Reflow Oven Related Knowledge

Reflow oven related knowledge

Reflow soldering is used for SMT assembly, which is a key part of SMT process. Its function is to melt the solder paste, make the surface assembly components and PCB firmly bonded together. If it can not be well controlled, it will have a disastrous impact on the reliability and service life of the products. There are many ways of reflow welding. The earlier popular ways are infrared and gas-phase. Now many manufacturers use hot air reflow welding, and some advanced or specific occasions use reflow methods, such as hot core plate, white light focusing, vertical oven, etc. The following will make a brief introduction to the popular hot air reflow welding.

1. Hot air reflow welding

Now, most of the new reflow soldering furnaces are called forced convection hot air reflow soldering furnaces. It uses an internal fan to blow hot air to or around the assembly plate. One advantage of this furnace is that it gradually and consistently provides heat to the assembly plate, regardless of the color and texture of the parts. Although, due to different thickness and component density, the heat absorption may be different, but the forced convection furnace gradually heats up, and the temperature difference on the same PCB is not much different. In addition, the furnace can strictly control the maximum temperature and temperature rate of a given temperature curve, which provides a better zone to zone stability and a more controlled reflux process.

2. Temperature distribution and functions

In the process of hot air reflow welding, the solder paste needs to go through the following stages: solvent volatilization; flux removal of oxide on the surface of weldment; solder paste melting, reflow and solder paste cooling, and solidification. A typical temperature curve (Profile: refers to the curve that the temperature of a solder joint on PCB changes with time when passing through the reflow furnace) is divided into preheating area, heat preservation area, reflow area, and cooling area. (see above)

① Preheating area: the purpose of the preheating area is to preheat PCB and components, achieve balance, and remove water and solvent in solder paste, so as to prevent solder paste collapse and solder spatter. The temperature rise rate shall be controlled within a proper range (too fast will produce thermal shock, such as cracking of multilayer ceramic capacitor, splashing of solder, forming solder balls and solder joints with insufficient solder in the non-welded area of the whole PCB; too slow will weaken the activity of flux). Generally, the maximum temperature rise rate is 4 ℃ / sec, and the rising rate is set as 1-3 ℃ / sec, which is the standard of ECs Is less than 3 ℃ / sec.

② Heat preservation (active) zone: refers to the zone from 120 ℃ to 160 ℃. The main purpose is to make the temperature of each component on the PCB tend to be uniform, reduce the temperature difference as much as possible, and ensure that the solder can be completely dry before reaching the reflow temperature. By the end of the insulation area, the oxide on the solder pad, solder paste ball, and component pin shall be removed, and the temperature of the whole circuit board shall be balanced. The processing time is about 60-120 seconds, depending on the nature of the solder. ECS standard: 140-170 ℃, max120sec;

③ Reflow zone: the temperature of the heater in this zone is set at the highest level. The peak temperature of welding depends on the solder paste used. It is generally recommended to add 20-40 ℃ to the melting point temperature of solder paste. At this time, the solder in the solder paste begins to melt and flow again, replacing the liquid flux to wet the pad and components. Sometimes, the region is also divided into two regions: the melting region and the reflow region. The ideal temperature curve is that the area covered by the “tip area” beyond the melting point of the solder is the smallest and symmetrical, generally, the time range over 200 ℃ is 30-40 sec. The standard of ECS is peak temp.: 210-220 ℃, time range over 200 ℃: 40 ± 3sec;

④ Cooling zone: cooling as fast as possible will help to get bright solder joints with full shape and low contact angle. Slow cooling will lead to more decomposition of the pad into the tin, resulting in gray and rough solder joints, and even lead to poor tin staining and weak solder joint adhesion. The cooling rate is generally within – 4 ℃ / sec, and it can be cooled to about 75 ℃. Generally, forced cooling by cooling fan is required.

3. Various factors affecting welding performance

Technological factors

Welding pretreatment method, treatment type, method, thickness, number of layers. Whether it is heated, cut or processed in other ways during the time from treatment to welding.

Design of welding process

Welding area: refers to the size, gap, gap guide belt (wiring): shape, thermal conductivity, the heat capacity of the welded object: refers to the welding direction, position, pressure, bonding state, etc

Welding conditions

It refers to welding temperature and time, preheating conditions, heating, cooling speed, welding heating mode, carrier form of the heat source (wavelength, heat conduction speed, etc.)

welding material

Flux: composition, concentration, activity, melting point, boiling point, etc

Solder: composition, structure, impurity content, melting point, etc

Base metal: composition, structure and thermal conductivity of base metal

Viscosity, specific gravity and thixotropic properties of solder paste

Substrate material, type, cladding metal, etc.

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