1. Knowledge of SMT reflow (reflow furnace) equipment

 

 

 

1. Understand the reflow oven

 

 

 

The reflow oven is often called a reflow oven or a reflow soldering machine. It is an equipment used to complete reflow soldering. After connecting it to the placement machine on the SMT production line, the SMD is placed on the pre-printed solder paste pad of the PCB board by the placement machine, and then the PCB board is sent to the reflow furnace. The high temperature in the furnace causes the solder paste to After melting and reflowing, and then cooling the PCB board, the solder solidifies, finally achieving a stable and firm bond between the SMD and the pad. The schematic diagram of the appearance of the reflow oven is shown in the figure below. The appearance will be slightly different depending on the manufacturer and type.

 

 

 

 

 

1) Classification of reflow ovens

 

 

 

According to the different heat sources and heating methods of the reflow oven, the reflow oven can be divided into infrared reflow oven, hot air reflow oven, infrared/hot air reflow oven, laser reflow machine and gas soldering reflow oven (also known as For vapor phase reflow oven) and other types.

 

 

 

The infrared reflow oven (IR furnace) appeared in the 1980s. It is an integrated reflow method that uses infrared radiation emitted by a rod-shaped or flat heating element as a heat source to generate heat and generate convection heating inside the machine. This method is prone to temperature differences due to the influence of the size of the components, PCB thickness, size and number of layers, and there is a shadowing effect, that is, the temperature difference is relatively large under the shadow of the components.

 

 

 

The most widely used one at present is the all-hot air reflow oven that began to appear in the 1990s. Its heating principle is to use an electric heater plate to heat and blow it up and down with a fan, so that the hot air circulates inside the furnace body to heat the PCB board. As shown below. In this method, due to the constant flow of hot air, the heating on the PCB board will be uneven. In order to heat the PCB board surface evenly, the configuration of the electric heating plate and the circulation method of hot air are both important. The advantage of this type of reflow oven is uniform heating, which can make the temperature of the PCB board and components close to the temperature of the gas in the temperature zone, overcoming the temperature difference problem and shielding effect of the infrared reflow oven. Because it uses air circulation, its operating cost is low and there will be no overheating. However, due to the poor heat transfer efficiency of air, if you want to blow away the cold air layer on the PCB board and increase its temperature, you need to increase the flow rate of the hot air. If there are small parts on the PCB board, they may be blown away. If position deviation occurs, it will affect the welding quality in severe cases.

 

 

 

 

 

The infrared/hot air reflow oven is based on the infrared reflow oven and adds hot air circulation to make the temperature in the furnace more uniform. It is currently an ideal and advanced method and overcomes the shortcomings of the first two reflow ovens. As equipment costs further reduce, the application of this equipment will become more and more widespread.

 

 

 

The heat source of the laser reflow soldering machine comes from the laser beam. It is a local heating spot welding technology. It is mainly used for the production of small and medium-sized batches of BGA, the welding of heat-sensitive devices and the welding of circuit devices in military and space electronic equipment. . Compared with traditional reflow soldering technology, its heating is concentrated and fast, but its production cost is high and the welding efficiency is low. It has not yet been put into large-scale application.

 

 

 

Gas soldering reflow ovens are widely used in the United States. The heating method is to heat an inert organic solvent liquid (such as perfluoride liquid) with a large heat transfer coefficient, so that the steam obtained after vaporization contacts the lower temperature PCB board. Therefore, Condensation heat is generated. The basic process is: use an electric hot plate to heat the inert liquid to reach the boiling point to form saturated steam, and then soak the PCB board in the saturated steam, so that the PCB board absorbs heat to reach the evaporation temperature, and the steam condensed on the surface of the PCB board becomes a heat source. And heat the PCB board. Its main advantages are: because it uses inert gas, it almost never oxidizes; because the heating temperature is fixed, it can be welded at lower temperatures to prevent parts from being damaged by heating; solvent vapor can reach every corner of the PCB board. The heat conduction is uniform and high-quality welding can be completed regardless of the product geometry. The disadvantages are that the liquid vaporization loss is large, the solvent is expensive, and the production cost is high.

