Detailed introduction to the principle and process of reflow soldering

Reflow soldering is the process of soldering components onto a PCB board, which involves surface mounted components. Reflow soldering relies on the action of hot air flow on the solder joint, and the adhesive flux undergoes physical reactions under a certain high-temperature air flow to achieve SMD welding; The reason why it is called "reflow soldering" is because the gas circulates in the welding machine to generate high temperature and achieve the welding purpose. Below is a detailed introduction to the principle and process of reflow soldering by JEENOCE.

1、 Introduction to the principle of reflow soldering

The welding process in the reflow soldering furnace is divided into four stages. The circuit board with SMT components attached is transported through the reflow soldering furnace guide rail, passing through the preheating zone, insulation zone, welding zone, and cooling zone of the reflow soldering furnace. After passing through the four temperature zones of the reflow soldering furnace, a complete solder joint is formed.

The working principle of the preheating zone of the reflow soldering furnace:

Preheating is a heating behavior carried out to activate the solder paste and avoid rapid high-temperature heating during tin immersion, which may cause component defects. The goal of this area is to heat up the PCB at room temperature as soon as possible, but the heating rate should be controlled within an appropriate range. If it is too fast, it may cause thermal shock, and the circuit board and components may be damaged. If it is too slow, the solvent evaporation will not be sufficient, affecting the welding quality. Due to the fast heating speed, there is a large temperature difference in the reflow soldering furnace in the rear section of the temperature zone. To prevent damage to components caused by thermal shock, a large heating rate of 4 ℃/S is generally specified, and a rising rate of 1-3 ℃/S is usually set.

The working principle of the insulation area of the reflow soldering furnace:

The main purpose of the insulation stage is to stabilize the temperature of various components in the reflow soldering furnace and minimize the temperature difference as much as possible. Give enough time in this area to catch up with the temperature of larger components and ensure that the flux in the solder paste is fully evaporated. At the end of the insulation section, oxides on the solder pads, solder balls, and component pins are removed by the action of the flux, and the temperature of the entire circuit board reaches equilibrium. It should be noted that all components on the SMA should have the same temperature at the end of this section, otherwise entering the reflux section will result in various poor soldering phenomena due to uneven temperatures in each part.

The working principle of the reflow soldering zone in the reflow soldering furnace:

When the PCB enters the reflow zone, the temperature rapidly rises, causing the solder paste to reach a molten state. The melting point of lead solder paste 63sn37PB is 183 ℃, and the melting point of lead solder paste 96.5Sn3Ag0.5Cu is 217 ℃. The temperature of the heater is set high in this area, causing the temperature of the components to rapidly increase. The value temperature of the reflow soldering curve is usually determined by the melting point temperature of the solder, the heat resistance temperature of the assembled substrate and components. The welding temperature in the reflow section varies depending on the type of solder paste used. Generally, the high temperature for lead is between 230-250 ℃, while for lead, it is between 210-230 ℃. Low temperature can easily lead to cold contact and insufficient wetting; If it is too high, the epoxy resin substrate and plastic parts are prone to coking and delamination, and excessive eutectic metal compounds will form, leading to brittle welding points and affecting welding strength. In the reflow soldering area, special attention should be paid to the reflow time not being too long to prevent damage to the reflow soldering furnace, which may also have adverse effects on the performance of electronic components or cause circuit boards to be burnt.

Working principle of the cooling area of the reflow soldering furnace:

At this stage, the temperature is cooled below the solid-phase temperature to solidify the solder joint. The cooling rate will have an impact on the strength of the solder joint. If the cooling rate is too slow, it will lead to the production of excessive eutectic metal compounds and the formation of large grain structures at the welding points, resulting in a decrease in the strength of the welding points. The cooling rate in the cooling zone is generally around 4 ℃/S, and cooling at 75 ℃ is sufficient.

The circuit board that has been coated with solder paste and installed with SMT components is transported through the guide rail of the reflow soldering furnace, and after passing through the four temperature zones above the reflow soldering furnace, it forms a complete soldered circuit board. This is also the entire working principle of the reflow soldering furnace.

2、 Introduction to reflow soldering process

1. Reflow soldering process requirements

Reflow soldering technology is not unfamiliar in the field of electronic manufacturing. The components on various boards used in our computers are soldered to circuit boards through this process. The advantage of this process is that the temperature is easy to control, oxidation can be avoided during the welding process, and manufacturing costs are also easier to control. This device 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.

1. It is necessary to set a reasonable reflow soldering temperature curve and regularly conduct real-time testing of the temperature curve.

2. Welding should be carried out according to the welding direction specified in the PCB design.

3. Strictly prevent vibration of the conveyor belt during the welding process.

4. It is necessary to inspect the welding effect of the block printed board.

5. Whether the welding is sufficient, whether the surface of the solder joint is smooth, whether the shape of the solder joint is half moon shaped, the situation of solder balls and residues, and the situation of continuous welding and virtual welding. Also, check for color changes on the surface of the PCB. Adjust the temperature curve based on the inspection results. During the entire production process, welding quality should be regularly checked.

2. Factors affecting reflow soldering process:

a. Usually, PLCC, QFP and individual discrete sheet metal components have a larger heat capacity, making it more difficult to weld large-area components than small ones.

b. In the reflow soldering furnace, the conveyor belt not only performs reflow soldering on the conveyed products, but also becomes a heat dissipation system. In addition, the heat dissipation conditions at the edges and centers of the heating part are different, and the temperature at the edges is generally lower. In addition to different temperature requirements in each temperature zone, the temperature on the same loading surface also varies in the furnace.

c. The impact of different product loading capacities. The adjustment of the temperature curve for reflow soldering should consider achieving good repeatability under no-load, load, and different load factors. The load factor is defined as: LF=L/(L+S); Where L=length of assembled substrate, S=spacing of assembled substrate. The greater the load factor, the more difficult it is to achieve good repeatability in reflow soldering process. The typical range of high load factor for reflow soldering furnaces is 0.5~0.9. This depends on the product situation (component welding density, different substrates) and the different models of the reflow furnace. To achieve good welding results and repeatability, practical experience is crucial.

3. What are the advantages of reflow soldering technology

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

b. 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.

c. In reflow soldering technology, the solder is only used for secondary purposes and there is no possibility of reuse. Therefore, the solder is very clean and free of impurities, ensuring the quality of the solder joints.

4. Introduction to reflow soldering process flow

The reflow soldering process involves surface mounted boards, which have a complex process and can be divided into two types: single-sided mounting and double-sided mounting.

a. Single sided mounting: pre coated solder paste → mounting (divided into manual mounting and machine automatic mounting) → reflow soldering → inspection and electrical testing.

b. Double sided mounting: A-side pre coated solder paste → SMT (divided into manual mounting and machine automatic mounting) → Reflow soldering → B-side pre coated solder paste → SMT (divided into manual mounting and machine automatic mounting) → Reflow soldering → Inspection and electrical testing.

The simple process of reflow soldering is "screen printing solder paste patch reflow soldering, the core of which is the accuracy of screen printing. The yield of the patch is determined by the PPM of the machine, and reflow soldering is to control the temperature rise, high temperature, and low temperature curves."