What are the advantages and disadvantages of reflow soldering, and what problems may occur with reflow soldering

What are the advantages and disadvantages of reflow soldering, and what problems may occur with reflow soldering

1.Advantages of reflow soldering

(1)The advantage of this process is that it makes the temperature easy to control, prevents oxidation during the welding process, and makes manufacturing costs easier to control.

(2)It has an internal heating circuit that heats nitrogen to a sufficiently high temperature and blows it towards the circuit board where the components have already been attached, allowing the solder on both sides of the components to melt and bond with the motherboard.

(3)When reflow soldering technology is used for soldering, it is not necessary to immerse the printed circuit board in molten solder, but to complete the soldering task through local heating. Therefore, the soldered components are less susceptible to thermal shock and will not be damaged due to overheating.

(4)Due to the fact that welding technology only requires the application of solder at the welding site and local heating to complete the welding, welding defects such as bridging are avoided.

(5)In reflow soldering technology, the solder is disposable and there is no possibility of reuse. Therefore, the solder is pure and free of impurities, ensuring the quality of the solder joints.

2.Disadvantages of reflow soldering

The temperature gradient is difficult to grasp (the specific temperature range of the four working areas).

3.Introduction to reflow soldering process

Reflow soldering is a complex process for surface mounted boards.

However, in simple terms, it can be divided into two types: single-sided mounting and double-sided mounting.

A. Single sided mounting: pre coated with adhesive paste ->mounting (divided into manual mounting and machine automatic mounting) ->reflow soldering ->inspection and electrical testing.

B. Double sided mounting: A-side pre coated adhesive tape -->patch (divided into manual mounting and machine automatic mounting) -->reflow soldering -->B-side pre coated adhesive tape -->patch -->reflow soldering -->inspection and electrical testing.

The simplest process of reflow soldering is "silk screen solder paste" ->"surface mount" ->"reflow soldering". Its core is the accuracy of silk screen printing, and the yield of surface mount is determined by the PPM of the machine.

Reflow soldering requires controlling the temperature rise, maximum temperature, and temperature drop curves.

4.Possible accidents that may occur during reflow soldering

(1) Bridging

During the heating process of reflow soldering, solder collapse may also occur.

This situation occurs in both preheating and main heating scenarios. When the preheating temperature is within the range of tens to 100 degrees, the solvent, which is one of the components in the solder, will reduce its viscosity and flow out.

If the trend of its outflow is very strong, it will simultaneously squeeze out the gold containing particles outside the welding area. If it cannot return to the welding area during melting, it will also form trapped solder balls.

In addition to the above factors, factors such as whether the SMD component end electrodes are flat and good, whether the circuit board wiring design and solder zone spacing are standardized, the choice of solder mask coating method and its coating accuracy will all be the reasons for bridging.

(2) Monument element floating high (Manhattan phenomenon)

The warping of chip components caused by rapid heating during reflow soldering.

This is because the rapid heating causes a temperature difference between the two ends of the component. After the solder on one side of the electrode is completely melted, it obtains good wetting, while the solder on the other side is not completely melted, causing poor wetting, which promotes the component to stand upright.

Therefore, when heating reflow soldering, it is necessary to consider the time factor to form a balanced temperature distribution in the horizontal direction of heating and avoid the generation of rapid heat in reflow soldering.

(3) Poor wetting

Poor wetting refers to the process of reflow soldering where the solder and the solder area (copper foil) of the circuit substrate or the external electrodes of the SMD do not form a reaction layer with each other, resulting in solder leakage or insufficient soldering.

Most of the reasons are caused by contamination or sticking of solder mask on the surface of the welding area, or the formation of a metal oxide layer on the surface of the bonded material.

For example, the presence of sulfides on the surface of silver and oxides on the surface of tin can lead to poor wetting.

In addition, when the residual aluminum, zinc, cadmium, etc. in the solder exceed 0.005%, the activation degree may be reduced due to the moisture absorption effect of the solder, which can also lead to poor wetting.

Therefore, anti fouling measures should be taken on the surface of the welding substrate and the surface of the components. Select appropriate solder and set a reasonable welding temperature curve for reflow soldering.