 

 

 

In addition, when molten solder is exposed to air, rapid oxidation occurs, forming a thin layer of tin oxide and lead oxide, collectively called oxide scale. In order to avoid this situation, the more advanced reflow soldering equipment currently has two working modes: air/nitrogen. The nitrogen mode refers to filling the reflow furnace with an inert gas (nitrogen), and the nitrogen protects the metal surface during the heating process. Not oxidized, it also ensures proper flux activity. Comparing the wetting power of different fluxes in air and nitrogen environments, it can be found that nitrogen coverage can improve the process and reduce flux residues, minimizing the probability of circuit test failure. Many studies on lead-free soldering It shows that nitrogen can improve the welding quality, avoid the contact between welding parts, solder and oxygen, overcome their oxidation at high temperatures, and effectively improve the wetting performance. According to the different nitrogen filling areas, it is divided into two types: tunnel type and hood type. The tunnel type means that the preheating and welding areas are all sealed in a nitrogen-filled pipe, which can reduce the contact probability between the solder and oxygen to 5×10-6; the hood type means that only nitrogen is installed on the wave peak of the welding part. The nitrogen gas only covers the welding part of the solder overflow port. In this structure, the chance of solder contact with oxygen is twice as high as that of the tunnel type. The former is more beneficial to improving welding quality and reducing oxidation of solder, but consumes more nitrogen; the latter consumes less nitrogen and is more convenient for maintenance.

 

 

 

Although using nitrogen shielded welding has many advantages, it also increases production costs. In the lead-free process, it is sometimes easy to cause welding short circuits and "monuments". Therefore, whether to use nitrogen working method must comprehensively consider all aspects of the impact that may have on specific production.

 

 

 

2) Composition of reflow furnace equipment

 

 

 

The reflow furnace consists of an external computer control system and a furnace body, as shown in Figure 4-1. The furnace body is composed of two sealed boxes, upper and lower, with a transmission track in the middle. The overall structure of the full hot air reflow oven is shown in the figure below, which consists of a heating system, a cooling system, a flux recovery system, a PCB board conveying system, a nitrogen system and an electrical system. The functions of each part are detailed as follows.

 

 

 

 

 

(1) Computer control system. The computer control system is the backbone of the reflow furnace. The quality of its selected components, operating methods, operational flexibility, and functions all directly affect the use of the equipment. The computer control system uses PLC (programmable logi ccontroller, programmable controller) as the main control unit to monitor the working conditions of the entire reflow furnace. Its main functions are as follows: complete the temperature control of all controllable temperature zones; complete the transmission part Speed detection and control realizes inorganic speed regulation; cooperates with the temperature tester to realize PCB online temperature testing, and can store, call, and print; set parameters can be set and modified in real time, and can be stored and printed; internal control parameters can be modified in real time ; Display the working status of the equipment, with convenient human-machine dialogue function; with self-diagnosis system and sound and light alarm system.

 

 

 

(2) Heating system. The function of the heating system is to heat the PCB board from normal temperature to the melting of the solder paste. The heating system of the full hot air reflow oven mainly includes motor, heating tube, wind wheel, rectifier plate and other parts. Its structure is shown in the figure below. Each heating zone of the reflow furnace furnace has two heating temperature zones, upper and lower. Each heating temperature zone is equipped with a heating tube made of heating wire. As the upper and lower hot air motors drive the wind wheel to rotate, a The air blowing force causes the heat generated by the heating tube to form hot air, which passes through the specially structured air duct and is blown out through the rectifier plate, so that the hot air is evenly distributed in the temperature zone, thereby completing the heating of the PCB board on the intermediate conveyor belt. The speed of the hot air motor of some reflow furnaces can be adjusted. The faster the motor speed, the greater the air blowing force generated and the stronger the heat exchange capability. Therefore, good reflow furnace equipment should use high-performance motors.

