With the continuous development of electronic technology, PCB (Printed Circuit Board) as an indispensable part of electronic equipment, its production and welding process is becoming more and more mature. The PCB soldering process is an important step in connecting electronic components with the PCB board, and its quality and stability directly affect the performance and reliability of the entire electronic device. This article will present a comparison of these four welding processes – soldering iron, reflow soldering, wave soldering and laser soldering.
Soldering Iron Process
An electric soldering iron is used as a heating tool to melt the solder onto the object to be welded. The melting point of the solder is generally lower than that of the soldered object, so during the heating process, the solder will melt before the soldered object, thus forming a molten pool. Traditional welding processes are often used to join simple metal and plastic parts due to their relative simplicity.
The melting point of solder is relatively low, and it can be melted at about 300 °C, which is convenient for the welding of the welded workpiece. Soldering iron welding is the use of a heated soldering iron (also known as a heat gun, hot air welding machine) to heat and melt the part to be welded of the workpiece, and cooperate with the transmission of wire tapping to realize the soldering or soldering of electronic components.
Its disadvantages are the need to replace the soldering iron tip regularly and the welding accuracy is relatively low。 Besides, it is difficult to achieve an accurate welding position due to the limitation of the size and operating skills of the soldering iron.
Electric soldering iron welding method
1. Check whether the soldering iron is well grounded before using it.
2. Before welding, pay attention to whether the power of the soldering iron matches the soldering point.
3. Wash the tip with a sponge and plate it with tin.
4. Wipe the components to be welded clean and plated with tin, and then weld them, the temperature should not be too high during welding, and the time should not be too long.
5. Pay attention to the permeability of the solder joint, the spacing between the dots, and the matching of rosin and solder paste.
6. After welding, it is necessary to check whether the welded parts have virtual welding, no welding, wrong welding, and short circuit.
Reflow Soldering Process
The components on the various boards used in our computer are soldered to the circuit board by reflow soldering, and there is a heating circuit inside this device, which heats the air or nitrogen to a high enough temperature and blows it to the circuit board that has been pasted to the component, so that the solder on both sides of the component melts and bonds with the motherboard. The advantages of this process are that the temperature is easy to control, oxidation is avoided during welding, and manufacturing costs are easier to control.
Requirements for SMT reflow temperature control
Ensure that the temperature curve of SMT reflow soldering machine meets the product temperature requirements during use, and ensure the welding quality of chip processing products. What are the requirements for temperature control for SMT chip reflow soldering?
First, when the reflow welding is started, the temperature of each temperature zone is stable, and after the link speed is stable, the furnace temperature curve can be tested, and the furnace temperature is generally 20~30 minutes from cold to stable.
Second, the production line technicians should record the furnace temperature setting and connection speed every day or each batch of products, regularly measure the measurement and control files of the furnace temperature curve, and monitor the normal operation of reflow soldering. The Centre is responsible for patrols.
Third, the setting requirements of lead-free paste temperature curve:
1. The setting of the temperature curve is mainly based on: Recommended curve provided by the ointment supplier；materials；dimensions and thickness of printed plates; part density and dimensions, etc.
2. Lead-free furnace temperature requirements:
(1) The number of bonding points is greater than 100, and the size of C, QFN, BGA and PAD is below 3MM to 6MM, the measured peak temperature is controlled at 245 degrees to 247 degrees.
(2) Multi-density IC, QFN, BGA OR PCB PRODUCTS with A thickness greater than 2MM and a pad size greater than 6MM, the peak temperature can be controlled at 247-252 degrees according to actual needs.
(3) If FPC flexible PCB, aluminum substrate and other plates or parts have special requirements, they must be adjusted according to actual needs (such as the process of the product, it is managed according to the process flow).
Note: If the product is abnormal in actual operation, the basic requirements of the SMT technician 3.3 temperature curve should be timely reported to the technician.
Preheating area: preheating inclination angle 1~3 °C, temperature increase 140~150 °C; Room temperature: the temperature is 150~200 °C, lasting 60~120 seconds.
Return wind area: the temperature is above 217 °C for 40~90 seconds, and the temperature is 230~255 °C.
Cooling zone: cooling inclination angle (except PPC and aluminum substrate) / temperature (depending on the situation) below 1~4°C What is included in the note?
