1:The process flow of selective soldering includes: flux spraying, circuit boardpreheating, dip soldering and drag soldering. Flux coating process In selectivesoldering, the flux coating process plays an important role. At the end of thesoldering heat and soldering, the flux should be active enough to preventbridging and prevent oxidation of the circuit board. Flux spraying is carriedby the X/Y manipulator to pass the circuit board over the flux nozzle, and theflux is sprayed to the soldering position of the pcb circuit board. Solderingcircuit board skills

 2: Microwave peak selection after the reflowprocess. The important thing is that the flux is sprayed accurately, and themicro-hole spray type does not contaminate the area outside the solder joints.Micro-dot spraying. The diameter of the small flux dot pattern is greater than2mm, so the position accuracy of the flux deposited on the circuit board is±0.5mm, so that the flux can always cover the welded part. Soldering circuitboard skills 3: The process characteristics of selective soldering can beunderstood by comparing with wave soldering. The obvious difference is that inwave soldering, the lower part of the circuit board is completely immersed inliquid solder, while in selective soldering, Some specific areas are in contactwith the solder wave. Since the circuit board itself is a poor thermallyconductive medium, it does not heat and melt the solder joints adjacent tocomponents and areas of the circuit board when soldering. Flux must also bepre-applied before soldering. Compared with wave soldering, the flux is onlyapplied to the lower part of the circuit board to be soldered, rather than theentire pcb circuit board. In addition, selective soldering is only suitable forthe soldering of plug-in components, selective soldering is a new method, and athorough understanding of selective soldering process and equipment isnecessary for successful soldering. Precautions for circuit board soldering

1.   After getting the bare PCBboard, you should first perform a visual inspection to see if there areproblems such as short circuits and open circuits, and then familiarizeyourself with the schematic diagram of the development board, compare theschematic diagram with the PCB silk screen layer, and avoid the schematicdiagram and the PCB does not match.

 2. After the materials required for PCBwelding are ready, the components should be classified. All components can bedivided into several categories according to their size, which is convenientfor subsequent welding. A complete bill of materials needs to be printed.During the welding process, if an item is not welded, use a pen to cross outthe corresponding option, which is convenient for subsequent weldingoperations. Before welding, anti-static measures such as wearing anelectrostatic ring should be taken to avoid damage to components caused bystatic electricity. After the equipment required for welding is ready, the tipof the soldering iron should be clean and tidy. It is recommended to use aflat-angle soldering iron for the initial soldering. When soldering componentssuch as 0603-type packages, the soldering iron can better contact the pads,which is convenient for soldering. Of course, for experts, this is not aproblem.

3.When selecting components for welding, the components should be welded in orderfrom low to high and from small to large. So as not to bring inconvenience tothe welding of smaller components by welding larger components. Priority isgiven to soldering IC chips.

4.Before soldering the integrated circuit chip, it is necessary to place the chipin the correct direction. For the chip silk screen layer, generally therectangular pad represents the starting pin. When welding, one pin of the chipshould be fixed first, and the position of the components should be fine-tunedand then the diagonal pins of the chip should be fixed, so that the componentsare accurately connected and then welded.

5.There is no positive and negative pole for the SMD ceramic capacitor and theZener diode in the voltage regulator circuit. The positive and negative polesneed to be distinguished for light-emitting diodes, tantalum capacitors andelectrolytic capacitors. For capacitor and diode components, generally the endwith a prominent mark should be negative. In the package of SMD LEDs, thedirection along the lamp is the positive-negative direction. For the packagedcomponents identified by the silk screen as the diode circuit diagram, thenegative end of the diode should be placed at one end of the vertical line.

6.For crystal oscillators, passive crystal oscillators generally have only twopins, and there is no positive or negative distinction. Active crystaloscillators generally have four pins. Pay attention to the definition of each pinto avoid welding errors.

7.For the welding of plug-in components, such as power module related components,the pins of the components can be modified before welding. After the componentsare placed and fixed, the solder is generally melted by a soldering iron on theback and then the pads are integrated into the front. You don't have to put toomuch solder, but the components should be stabilized first.

8.The PCB design problems found during the welding process, such as installationinterference, incorrect pad size design, component packaging errors, etc.,should be recorded in time for subsequent improvement.

 9. After welding, use a magnifying glass tocheck the solder joints to check whether there is any virtual welding and shortcircuit.

 10. After the welding of the circuit board iscompleted, the surface of the circuit board should be cleaned with a cleaningagent such as alcohol to prevent the iron filings attached to the surface ofthe circuit board from short-circuiting the circuit, and at the same time tomake the circuit board cleaner and more beautiful.