 

 

 

 

 

In addition, the temperature climbing ability of the heating system is the main factor that determines the preheating of the PCB board. The temperature climbing ability is low, and it is easy to cross temperature between various temperature zones. When the insulation between each heating temperature zone is good, heat is basically not transferred between the metal and the gas, but only transferred between the conveyor belt and the PCB board, which can increase the speed of temperature climb. In this case Under certain circumstances, the preheating temperature can be appropriately reduced to avoid changes in the performance of components on the PCB caused by high preheating temperatures.

 

 

 

(3) Cooling system. In reflow soldering, the last process that the PCB board goes through is cooling. The function of the cooling system is to cool the PCB board temperature below the solid phase temperature to solidify the solder joints. The cooling system has a great impact on the quality of welding. It determines the crystalline form and internal structure of the solder joints, affects the reliability of the solder joints, and also has a certain impact on the appearance of the solder joints, so the cooling rate must be strictly controlled. In order to further improve the quality of lead-free soldering, various equipment manufacturers have currently developed dual-module cooling zones that combine cooling zones and flux management. Each cooling module is independently controllable, and the temperature of the cooling zone can be digitally displayed and adjusted.

 

 

 

Reflow ovens that work under natural air conditions use the surrounding air as a cooling medium, which can use a large amount of gas at a low cost and effectively cool the PCB board. In the environment where nitrogen is used for welding protection, the cooling process must be carried out in a controlled environment, usually using a heat exchanger, using gas or liquid as the heat exchange medium, and the cooling air flow must also be circulated in the furnace to reduce Overall nitrogen consumption.

 

 

 

The current mainstream technology of the cooling system is the forced cooling method with controllable cooling speed. The cooling method mostly uses circulating water cooling plus air cooling. The main body adopts a water circulation structure for heat replacement. The circulating water comes from an external chiller, which can meet various needs. Lead-free rate requirements, additionally include cooling fans and flux recycling outlets. The function of the cooling fan is to cool the PCB board before it comes out of the soldering furnace. It is generally equipped with a cooling fan speed control knob, which can control and adjust the fan speed according to the air volume required for cooling the PCB board to achieve the required Wind speed. In the cooling system, the flux is also prone to condensation as the temperature decreases, as shown in the figure below. Therefore, flux condensation on the flux filter must be checked and cleaned regularly, otherwise excessive accumulation of condensation will lead to a decrease in cooling capacity.

 

 

 

 

 

At present, a new cooling method has emerged - thin air flow cooling. Its advantages are that it can reduce the circulating air flow, reduce the mixing with the flux gas from the preheating zone and the welding zone, reduce the flux cooling rate, improve the cooling efficiency, and provide Stable cooling performance can make the solder joints brighter, the microstructure better, and the reliability higher. Cooling water passes through a vertically mounted heat exchanger, allowing flux residues to drain naturally by gravity into a flux collection tank. The cooling water connection is done through quick connectors.

 

 

 

(4) Flux recovery system. In the lead-free process, higher soldering temperatures will cause a large amount of flux to volatilize. If these volatilized fluxes are not effectively treated, they will stain the machine and, in severe cases, the PCB board in production, causing Product scrapped. In addition, a large amount of water vapor, toxic gases and dust will be generated during the welding process, mainly from the PCB board. After testing, it was found that during the reflow soldering process, the weight of a PCB board is reduced by about 0.3g. Calculated from this calculation, if 10,000 boards are welded, about 3kg of dirt will be produced. Therefore, a flux and welding waste recovery device must be added to the equipment, and an exhaust filtration device is usually used to circulate the gas in the furnace and discharge the exhaust gas.

 

 

 

The schematic diagram of a flux recovery system is shown in the figure below. A filter system is arranged below the condensation point where the flux reaches the cooling zone. Large particles and flux are filtered out after passing through the filter, and activated carbon is used to filter out other pollutants. , the remaining gas reaches the exhaust exhaust outlet through the exhaust channel and is discharged.

