What is reflow soldering? Introduction to its principle and process

Reflow soldering is a common surface mount technology used to solder surface mount components (SMDs) onto printed circuit boards (PCBs) in electronic product manufacturing. The process of reflow soldering is to place the mounted components on the PCB, and then send the entire assembly into the reflow furnace for heating, melting the solder paste and soldering the mounted components onto the PCB.

1、 Introduction to reflow soldering principle

Reflow soldering technology is not unfamiliar in the field of electronic manufacturing. The components on various boards used in our computers are soldered to the circuit board through this process. There is a heating circuit inside this device that heats air or 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. 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.

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. This device has an internal heating circuit that heats air or 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.

2、 Reflow soldering process

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.

1. Single sided mounting

Pre coated solder paste → SMT (divided into manual and machine automatic SMT) → reflow soldering → inspection and electrical testing.

2. Double sided mounting

A-side pre coated solder paste → SMT (divided into manual and machine automatic SMT) → reflow soldering → B-side pre coated solder paste → SMT (divided into manual and machine automatic SMT) → reflow soldering → inspection and electrical testing.

3、 Development of reflow soldering technology

1. Hot plate and push plate hot plate conduction reflow soldering:

This type of reflow soldering furnace relies on a heat source under the conveyor belt or push plate to heat up the components on the substrate through thermal conduction. It is used for single-sided assembly of thick film circuits using ceramic (Al2O3) substrates. Only by sticking the ceramic substrate on the conveyor belt can sufficient heat be obtained. Its structure is simple and the price is cheap. Some thick film circuit factories in our country introduced such equipment in the early 1980s.

2. Infrared radiation reflow soldering:

This type of reflow soldering furnace is also mostly conveyor belt type, but the conveyor belt only serves as a support and conveying substrate. Its heating method mainly relies on infrared heat sources to heat in a radiation manner. The temperature in the furnace is more uniform than the previous method, with larger mesh holes, suitable for reflow soldering heating of double-sided assembled substrates. This type of reflow soldering furnace can be said to be the basic type of reflow soldering furnace. It is widely used in our country and the price is relatively cheap.

3. Infrared heating air (Hot air) reflow soldering:

This type of reflow soldering furnace is based on the IR furnace, and hot air is added to make the temperature inside the furnace more uniform. When using infrared radiation heating alone, people find that in the same heating environment, different materials and colors absorb heat differently, that is, the Q value in equation (1) is different, resulting in a temperature rise Δ T is also different, for example, the packaging of SMDs such as ICs is black phenolic or epoxy, while the lead wire is white metal. When heated alone, the temperature of the lead wire is lower than its black SMD body. Adding hot air can make the temperature more uniform, while overcoming differences in heat absorption and poor shading, IR+Hot air reflow soldering furnaces have been widely used internationally.

4. Nitrogen filled (N2) reflow soldering:

With the increase of assembly density and the emergence of Fine pitch assembly technology, nitrogen filled reflow soldering technology and equipment have emerged, improving the quality and yield of reflow soldering, and have become the development direction of reflow soldering. Nitrogen reflow soldering has the following advantages:

(1) Prevent and reduce oxidation

(2) Improve welding wetting force and accelerate wetting speed

(3) Reduce the generation of solder balls, avoid bridging, and achieve the listed welding quality

4、 Knowledge related to reflow soldering

1. Reflow soldering preheating

The purpose of this area is to heat up the PCB at room temperature as soon as possible to achieve the second specific goal, but the heating rate should be controlled within an appropriate range. If it is too fast, it may cause thermal shock and damage to the circuit board and components; If it is too slow, the solvent evaporation is not sufficient, which affects the welding quality. Due to the fast heating speed, there is a large temperature difference in the later stage of SMA in the temperature range. To prevent damage to components caused by thermal shock, the maximum speed is generally set at 4 ℃/s. However, the usual rate of increase is set at 1-3 ℃/s. The typical heating rate is 2 ℃/s.

2. Reflow soldering insulation

The insulation section refers to the area where the temperature rises from 120-150 ℃ to the melting point of the solder paste. Its main purpose is to stabilize the temperature of various components within SMA and minimize temperature differences 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, 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.

3. Reflow soldering reflow

The temperature of the heater is set to the highest in this area, causing the temperature of the components to quickly rise to the peak temperature. The peak welding temperature in the reflow section varies depending on the type of solder paste used, and it is generally recommended to add 20-40 ℃ to the melting point temperature of the solder paste. For solder paste with a melting point of 183 ℃ and Sn62/Pb36/Ag2 solder paste with a melting point of 179 ℃, the peak temperature is generally between 210-230 ℃, and the reflow time should not be too long to prevent adverse effects on SMA. The ideal temperature curve is the minimum area covered by the "tip zone" that exceeds the melting point of the solder.