Welding knowledge: reflow technical index and defect analysis and treatment

Date：2022-02-28 15:21:45Views：1450

Reflow soldering is to realize the mechanical and electrical connection between the solder end or pin of surface assembled components and the PCB pad by remelting the paste solder pre allocated to the PCB pad. Reflow soldering technology is no stranger in the field of electronic manufacturing. The components on various boards and cards used in our computer are welded to the circuit board through this process. There is a heating circuit inside the equipment, which heats the air or nitrogen to a sufficiently high temperature and blows it to the circuit board with the components pasted, so that the solder on both sides of the components melts and bonds with the main board. Next, let's introduce the technical indicators and defect analysis and treatment of circuit board reflow soldering. Let's have a look.

Main technical indexes of reflow soldering:

1. Temperature control accuracy of reflow welder: it shall reach ± 0.1-0.2 ℃.

2. Transverse temperature difference of conveyor belt of reflow welding machine: the traditional requirement is less than ± 5 ℃, and the lead welding requirement is < ± 2 ℃.

3. Temperature curve test function of reflow welder: if the reflow welder does not have this configuration, the temperature curve collector shall be purchased.

4. The maximum heating temperature of reflow welder is generally 210-235 ℃. If lead-free solder or metal substrate is considered, it should be more than 250 ℃.

5. Number and length of heating zone of reflow welding machine: the longer the length of heating zone and the more the number of heating zones, the easier it is to adjust and control the temperature curve. For lead-free welding, the temperature zone above 7 shall be selected.

6. Width of conveyor belt of reflow welding machine: it shall be determined according to the size of large and small PCB.

7. Cooling efficiency of reflow welding machine: it shall be determined according to the complexity and reliability requirements of products. For products with complex and high reliability requirements, high cooling efficiency shall be selected.

Analysis of reflow defects in circuit board welding：

1. The wettability of reflow soldering is poor, which is reflected in the poor tin consumption of PCB pads or component pins.

Cause: the PCB pad of component pin has been oxidized and polluted; Excessive reflux temperature; The quality of solder paste is poor. It will lead to poor wettability and false welding in serious cases.

2. The amount of tin after reflow soldering is small, which is manifested in that the solder joint is not full and the root meniscus of IC pin is small.

Causes: the printing template window is small; Wick phenomenon (temperature curve difference); The metal content of solder paste is low. These will lead to small amount of tin and insufficient solder joint strength.

3. After reflow soldering, the pin is damaged, which is reflected in the poor coplanarity or bending of the device pin, which directly affects the welding quality.

Cause: damaged during transportation / taking and placing. For this reason, components and parts, especially fqfp, should be kept carefully

4. After reflow soldering, the pollutants cover the solder paste, which occurs from time to time in production.

Causes: paper pieces from the site, foreign matters from the winding tape, touching PCB pads or components by hand, and wrong position of character printing drawing. Therefore, during production, attention should be paid to the cleanness of the production site and the process should be standardized

5. Insufficient solder paste after reflow soldering is a common phenomenon in production.

Cause: PCB Printing / printing after the machine stops; Change of printing process parameters; The steel plate window is blocked; The quality of solder paste has deteriorated. The above reasons will lead to insufficient tin, so the problem should be solved.

6. After reflow soldering, the solder paste is angular, which often occurs in production and is not easy to find. If it is serious, it will be welded continuously.

Cause: the screen lifting speed of the printing press is too fast; The hole wall of the template is not smooth, which is easy to make the solder paste treasure.

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