 

 

 

 

 

Other recycling methods are also possible, for example, a new recycling system includes a two-stage filtration/separation system and integrated self-cleaning function. The first stage of filtration utilizes a mesh-shaped filter, which is contained in a box. On its way into the box, the flux vapor undergoes an expansion process, which increases the pressure and creates small droplets that, if large enough, fall out of the air stream. Pass the remaining steam through the filter, which will separate the large, heavy particles from the steam. These particles are mainly composed of entangled metals, resins and rosin, and they remain adhered to the outside of the filter. This in some cases helps eliminate the possibility of highly viscous and difficult-to-remove residues from penetrating down into the system. Cleaning of the filter is accomplished by an additional motor that rotates the filter periodically. The centrifugal force exerted on the particles overcomes the adhesion forces holding them to the filter and throws them towards the walls of the chamber. Because it is not incorporated with an active cooling system, the system maintains a certain temperature as the gas passes through the chamber, which allows the heavier liquid that sticks to the walls of the chamber to drip down to the drain located at the bottom of the chamber. in the jar.

 

 

 

The second stage of filtration consists of a packing filled with stainless steel balls contained in a box. Composed mainly of alcohol and solvents, the small mass of particles contained in the steam that has passed the first layer of filtration will again undergo expansion, thereby increasing the volume of the droplets. Then, the steam passes through the filling layer and collides with the steel balls multiple times. Since the liquid contained in the steam will spread on the surface of the steel balls, these steel balls are determined to be wettable. Therefore, in the initial collision of the particle and the steel ball, different kinds of crystal nuclei are generated, and the steel ball is covered with a thin film of liquid. Once the ball is completely covered by the film, particles contained in the vapor collide with the liquid film. Since these are similar substances but produced by different kinds of crystal nuclei, the liquid also grows in size, forming droplets that flow into the flux collection tank, waiting to be cleaned.

 

 

 

Exhaust channels can also be set up in each temperature zone to circulate the gas in the furnace in a unique way. Concentrated pollutants can be discharged through a standard pipe, thereby preventing flux airflow from entering the cooling zone. The advantage of this method is that no filter is required, and flux gases, solid particulate contaminants, and exhaust gases can all be removed.

 

 

 

The furnace gas circulation device is usually connected to the flux recovery system. The control process of gas circulation mainly includes two aspects: gas injection in the furnace and exhaust gas emission.

 

 

 

(5) PCB board delivery system. The function of the PCB board conveying system is a transmission device that transports the PCB board from the entrance to the exit of the reflow oven at a certain transmission speed. There are three main transmission methods of common reflow oven conveyor systems on the market: chain type, mesh belt type and chain/mesh belt type. The chain type uses a stainless steel chain with adjustable chain width to place PCB boards of various widths on it for transmission. It is generally used for single/double panel welding. Mesh belt conveyor overcomes the defect that larger width PCB boards may be dented due to heat, but it is more suitable for single-panel welding. Therefore, the most widely used transmission method at present is the chain/mesh belt dual transmission mechanism. This mechanism combines the advantages of the two aforementioned methods and is suitable for the transmission of various PCB boards, as shown in the figure below. In addition, the conveyor belt must run smoothly. The vibration of the conveyor belt will cause the displacement of smaller components, suspension bridges, cold welding, etc.Welding quality defects.

 

 

 

 

 

The chain/mesh belt conveyor system mainly includes rails, mesh belts (central support), chains, transport motors, track width adjustment devices, transmission speed control mechanisms, etc.

 

 

① Track: Generally made of aluminum alloy, it is used to control the transfer direction of PCB in the reflow furnace.

 

 

 

② Chain net: It has the function of preventing the board from falling off.

 

 

 

③ Chain: Its speed can be set by software and is used to transport PCB boards. It has an automatic lubrication device, and the lubrication device is automatically controlled by the computer.

