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PCBA Tech
SMT processing important materials and solder joint quality
PCBA Tech
SMT processing important materials and solder joint quality

SMT processing important materials and solder joint quality

2021-11-10
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Author:Downs

1. SMT patch processing technology solder paste

Solder paste is a paste formed by uniformly stirring alloy solder powder and paste flux. It is an indispensable soldering material in the SMT chip processing process. It is widely used in reflow soldering. The solder paste has a certain degree of stability at room temperature. Viscosity, it can initially stick the electronic components in the predetermined position. At the soldering temperature, as the solvent and some additives volatilize, the soldered components and the PCB will be interconnected to form a permanent connection.

At present, most SMT patch processing plants apply the solder paste method by using the silk screen omission printing method, which has the advantages of simple operation and immediate use after rapid printing. However, there are also defects such as difficulty in ensuring the reliability of solder joints, easy to cause false soldering, waste of solder paste, and high cost.

pcb board

2. SMT patch processing technology patch adhesive

Patch glue, also known as SMT adhesive, SMT red glue, usually red (also yellow or white) paste is uniformly distributed with hardeners, pigments, solvents and other adhesives, mainly used to The device is fixed on the printed circuit board, and is generally distributed by dispensing or stencil printing. After the components are pasted, they are placed in an oven or reflow oven to heat and harden.

The difference between the patch adhesive and the solder paste is that it solidifies after being heated, and its freezing point temperature is 150 ℃, and it will not melt after heating. That is to say, the thermal hardening process of the patch adhesive is irreversible. The use effect of SMT patch adhesive will be different due to different thermal curing conditions, connected objects, equipment used, and operating environment. When using it, you should choose the patch adhesive according to the printed circuit board assembly (PCBA, PCA) process. .

3. Flux for SMT chip processing technology

Flux is the carrier of tin powder, and its composition is basically the same as that of general flux. In order to improve the printing effect, it is sometimes necessary to add an appropriate amount of solvent. Through the action of the active agent in the flux, it can eliminate the surface of the soldered material and the tin powder itself. Oxide, make the solder quickly diffuse and adhere to the surface of the metal to be soldered. The composition of the flux plays a decisive role in the expansibility, wettability, collapse, viscosity change, cleanability and shelf life of the solder paste.

SMT chip processing solder joint quality

1. Judgment of Welding

1. Use online tester professional equipment for testing.

2. Visual inspection or AOI inspection. When you find that the solder joints have too little solder material, the solder penetration is poor, or there are cracks in the middle of the solder joints, or the surface of the solder is convex spherical, or the solder does not melt with the SMD, etc., you need to pay attention to it, even if it is a slight condition Causes hidden dangers, it should be immediately judged whether there is a lot of false welding problem. The method of judgment is: to see if there are many solder joints at the same position on the PCB, such as the problem on some PCBs, it may be due to scratching of the solder paste, pin deformation, etc., such as on many PCBs. There is a problem with the same position. At this time, it is likely to be caused by a bad component or a problem with the pad.

Second, the cause of the virtual welding and its solution

1. The pad design is defective. The existence of through-holes in the pads is a major disadvantage of PCB design. If it is not necessary, it is not necessary to use them. The through-holes will cause the loss of solder and lead to insufficient soldering materials. The spacing and area of the pads also need to be standardized, otherwise the design should be corrected as soon as possible.

2. The PCB board is oxidized, that is, the pad is black and does not shine. If there is oxidation, you can use an eraser to remove the oxide layer to make it bright again. If the PCB board is damp, it can be dried in a drying box if suspected. The PCB board has oil stains, sweat stains and other pollution, so use absolute ethanol to clean it up.

3. For PCBs that have been printed with solder paste, the solder paste is scraped and rubbed, which reduces the amount of solder paste on the relevant pads and makes the soldering material insufficient. Should be added immediately. You can use a dispenser or use a bamboo stick to pick a little supplement.