The use of reflow soldering instead of wave soldering in the SMT mixed assembly process is one of the current development trends of SMT process technology. This article describes the investigation and preliminary testing of this process, and makes a summary of the process testing. Since this process has just begun to be researched and is not yet mature, it was proposed at this seminar for discussion with peers.

2. Characteristics of pure reflow soldering process.
3. This process replaces wave soldering with one or two reflows (Reflow 2 and Reflow 3).
4. Advantages (compared to wave soldering): (1) High reliability and good welding quality. (2) There are fewer welding defects such as false welding and bridging, and the workload of repairing the board is reduced. (3) The PCB board surface is clean and the appearance is obviously better than that of wave soldering. (4) The process is simplified. It eliminates the process of dotting (or printing) patch glue, wave soldering process, and cleaning process. Since the above steps are omitted, operation and management are simplified. The fewer materials and equipment used in the same product, the better it is managed. Moreover, the operation of the reflow soldering furnace is much simpler than that of the wave cresting machine. (5) Reduce costs and increase benefits. After adopting this process, wave soldering equipment and cleaning equipment, wave soldering cleaning workshop, wave soldering and cleaning staff, and a large amount of wave soldering standard materials and cleaning agent materials are eliminated. Although the price of no-clean solder paste is slightly higher than that of non-clean solder paste, in general it can greatly reduce costs and increase benefits.
5. Application status of using reflow soldering instead of wave soldering when SMT mixed assembly
6. This technology is currently mainly used in the production of color TV tuners (electronic tuners).
7. Currently, it has been successful in companies such as ALPS and SONY in Japan and has been widely promoted in the ESC industry. After the reform, welding quality and reliability have been significantly improved.
8. my country’s Shanghai Modern Technology Development Company imitated SONY’s three-dimensional needle tube printing press. This process has been used in the following domestic tuner manufacturers: (and has also achieved good benefits). (1) Wuxi Radio Factory No. 6. (2) Shanghai Jinling Radio Factory. (3) Chengdu 8800> Factory. (4) Chongqing Testing Instrument Factory. (5) Shenzhen Dongguan Tuner Factory.
9. Process test situation of using reflow soldering instead of wave soldering when SMT mixed assembly
10. Samples and materials: (1) Sample 1: There are 20 sockets, 70 resistors, 42 ceramic capacitors, 1 resistor row, 4 tantalum capacitors, and 32 polyester capacitors on the 80mmX236mm printed board. . (2) Sample 2: There are 3 sockets, 6 domestic plastic components, 5 transistors, 2 diodes, 12 resistors, 5 ceramic capacitors, 2 diodes, and 4 aluminum on the 63mm Electrolytic capacitor. (3) Use 55-2063Sn/Pb solder joints from Beijing Nonferrous Metal Processing Factory.
11. Test method: (1) First test whether the discrete components can withstand the temperature impact of the reflow soldering furnace; during the test, lower the temperature under the soldering area of the reflow soldering furnace as much as possible, and lower the temperature below the soldering area as much as possible. high. The temperatures in the drying and preheating zones should also be appropriately lowered. Find an optimal temperature curve that enables separate welding of the THC pins and PCB pads without damaging the quality of the THC itself. (2) Apply solder paste (test with three methods): a. Print solder paste on the THC component side (A side). (Use a 0.5mm thick copper template) b. Print solder paste on the soldering surface (side B). (Use a 0.5mm thick copper template) c. Use a soldering paste machine to apply solder paste on the THC component surface. (Using a manual dispensing machine) Solder paste volume analysis: According to the plug-in component (THC), in addition to the upper and lower pads of the PCB, there are also vias in the thickness direction of the PCB that need to be filled with solder, and the component pins are soldered to both sides of the PCB. A half-moon-shaped solder joint is also formed at the intersection of the disks, so the amount of solder paste required is much more than that of surface mount SMC/SMD. It is estimated to be about 3-4 times that of SMC/SMD (the amount of solder paste is related to the diameter of the PCB jack and the pad size of the jack—in direct proportion). According to the above analysis, when using the printing process, 0.5-0.8 needs to be processed mm thick template. Since it is difficult to process thick stencils, we processed 0.5mm thick copper stencils as test boards. When using the spot solder paste process, the appropriate amount of solder paste must also be mastered. (3) Insert the formed THC (short insertion). Divide sample 1 into two densities for insertion: a. Plug in all the THC. b. Only insert 1/4 of the THC component. (4) Reflow soldering (using K6025 infrared reflow furnace) Analyze the different requirements for soldering THC and SMC/SMD in a reflow soldering furnace: a. When SMC/SMD is reflowed, both the component surface and the welding surface are on the furnace. Since the SMC/SMD design has considered that the component surface and the welding surface are on the same side, the outer packaging material can withstand the reflow heat shock. Therefore, high temperature is required above the furnace, and the temperature below the furnace needs to be lowered (especially when double-sided welding). b. The reflow soldering situation of THC is different from that of SMC/SMD – the component surface is on top and the welding surface is on the bottom. Since THC is designed based on traditional manual soldering and wave soldering, the component surface and welding surface are on both sides of the PCB. Some elements The outer packaging material of the device is not subject to high temperature shock, so the temperature under the furnace (soldering surface) is lowered. Based on the above analysis, adjust the reflow temperature curve. Find out the optimal temperature and speed that can not only ensure the quality of the solder joints under the PCB, but also prevent the discrete components on the PCB from discoloration and deformation.
