Introduction to the principle and process of reflow soldering

(1) Principle of reflow soldering

Due to the continuous miniaturization of electronic product PCB boards, chip components have appeared, and traditional welding methods have been unable to meet the needs. Reflow soldering is used in the assembly of hybrid integrated circuit boards, and most of the components assembled and welded are chip capacitors, chip inductors, mounted transistors and diodes. With the development of the entire SMT technology becoming more and more perfect, the emergence of a variety of chip components (SMC) and mounting devices (SMD), the reflow soldering process technology and equipment as part of the mounting technology have also been developed accordingly, and their applications are becoming more and more extensive. It has been applied in almost all electronic product fields. Reflow soldering is a soft solder that realizes the mechanical and electrical connection between the solder ends of surface mounted components or the pins and the printed board pads by remelting the paste-loaded solder that is pre-distributed on the printed board pads. weld. Reflow soldering is to solder components to the PCB board, and reflow soldering is to mount devices on the surface. Reflow soldering relies on the action of hot air flow on solder joints, and the jelly-like flux undergoes physical reaction under a certain high temperature air flow to achieve SMD soldering; so it is called “reflow soldering” because the gas circulates in the welding machine to generate high temperature to achieve the purpose of soldering. .

(2) The principle of reflow soldering machine is divided into several descriptions:

A. When the PCB enters the heating zone, the solvent and gas in the solder paste evaporate. At the same time, the flux in the solder paste wets the pads, component terminals and pins, and the solder paste softens, collapses, and covers the solder paste. plate to isolate pads and component pins from oxygen.

B. When the PCB enters the heat preservation area, the PCB and components are fully preheated to prevent the PCB from suddenly entering the high temperature area of ​​welding and damaging the PCB and components.

C. When the PCB enters the welding area, the temperature rises rapidly so that the solder paste reaches a molten state, and the liquid solder wets, diffuses, diffuses, or reflows the pads, component ends and pins of the PCB to form solder joints.

D. The PCB enters the cooling zone to solidify the solder joints; when the reflow soldering is completed.

(3) Process requirements for reflow soldering machine

Reflow soldering technology is not unfamiliar in the field of electronic manufacturing. Components on various boards used in our computers are soldered to circuit boards through this process. The advantages of this process are that the temperature is easy to control, oxidation can be avoided during the soldering process, and the manufacturing cost is easier to control. There is a heating circuit inside this device, which heats nitrogen gas to a high enough temperature and blows it to the circuit board where the components have been attached, so that the solder on both sides of the components is melted and then bonded to the motherboard.

1. Set a reasonable reflow soldering temperature profile and do real-time testing of the temperature profile regularly.

2. Weld according to the welding direction of PCB design.

3. Strictly prevent the conveyor belt from vibrating during the welding process.

4. The welding effect of a printed board must be checked.

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, the situation of solder balls and residues, the situation of continuous welding and virtual welding. Also check the PCB surface color change and so on. And adjust the temperature curve according to the inspection results. Welding quality should be checked regularly throughout the production run.

(4) Factors affecting the reflow process:

1. Usually PLCC and QFP have larger heat capacity than discrete chip components, and it is more difficult to weld large-area components than small components.

2. In the reflow oven, the conveyor belt also becomes a heat dissipation system when the conveyed products are reflowed repeatedly. In addition, the heat dissipation conditions at the edge and the center of the heating part are different, and the temperature at the edge is low. In addition to different requirements, the temperature of the same loading surface is also different.

3. The influence of different product loadings. The adjustment of the temperature profile of reflow soldering should take into account that good repeatability can be obtained under no-load, load and different load factors. The load factor is defined as: LF=L/(L+S); where L=the length of the assembled substrate and S=the spacing of the assembled substrate. The higher the load factor, the more difficult it is to obtain reproducible results for the reflow process. Usually the maximum load factor of the reflow oven is in the range of 0.5~0.9. This depends on the product situation (component soldering density, different substrates) and different models of reflow furnaces. Practical experience is important to obtain good welding results and repeatability.

(5) What are the advantages of reflow soldering machine technology?

1) When soldering with reflow soldering technology, there is no need to immerse the printed circuit board in molten solder, but local heating is used to complete the soldering task; therefore, the components to be soldered are subject to little thermal shock and will not be caused by overheating damage to components.

2) Since the welding technology only needs to apply solder on the welding part and heat it locally to complete the welding, welding defects such as bridging are avoided.

3) In the reflow soldering process technology, the solder is only used once, and there is no reuse, so the solder is clean and free of impurities, which ensures the quality of the solder joints.

(6) Introduction to the process flow of reflow soldering machine

The reflow soldering process is a surface mount board, and its process is more complicated, which can be divided into two types: single-sided mounting and double-sided mounting.

A, single-sided mounting: pre-coating solder paste → patch (divided into manual mounting and machine automatic mounting) → reflow soldering → inspection and electrical testing.

B, Double-sided mounting: Pre-coating solder paste on A side → SMT (divided into manual placement and automatic machine placement) → Reflow soldering → Pre-coating solder paste on B side → SMD (divided into manual placement and machine automatic placement) placement) → 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 silk screen printing, and the yield rate is determined by the PPM of the machine for patch soldering, and reflow soldering is to control the temperature rise and high temperature. and the decreasing temperature curve.”

(7) Reflow soldering machine equipment maintenance system

Maintenance work that we must do after reflow soldering is used; otherwise, it is difficult to maintain the service life of the equipment.

1. Every part should be checked daily, and special attention should be paid to the conveyor belt, so that it cannot be stuck or fallen off

2 When overhauling the machine, the power supply should be turned off to prevent electric shock or short circuit.

3. The machine must be stable and not tilted or unstable

4. In the case of individual temperature zones that stop heating, first check that the corresponding fuse is pre-distributed to the PCB pad by remelting the paste

(8) Precautions for reflow soldering machine

1. In order to ensure personal safety, the operator must take off the label and ornaments, and the sleeves should not be too loose.

2 Pay attention to high temperature during operation to avoid scald maintenance

3. Do not arbitrarily set the temperature zone and speed of reflow soldering

4. Ensure that the room is ventilated, and the fume extractor should lead to the outside of the window.

Post time: Sep-07-2022