can carry the components to be welded to move to different angles to align the welding nozzle, providing a new space for welding technology. As electronic products are developing towards miniaturization, reflow soldering technology has become the mainstream of mass production. However, some electromechanical devices, such as switches, connectors and some sockets, must withstand mechanical stress by using conventional peaks, which will damage the firm connection of device through-hole interconnection. However, in some application fields, the reflow soldering process of through-hole solder paste is not applicable due to its high reflow soldering temperature and high cost. Through hole devices can be welded by different original methods, and different welding processes and costs can be compared to make decisions. For example, through-hole devices can be welded manually, but they are often not used because of quality standards and costs. Today's electronic components are constantly changing. Process engineers should understand the new technology of welding process, quality standards, lead-free solder, miniaturization and other related information. Circuit components can be welded by three selective welding methods; First, template wave soldering, which designs a special template for each circuit component to protect the soldered surface mount devices; Second, selective welding equipment - drag welding process, the mechanical arm carries the PCB to be welded and immerses it into the solder wave of the fixed position nozzle group (multiple solder waves). These three selective welding processes can be used for small batch or mass production

2. Flux

coating process in the above three welding processes, flux coating process plays an important role. At the end of welding heating and process, the flux can prevent the oxidation of PCB, and the flux should also have sufficient activity. Prevent bridging when PCB is separated from solder. In order to meet these requirements, some important factors related to the flux coating process must be considered; Correct coating amount of flux at the position to be welded of upcb u flux coating should be uniform u component preheating I preparation for the next welding process u welding temperature and contact time u component cleaning after welding

3 Template wave soldering

wave soldering process has many special flux series with different specifications. Example; For lead/tin welding process, the commonly used non cleaning flux, it is recommended that its solid content be close to 2%, and the flux should have good spreadability and surface tension. In the wave soldering process, the flux spreading is automatically completed by using the pneumatic nozzle. All spray fluxes are deposited on PCB. The pattern of nozzle spray is round or oval to ensure that the flux can be applied before and after the assembly. Due to the use of pneumatic automatic spray, some flux will splash back. In our lead/tin experiment, low solid content, cleaning free flux is selected. Before welding, the solder coated on PCB needs to be preheated at 70 ℃ -100 ℃. Flux density 0.8g/ml, solid content 1.5% Tin/silver/copper based lead-free solder uses water-soluble, organic free solder. Before welding, the solder coated on PCB needs to be preheated at 100 ℃ - 130 ℃. Flux density: 0995g/ml, solid content: 1.8%. The minimum amount of flux for PCB welding surface of lead tin solder is 1600 μ g/in2。 No cleaning, the solid content of alcohol based flux is 1.5%. The amount of flux coated on the test board is:

◆ test sample area 100*160mm2=24.8mm2

◆ amount of flux per PCB (wet) = amount of solid flux × Solid flux required per board =100/0.5*24.8*0.0016=2.645g

◆ flux required = density × (flux amount +30% loss) × 2.645=2.751mi/single board

◆ the coating amount of lead-free solder is less than 40%, and the decomposition speed in water is much faster than that in alcohol

◆ the amount of flux (wet) per PCB is 1.8*24.8*0.001=1.377g

◆ the actual amount of flux is 0.997*1.3*1.377=1/784ml/single board 4 Selective 6. Turn on the digital universal experimental machine first. Welding process selective welding process is to use a micro drop flux sprayer, because the flux coated on the PCB to be welded is very small, and the position of the coated flux is the position where the PCB contacts with the solder melt. There is no need to apply flux in other places. There is no special flux for selective flux. The flux of wave soldering can be used, and the flux dosage is the same. In the selective welding process, the amount of flux required for each welding part is:

◆ area of the part to be welded (lead/tin) =3* (8*5) + 16*6+32*6+50*6=708mm2

◆ the amount of flux required for each plate is 100/1.5 * 1.097 * 0.00016 = 0.117g