 

 

 

④ Track width adjustment device: In addition to the transmission method, another important aspect of the conveyor system is the adjustment range of the track spacing. The processing size range of the reflow furnace is determined by the maximum track spacing that the conveyor system can adjust. The larger the adjustment range of the track spacing, the more adaptable it is to the transmission of PCB boards of various sizes. The track width adjustment device consists of two racks (located at the entrance and the middle), a transmission rod, two gears, chains, two front and rear screw rods, brackets, motors, etc. to adjust the track width. When adjusting the track width, the two screw rods drive the track forward and backward, and the transmission rod makes the middle and both ends of the track move synchronously to ensure that the width of the front, rear and middle of the track is consistent. The track width adjustment device is shown in the figure below.

 

 

 

 

 

⑤ Transmission speed control mechanism: used to control the speed of transporting PCB boards. The speed adjustment range of the transmission speed is generally 0.1~1.2m/min, using stepless speed adjustment. The more commonly used method is "frequency converter + full closed-loop control", as shown in the figure below.

 

 

 

 

 

(6) Nitrogen system. In order to prevent oxidation and increase wettability, nitrogen protection is often required in today's reflow soldering, and the oxygen concentration is generally required to be below 1000ppm. The use of nitrogen protection is beneficial to the improvement of welding process and welding quality, but it also increases the cost. Therefore, the consumption of nitrogen should be reduced while ensuring the welding quality. In furnaces where there is no forced convection and the gas flow is flake-like, it is relatively easy to control gas consumption. There are several methods that can be used to reduce nitrogen consumption, including reducing the openings at both ends of the reflow oven, using blank baffles or other similar arrangements to block the unused parts of the inlet and outlet openings, etc. The purpose of this is to Control the air flow out of the reflow oven and minimize mixing of nitrogen with outside air. Another method is to use the principle that hot nitrogen will form a gas layer above the air, and the two layers of gas will not mix. Design the heating chamber in the reflow furnace so that its position is higher than the entrance and exit of the furnace, so that Nitrogen naturally forms a gas blanket that reduces the amount of gas input required to maintain a certain gas purity.

 

 

 

(7) Electrical system. The electrical system of the reflow furnace is the general term for all mechanical and electrical components in the equipment, and it is controlled by the computer control system. The most typical devices include relays, air switches, etc.

 

 

 

Solid state relay (SSR) is a contactless switching device with relay characteristics that uses semiconductor devices to replace traditional electrical contacts as switching devices. The single-phase SSR is a four-terminal active device, two of which are input control terminals, and the other two are output terminals. The input terminal and the output terminal are optically isolated. After a DC or pulse signal is applied to the input terminal to a certain current value, The output terminal can change from off state to on state. There are no mechanical contact parts during the switching process. Therefore, in addition to the same functions as electromagnetic relays, solid-state relays are also compatible with logic circuits, resistant to vibration and mechanical shock, have unlimited installation positions, and have good moisture-proof, mildew-proof, and anti-corrosion properties. It also has excellent performance in terms of explosion-proof and ozone pollution prevention. Its characteristics include small input power, high sensitivity, small control power, good electromagnetic compatibility, low noise and high operating frequency. It has been widely used in computer peripheral interface equipment, temperature regulation, speed regulation, light dimming, motor control, electric furnace heating control, electric power, petrochemicals, medical equipment, financial equipment, instrumentation, traffic signals and other fields. The appearance of the relay used in the reflow oven is as shown in the figure below.

 

 

 

 

 

The air switch is also called a low-voltage circuit breaker or automatic switch, as shown in the figure below. It is an electrical appliance that can not only use a manual switch, but also automatically perform voltage loss, undervoltage, overload and short circuit protection. It can be used to distribute electric energy, avoid frequently starting asynchronous motors, and protect power lines and motors. When they suffer from severe overload, short circuit, undervoltage, etc., they can automatically cut off the circuit. Its function is equivalent to that of a fuse. The combination of a switch and an overheating or underheating relay, etc., generally does not require changing parts after breaking the fault current, and has been widely used.