After the five plates pass through the stove, the gloss, solderability and soldering properties of each plate must be thoroughly inspected.
Product use management: use grease in strict accordance with product technology and user requirements.
Measure the furnace temperature once per shift, measure the furnace temperature after changing the line, measure the production model requirements of each shift, and confirm that there are plates or other impurities in the furnace when adjusting the quality, and confirm that the width of the inlet and outlet are consistent.
The furnace temperature is tested every time the temperature parameter is changed. The above are the requirements for SMT chip processing reflow soldering temperature control.
Wave Soldering Process
Wave soldering is to make the welding surface of the plugboard directly contact with high-temperature liquid tin to achieve the purpose of welding. The process is to insert the component into the corresponding component hole→ pre-apply flux→ pre-bake (temperature 90-100°C, length 1-1.2m), →wave soldering (220-240°C) → remove excess inserts → inspection.
Traditional manual soldering requires many people to adopt point-to-point welding for each point of the circuit board. The choice of welding is the assembly line industrial mass production mode, different sizes of welding nozzles can use drag welding batch welding, usually welding efficiency can be more than dozens of times higher than manual welding (depending on the design of the specific circuit board).
Its disadvantages are only applicable to the PCB assembly process of through-hole design, SMT, CABLE WIRES are not applicable, so the application range is more limited, because soldering requires the use of flux and produces solder dross, and the later production cost is high.
The selective wave soldering operation process generally consists of three parts: flux spraying, preheating and soldering. Through the program setting of the equipment, the flux spraying work can be completed sequentially on the solder joints to be soldered, and then the solder joints are preheated by the preheating module, and then the soldering module solders them point by point.
Wave soldering operation process requirements
1. Strictly control the computer parameter setting of the wave soldering machine according to the parameters given by the wave soldering production process;
2. Record the operating parameters of the wave soldering machine on time every day;
3. Ensure that the interval between the successive 2 Allegro plates placed on the conveyor belt of the spray wave soldering machine is not less than 5CM;
4. Check the flux spray situation of the wave soldering machine every hour, and it is necessary to spot check the 5S condition of the spray hood every time you turn to ensure that the flux will not drip onto the PCB board;
5. Check every hour whether the wave soldering machine has a flat wave peak, whether the nozzle is blocked by solder slag, and deal with the problem immediately;
6. If the operator finds that the parameters given by the process cannot meet the requirements during the production process, he shall not adjust the parameters by himself, and immediately tell the engineer to deal with it.
Laser Soldering Process
Laser soldering with laser as a heating source, laser heating solder joints or connection pads without leadless devices, through laser soldering special solder such as laser solder paste, solder wire, tin ring, solder ball or special solder sheet, heat conduction to the substrate, when the temperature reaches the melting point temperature of the solder solder, the solder melts, thereby forming a solder joint.
Multi-layer electrical mounting parts for precision electronic substrates. Since the traditional soldering process cannot be applied, the rapid progress of technology has been promoted. Existing soldering iron processes cannot process precision parts and can only be completed by laser welding. The biggest advantage of laser welding is non-contact soldering, that is, there is no need to contact with the substrate or electronic components.
The principle of laser soldering is to use laser as a heating light source, transmit optical fiber and laser welding head to cooperate with each other, focus the laser on the welding area, convert laser radiation energy into heat energy, melt tin material, and complete welding.
Laser welding is divided into solder paste, solder wire and solder ball laser welding according to the state of the tin material. Compared with traditional wave soldering, reflow soldering, manual soldering iron soldering and other soldering processes, the main excitation source of excitation soldering is a semi-conductive source (808-980nm).
Wave soldering is to let the welding surface of the plugboard directly contact with high-temperature liquid tin to achieve the purpose of welding, its high-temperature liquid tin maintains a bevel, and by a special device to make the liquid tin form a wave similar phenomenon, so called "wave soldering", the main material is solder bar.
Laser welding is a method of welding with a focused laser beam as an energy source to bombard the heat generated by the weldment.
Reflow soldering is mainly used to solder the circuit board that has been mounted on the component, melt the solder paste by heating to fuse and solder the patch component and the PCB pad, and then cool the solder paste through the cooling of reflow soldering to solidify the components and pads together.