12. Test results: Set temperature actual temperature Zone 1 temperature: 2000℃ 2000℃+50℃ Zone 2 temperature: 2800℃ 2800℃+50℃ Zone 3 temperature: 300℃ 2920℃+50℃ Zone 4 temperature: 5500℃ 5500 ℃+50℃ Conveyor belt speed: 35cm/min Note: The set temperature and actual temperature are the temperatures of the heating plate of the infrared reflow furnace. (1) Solder joint inspection results (use a 5x magnifying glass): a. For thick-leaded tantalum capacitors and thick-leaded resistors, the solder joints are perfectly acceptable. Due to the small gap between the leads and the jack, the solder joints on the soldering surface of the PCB are not full, and the solder does not crawl from the through holes to the pads on the PCB. b. For resistors with thin leads, the solder joints of resistor rows and sockets are not full. Due to the large gap between the leads and the jack, a large amount of solder needs to be filled in the jack, so the solder joints on the soldering surface of the PCB are not full, and the solder does not crawl from the through hole to the pad on the PCB. c. Sample 1 filled with THC: the solder paste has melted around the PCB and in the parts with sparse components, and the solder joints are bright; the solder paste on the soldering surface of the tantalum capacitor with a larger component body has melted, and on the pad above the jack on the component surface The solder paste has not been fully melted, and the solder paste on the component surface jack has not been fully melted; in the densely populated area in the middle of the PCB, the solder paste on the soldering surface and the component surface pad has not melted. d. Sample with only 1/4THC inserted. 1: The solder paste on all the THC pads on the upper and lower sides of the PCB is completely melted and the solder joints are bright. (2) The situation of discrete components on the PCB component side (side A) a. All discrete components on sample 1 (including resistors, resistor banks, ceramic capacitors and sockets), including all sockets, have no deformation or obvious discoloration. Phenomenon. One of the tantalum capacitors has a darker surface. According to analysis, the outer surface coating of the capacitor is an insulating varnish. The insulating varnish is baked at different temperatures and has different colors, which is not expected to affect its performance. b. The four aluminum electrolytic capacitors on sample 2 passed through the reflow oven twice, and one of them fell off and one cracked. It means that aluminum electrolytic capacitors cannot be reflow soldered. The three domestic plastic-sealed devices on the PCB were reflowed twice, and the tops were deformed and blistered. It shows that some domestic plastic sealed devices cannot be reflow soldered.
13. Test summary of using reflow soldering instead of wave soldering process when SMT mixed assembly
14. The scope of application of reflow instead of wave soldering process in SMT mixed assembly. After preliminary research and testing, this process does have many advantages, but not all SMT mixed assembly forms can adopt this process. (1) This process is only suitable for products that are mainly mounted and supplemented by insertion (most SMC/SMD, a small amount of THC). Because the THC in this process requires short insertion, the workload of component forming and insertion increases. (2) This process requires that the outer packaging material of THC can withstand the impact of reflow soldering (because traditional THC is designed according to the process requirements of manual soldering and wave soldering, there are no special requirements for the outer packaging material of components). Tests have proven that this process cannot be used for the following components: aluminum electrolytic capacitors, domestic plastic sealed components, etc.
15. Special requirements for equipment (1) Printing equipment: When double-sided mixed packaging, because there is already soldered SMC/SMD on the THC component surface, the solder paste cannot be printed with a flat stencil, and special three-dimensional tubular printing is required. Apply solder paste using a machine or spot solder paste machine. (2) Reflow soldering equipment: Because the temperature control of zones 1 and 2 of our existing K6025 infrared reflow soldering furnace cannot be controlled up and down separately. Although zone 3 of the welding zone is above and zone 4 is below, the temperature in zone 3 can be adjusted as low as possible, but the temperature above zone 1 and zone 2 cannot be adjusted down. The furnace has been adjusted to the extreme temperature, but the color of the packaging material of individual components still becomes darker. In addition, the welding results of Sample 1 filled with THC fully prove that the temperature of the infrared furnace is very uneven. According to the above test, a. When welding THC, the reflow furnace must have the function of independently controlling the temperature up and down in each temperature zone of the entire furnace. This is determined by the different conditions when soldering THC and SMC/SMD in a reflow oven. b. You must choose a hot air furnace or a hot air + far infrared furnace with a relatively uniform furnace temperature.
16. Special process requirements (1) The amount of solder paste used to weld THC is 3-4 times greater than that of SMC/SMD. (2) Method of applying solder paste: a. When mixing on one side, stencil printing, tubular printer printing or spot welding paste machine dispensing can be used. b. When double-sided mixed packaging, because there is already soldered SMC/SMD on the THC component surface, the solder paste cannot be printed with a flat stencil, and a special three-dimensional tubular printing or spot soldering paste machine is required to apply the solder paste. (3) Special requirements for THC pad design: The diameter of the THC jack and the size of the two plates of the jack are directly related to the demand for pads. Therefore, it is necessary to measure the pins of each component when designing the pad. a. The diameter of the jack needs to be designed according to the diameter of the lead pin, and the hole diameter cannot be too large. b. The pads on both sides of the jack should not be too large. (4) Short insertion must be used, and the pins cannot be too long when forming the components. Due to the surface tension when the solder paste melts, a layer of solder is also plated around the component leads. (5) The temperature curve of the reflow oven should be adjusted according to the specific conditions of THC. The temperature above the furnace should be adjusted as low as possible, and the temperature below the furnace should be suitable for the optimal temperature and speed at which the discrete components above are not damaged. (6) If there are aluminum electrolytic capacitors and domestic plastic-sealed devices on the PCB, the attached method should be used to solve the problem.

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