 

 

 

 

 

2. Understand temperature measurement equipment

 

 

 

When the PCB board passes through the reflow soldering machine, the curve of the temperature changing with time at a certain point on the surface-mounted device is called the time-temperature curve, or the furnace temperature curve for short. The furnace temperature curve is a crucial factor that affects the quality of reflow soldering. The furnace temperature can be measured using the temperature measurement equipment that comes with the reflow furnace, or a reflow soldering temperature curve tester sold on the market. In order to prevent errors in the temperature measurement equipment provided by the reflow oven, a reflow temperature curve tester can generally be used to accurately measure the furnace temperature curve, thereby helping to adjust the furnace temperature curve to the best state in real time. There are two types of thermometers on the market. One is a real-time thermometer, which transmits temperature/time data in real time and makes graphics; the other is a sampling and storage data thermometer, which uploads the data to the computer system after the measurement is completed. .

 

 

 

Temperature measurement equipment includes a temperature profiler, a thermocouple (as shown in Figure 1 below), as well as tools and solder paste parameter sheets used to attach the thermocouple to the PCB. A typical temperature curve tester is shown in Figure 2 below. The main body is a flat metal box with multiple micro thermocouple probes and a memory function. It is connected to the temperature measuring plate through thermocouples and passes through the furnace together to record the temperature curve. , and then connected to a computer, printer, etc., the data can be saved and printed.

 

 

 

 

Before measuring the temperature, first fix the thermocouple on the PCB board with glue or high-temperature solder such as silver/tin alloy. If you use solder, the solder joints should be as small as possible. You can also use a small amount of thermal compound (also called thermal paste or thermal grease) to cover the thermocouple, and then use high-temperature tape (such as Kapton, etc.) to adhere to the PCB board. The location of attachment is also chosen; it is usually best to attach the thermocouple tip between the PCB pad and the corresponding component pin or metal end. Then turn on the switch of the tester, and the tester enters the furnace together with the test substrate. After preheating, melting, and cooling, the process temperature is automatically sampled and recorded. Most testers can be connected to the computer system of the reflow oven using wireless transmission mode or data storage mode as needed, and the recorded data can be analyzed through a dedicated software system. The tester can also be connected to a printer to print out Temperature curves in multiple colors.

 

 

 

The temperature curve measured by the temperature curve tester comprehensively and accurately records the temperature and time parameters of each test point on the test substrate. According to the temperature curve, the speed of the heating controller and conveyor belt in each temperature zone is adjusted to achieve the optimal temperature and time process. parameters to obtain optimal reflow soldering quality. It is worth noting that if the parts of the reflow furnace are replaced or the process conditions are changed, the furnace temperature curve must be re-measured.

 

 

 

2. Preparation work before reflow furnace operation

 

 

 

Before running a reflow oven, it is usually necessary to make some necessary preparations, such as an in-depth understanding of the time-temperature curve, familiarity with the structure of the reflow oven equipment used, understanding the performance parameters of the solder paste used for welding and the type of PCB substrate, etc.

 

 

 

1. Understand the time-temperature curve

 

 

 

After the substrate printed with solder paste is reflowed, the solder paste particles on the substrate are melted at high temperature and then cooled and solidified, and the components are soldered to the corresponding positions on the PCB board. In this process, setting the temperature of each temperature zone in the reflow furnace reasonably to achieve the optimum is crucial to the quality of welding.

 

 

 

A typical time-temperature curve is shown above. Generally speaking, the ideal curve consists of four sections or intervals, with the first three zones heating and the last zone cooling. The four temperature zones are called preheating zone (also known as ramp zone), constant temperature zone (also known as uniform temperature zone), reflow zone (also known as reflow zone) and cooling zone.

 

 

 

The preheating zone generally accounts for 25% to 33% of the entire heating channel length and is used to increase the temperature of the PCB from the ambient temperature to the required active temperature. In this area, the temperature of the PCB continues to rise at a rate of no more than 2 to 5°C per second. If the temperature rises too fast, it will cause certain defects, such as fine cracks in ceramic capacitors; if the temperature rises too slowly, the solder paste will feel the temperature. Too much, not enough time for the PCB to reach active temperature.

