What is the process flow of reflow soldering?

Reflow soldering refers to a welding process that involves melting the solder paste pre printed on the PCB pad to achieve mechanical and electrical connections between surface assembled component solder joints or pins and the PCB pad.

1. Process flow

The process flow of reflow soldering: printing solder paste → SMT → reflow soldering.

2. Process characteristics

The size of solder joints is controllable. The desired solder joint size or shape requirements can be obtained through the size design of the solder pads and the amount of solder paste printed.

The application of solder paste is generally done through steel mesh printing. In order to simplify the process flow and reduce production costs, solder paste is usually only printed once on each welding surface. This feature requires that each component on the assembly surface can use a steel mesh (including steel mesh of the same thickness and stepped steel mesh) for solder paste distribution.

The reflow soldering furnace is actually a tunnel furnace with multiple temperature zones, and its main function is to heat PCBA. The components laid out on the bottom (B side) should meet certain mechanical requirements, such as BGA packaging, with a requirement of a component mass to pin contact area ratio of ≤ 0.05mg/mm2 to prevent the top components from falling off during welding.

When reflow soldering, the components are completely floating on the molten solder (solder joints). If the solder pad size is larger than the pin size, the component layout is heavier, and the pin layout is fewer, it is easy to shift under the surface tension of asymmetric molten solder or the blowing of forced convection hot air in the reflow soldering furnace.

Generally speaking, for components that can self correct their position, the larger the proportion of the solder pad size and the overlap area between the solder ends or pins, the stronger the positioning function of the component. We use this point to design specific pads for components with positioning requirements.

The formation of weld (spot) morphology mainly depends on the wetting ability and surface tension of the molten solder, such as 0.44mmQFP, and the printed solder paste pattern is a regular rectangular shape.

2、 Basic requirements for welding

No matter what welding technology we use, we should ensure that the basic welding requirements are met to ensure good welding results. High quality welding should meet the following 5 basic requirements.

1. Appropriate heat;

2. Good wetting;

3. Appropriate solder joint size and shape;

4. Controlled tin flow direction;

During the welding process, the welding surface does not move.

Appropriate heat refers to the requirement for all welding materials to have sufficient heat energy to melt and form intermetallic interfaces (IMCs), which is also one of the basic conditions for providing wetting. On the other hand, the heat must be controlled to a certain extent to ensure that the material in contact (not just the welding end) is not damaged by heat, and the formation of the IMC layer is not too thick (Note 2).

Wetting is not only a symbol of good weldability, but also an important condition for forming the final shape of the solder joint. Poor wetting phenomenon usually indicates that the structure of the solder joint is not ideal, including incomplete formation of IMC and poor filling of the solder joint. These issues will all affect the lifespan of solder joints.

To ensure sufficient lifespan of the solder joint, it is necessary to ensure that the shape and size of the solder joint meet the requirements of the welding end structure. A solder joint that is too small has insufficient mechanical strength to withstand the stress during use, and even the internal stress that exists after welding cannot be withstood. Once fatigue or creep cracking begins to occur during use, its fracture rate is also relatively fast. The poor shape of the solder joint can also lead to the phenomenon of choosing the lighter one over the heavier one, shortening the lifespan of the solder joint.

The controlled direction of tin flow is also an important part of the welding process. Melted solder must flow in the desired direction to ensure controlled formation of solder joints. The use of stolen solder pads and solder mask layers (green oil) in wave soldering process, as well as the phenomenon of tin absorption in reflow soldering process, are technical details related to the control of tin flow direction.

If the welding end moves during the welding process, it depends on the situation and time of movement, which not only affects the shape and size of the welding point, but also may cause virtual welding and inner hole situations. This will affect the quality and lifespan of the solder joints. So the design and process of the entire product must take into account that the welding end remains in a stationary state during the welding process.

In the reflow soldering process, in addition to the general welding conditions mentioned above, there is also a special point that the chemical components in the solder paste that have not worked after the printing process must be promptly evaporated. This is especially true in the double-sided welding process where the requirements for the first side are more stringent.