◆ actual amount of flux =0.8 * 1.3 * 0.117= 0.121ml/unit plate

◆ actual amount of flux (lead-free solder) =0.079mm l/board

5 Welding process

after the flux is coated, the next process is to dry the flux, evaporate the solvent, and obtain sufficient heat at the position to be welded to ensure reliable weld connection. In the conventional wave soldering equipment, three preheating zones are designed with a total length of 1.800m, and the configuration of the three preheating zones is as follows; Calrod heater in the first zone, strong convection heater in the second zone and infrared lamp heating zone in the third zone. The soldering pot is equipped with a double wave generator, and the soldering temperature is set to 250 ℃. The lead-free solder is a high melting point alloy. Therefore, in the welding temperature range, the wettability of lead-free solder is poor compared with lead-tin solder. In order to achieve reliable welding, the contact time or welding temperature needs to be increased. In this experiment, snag3.8cu0.5 solder was used, and the welding temperature was 260 ℃. The higher welding temperature is enough to ensure the high quality of welding, and the method of reducing the transmission speed is adopted to increase the contact time. It is bound to reduce production

6. Welding output

another indicator of selective welding process is the output that the process must meet the needs of users. Although the welding process is not the bottleneck of most assembly lines, the welding time is still important in the whole process. The experimental test board is arranged with 84 through-hole welds to be welded. The average time of manual welding is 2.7 seconds. Due to the high melting point and poor wettability of lead-free solder, the manual welding time is increased to 3.5 seconds

6.1 selective welding

selective welding equipment, flux spraying and preheating processes are completed in the transmission area. The welding time is defined as the longest time selected from the three processes of flux spraying, preheating and welding. Lead/tin welding process, the longest welding process. It takes 59.3 seconds for the single nozzle solder wave towing process. The multi nozzle solder wave soldering process takes 27.4 seconds. Using lead-free solder, it still takes 59.3 seconds for the single nozzle solder wave drag welding process. At this time, the components are still welded at the same speed (25mm/sec). The preheating time of multi nozzle solder wave immersion welding process is extended (VOC, water-soluble flux requires higher temperature), which increases the welding time to 36 seconds (preheating; 30 seconds, transmission time 6 seconds)

6.2 wave soldering

in the wave soldering process, the transmission speed of welding components is 20cm/sec, the interval between each plate is 160mm, and the final welding time is 16.5 seconds

7. Cost comparison

the cost of selective welding can be divided into several types; Consumption expenses; Flux, nitrogen, solder paste; Labor cost; Investment expenses (equipment, tool investment interest, depreciation; site. These cost calculations should be included in the process. The welding processing output of the product is related to the required processing time and labor cost. The special calculation model can be used to evaluate the ratio of the total number of welding points in each welding process to the annual cost.

7.1 labor cost

labor cost includes; man hours; programming time, maintenance time. Template wave soldering and transmission system Auxiliary operation time of the system (PCB loading/unloading, cleaning)

7.2 investment cost

floor mounted selective welding equipment is also divided into domestic configuration and imported configuration. The cost of the equipment includes; Equipment, multi nozzle template, spare parts. The wave soldering equipment shall also include the transmission system. The requirements for the transmission system shall be minimized for each assembly process. The service life of the transmission system is estimated at 20000 times. A good formwork depends on its design and size. The cost is ¥ and the average time of formwork installation is about 20 seconds

7.3 site

the site occupied by a selective welding equipment is about 5M3, the conventional wave soldering equipment needs 4m3, and the manual welding equipment can need 2m3 according to the national standard. It is impossible to use a general method to calculate the break even point of different welding processes. For example; Man hour cost, nitrogen consumption, area, etc. are just a range. In selective welding. Compared with wave soldering, its energy and labor costs are low, while compared with manual welding, its real benefit is the reduction of defect rate

8. Conclusion

selective welding process is a good substitute for template wave soldering, which can realize highly flexible circuit component welding without expensive transmission system. Although wave soldering can obtain high output, it is limited by the height of soldered devices on the welding surface and SMD devices. In addition to the high template and loading cost, the solder peak must be high enough to have proper solder pressure in the template window. The height of the wave crest is 12mm, which increases the production of tin slag and the cost of nitrogen consumption. Manual welding is a manual operation, so it is easy to produce defects, such as excessive soldering, improper through-hole tin filling, flux residue and thermal stress on the solder joint. Compared with the manual welding process with high labor cost, the selective welding equipment greatly reduces the defect rate and returns the investment well. Today, the number of solder joints of most products on the production line is about. The flexibility and programmable ability of selective welding process provide a new welding method for new product design and a new process approach for designers. (end)

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