 

 

 

The constant temperature zone has two purposes: one is to raise the PCB board, components and materials to a uniform temperature, close to the melting point of the solder paste, allowing it to transition to the reflow zone more easily; the other is to intensify the flux on the assembly. At the holding temperature, the aggravated flux begins to remove oxides from the pads and leads, leaving a cleaner surface for the solder to adhere to. Another transition occurs toward the peak temperature in the reflow zone, during which the temperature of the assembly rises above the solder melting point and the solder paste becomes liquid.

 

 

 

The function of the reflow zone is to increase the temperature of the PCB assembly from the active temperature to the recommended peak temperature. The active temperature is always a little lower than the melting point of the alloy, and the peak temperature is always at the melting point. The typical peak temperature range is 205~230℃. If the temperature in the reflow zone is set too high, the temperature rise slope will exceed 2~5℃ per second, or the reflow peak temperature will be higher than recommended. This situation can cause excessive curling, delamination or burning of the PCB and compromise the integrity of the component.

 

 

 

The cooling zone enables the formation of alloy solder joints, ultimately making the component pins and pads firmly integrated. The ideal cooling zone curve should be a mirror image of the reflow zone curve. The closer to this mirror image relationship, the closer the solid structure of the solder joint is, the higher the quality of the solder joint is, and the better the bonding integrity is.

 

 

 

In short, the more temperature zones in the reflow furnace, the more accurate the temperature curve can be and close to the ideal curve. Many reflow furnace equipment currently used are subdivided into eight temperature zones or even more based on the four temperature zones. With the increase of temperature zones, the outline of the actual time-temperature curve will be closer to the ideal curve.

 

 

 

In actual operation, the principle for establishing the reflow temperature curve is that the temperature rise rate before the welding zone should be as small as possible. After entering the second half of the welding zone, the heating rate should increase rapidly, and the time control of the maximum temperature of the welding zone should be short, so that PCB and SMD should be subject to thermal shock as little as possible. It must take a long time to adjust the temperature curve before production, and it should be adjusted according to the product characteristics. It is best to set the temperature curve according to the data provided by the solder paste supplier, and at the same time grasp the principle of temperature stress change inside the component. Under normal circumstances, the heating rate is less than 3°C per second, and the cooling rate is less than 5°C.

 

 

 

2. Inspect the reflow furnace equipment used

 

 

 

If it is the first time to use this equipment, you should first examine the basic conditions of the reflow oven before starting to operate the equipment. The main inspection points include: the heating method of the equipment, the number of temperature zones, whether the hot air motor speed and cooling fan speed can be adjusted, the supporting temperature measurement equipment, the supporting operation and maintenance guide of the equipment, etc.

 

 

 

3. Understand the properties of solder paste

 

 

 

The time-temperature curve varies depending on the type of solder paste used in reflow soldering, which mainly depends on the solder paste chemical composition. The most common formula types of solder paste include water-soluble (OA), rosin mildly activated (RMA) and no-clean solder paste. Generally speaking, the solder paste manufacturer will provide the best recommended temperature curve to optimize the soldering quality. Therefore, the temperature curve information can be obtained by contacting the solder paste manufacturer. When setting a temperature profile, you should first consult the solder paste supplier, review the component specifications, determine the best profile parameters for a specific process, then compare the profile parameters with the actual temperature profile measurement results, and finally take Measures to change machine settings to achieve optimal results for assembly.

 

 

 

4. Analyze the type of production substrate

 

 

 

The time-temperature curve also changes with the type of PCB substrate. The welding temperature curve must be set according to specific conditions such as the size of the device and the thickness of the PCB. The curve settings of flexible PCB substrates, single-sided PCB substrates, double-sided PCB substrates and PCB substrates containing BGA are discussed below.

 

 

 

(1) Flexible PCB substrate PCB made of soft insulating substrate is called soft PCB or flexible PCB. It is composed of insulating substrate, adhesive and copper conductor, as shown in the figure below. It adapts to the needs of today's electronic products developing towards high density, high reliability, miniaturization and lightweight, and also meets the strict economic requirements and the needs of market and technological competition. A significant advantage of using flexible PCB is that it can be more conveniently routed and connected in three-dimensional space, and can also be rolled or folded for use. As long as it is curled within the allowable curvature radius, it can withstand thousands to tens of thousands of uses without damage. In the same volume, the weight of flexible PCB can be reduced by 70% compared to wires and cables at the same carrying capacity, and the weight can be reduced by 90% compared with rigid PCB.

 

 

 

 

 

Due to its own characteristics, flexible PCB substrates generally have a small area and fewer components on them. Due to the use of special materials, the heat tolerance of the entire substrate including components is lower than that of ordinary PCBs. Therefore, the parameter settings of each temperature zone should be slightly lower during reflow soldering, and the speed frequency of the hot air motor should be adjusted lower.

 

 

 

(2) Single-sided PCB substrate Since there are components on only one side, relatively little heat is required during welding. The temperature zone of a single-sided PCB substrate should generally not be set too high.

 

 

 

(3) Double-sided PCB substrate The issue that must be paid attention to when reflow soldering double-sided PCB substrate is that when the front side is being soldered, the maximum temperature of the back side should not exceed the melting temperature of the solder paste. Therefore, when adjusting the temperature curve, thermocouples must be installed on the welding side and the reverse side.

 

 

 

(4) For PCB substrates containing BGA, in order to avoid damaging BGA devices, the preheating temperature should be controlled at 100~125°C. The heating rate and temperature holding time are both critical. The heating rate is controlled at 1~2°C per second. The welding temperature of BGA Compared with traditional SMD, its setting temperature is about 15°C higher, and the preheating temperature at the bottom of the PCB is controlled at about 160°C.

 

 

 

3. Inspection and maintenance before reflow furnace operation

 

 

 

1. Inspection of the surface of the reflow furnace

 

 

 

The inspection items and confirmation methods of the reflow furnace surface are shown in the table below.

 

 

 

 

 

2. Inspection of relevant devices in the conveyor system

 

 

 

The inspection work items and confirmation methods of the conveyor system are shown in the table below.

 

 

 

 

 

3. Inspection of relevant devices of the flux recovery system

 

 

 

The inspection work items and confirmation methods of the flux recovery system are shown in the table below.

 

 

 

 

 

4. Inspection of other system-related devices

 

 

 

The inspection items and confirmation methods of other system devices in the furnace cavity are shown in the table below.

 

 

 

 

 

4. Start-up and shutdown of the reflow oven

 

 

 

After the inspection work is completed, the reflow furnace can be started and operated for production. After production is completed or when it is necessary to shut down the machine for maintenance or repair, the correctmethod to close the reflow oven.

 

 

 

1. Boot steps

 

(1) Turn on the voltage regulator switch.

 

(2) Turn on the main power switch of the reflow oven.

 

(3) Turn on the internal power supply UPS of the reflow oven.

 

(4) Open the computer operating system.

 

(5) Select the required program and put it into production.

 

 

 

2. Shutdown steps

 

(1) Execute the COOLDOWN program.

 

(2) Wait about 45 minutes to keep the temperature in the furnace below 100°C.

 

(3) Execute SHUT OFF program.

 

(4) Exit the WINDOWS interface.

 

(5) Turn off the internal power supply DPS of the reflow oven.

 

(6) Turn off the main power switch.

 

(7) Turn off the regulated power supply.

 

 

 

3. Precautions for operating the reflow furnace

 

(1) After each maintenance, check whether there is any foreign matter in the furnace, and then start the working process after 15 minutes.

 

(2) When the reflow furnace is operating, do not extend your hands or body into the furnace.

 

(3) The furnace cover is not allowed to be opened during the production process.

 

(4) If any abnormality occurs during the production process, the power supply should be turned off in time for inspection and maintenance.