SMT (short for Surface Mounted Technology) is the most popular technology and process in electronic assembly industry at present.

What are the characteristics of SMT?

Electronic products have high assembly density, small volume and light weight. The size and weight of the patch component are only about110 of the traditional plug-in. After SMT, the volume of general electronic products is reduced by 40%~60% and the weight is reduced by 60%~80%.

High reliability and strong anti-vibration ability. The defect rate of solder joints is low.

Good high frequency characteristics. Reduce electromagnetic and radio frequency interference.

It is easy to realize automation and improve production efficiency. Reduce the cost by 30%~50%. Save materials, energy, equipment, manpower, time, etc. [Edit this paragraph] Why do you want to pursue miniaturization with SMT electronic products? The number of previously used perforated plug-ins cannot be reduced.

Electronic products are more versatile, and the integrated circuits (ic) used have no perforated components, especially large-scale and highly integrated IC, so surface mount components have to be adopted.

For mass production and automatic production, the factory should produce high-quality products with low cost and high output to meet customer demand and strengthen market competitiveness.

The development of electronic components, integrated circuits (ic) and diversified applications of semiconductor materials.

The revolution of electronic science and technology is imperative to catch up with the international trend. [edit this paragraph] printing of ]SMT basic process elements (or distribution->; mount->； (curing)-> Reflow soldering->; Cleaning-> Detection->; Repair something that is badly repaired.

Printing: Its function is to print solder paste or chip glue on the bonding pad of PCB, so as to prepare for the welding of components. The equipment used is a printing machine (solder paste printing machine), which is located at the forefront of SMT production line.

Glue dispensing: Since most of the circuit boards used now are double-sided patches, in order to prevent the components on the input surface from falling off due to the re-melting of solder paste during the second reflow soldering, a glue dispensing machine is installed on the input surface, which drips glue onto the fixed position of the PCB, and its main function is to fix the components on the PCB. The equipment used is a dispensing machine, which is located at the front end of SMT production line or behind the test equipment. Sometimes, due to the requirements of customers, the output surface also needs dispensing, but now many small factories do not use dispensing machines, so if the input surface components are large, they will use manual dispensing.

Installation: Its function is to accurately install the surface assembly components to the fixed position of PCB. The equipment used is mounter, which is located behind the printing machine of SMT production line.

Curing: Its function is to melt the patch adhesive, so that the surface mount component and PCB board are firmly bonded together. The equipment used is the curing furnace, which is located behind the mounter in the SMT production line.

Reflow soldering: Its function is to melt the solder paste, so that the surface assembly components and PCB board are firmly bonded together. The equipment used is a reflow oven, which is located behind the mounter in the SMT production line.

Cleaning: its function is to remove welding residues harmful to human body on the assembled PCB, such as flux. The equipment used is a cleaning machine, and the location can be not fixed, online or offline.

Inspection: Its function is to inspect the welding quality and assembly quality of the assembled PCB. The equipment used includes magnifying glass, microscope, on-line tester (ICT), flying needle tester, automatic optical inspection (AOI), X-ray inspection system, function tester, etc. This position can be configured in a suitable position on the production line according to the needs of detection.

Rework: Its function is to rework the PCB board whose fault has been detected. Tools used include soldering iron, maintenance workstation, etc. Configured at any position of the production line.

Integrated marketing communication of SMT

IMC is the abbreviation of intermetallic compound, which is translated as "interface alloy" by the author. Broadly speaking, it means that the interface between some metals in close contact with each other will produce an atomic migration interaction behavior, form a layer of "compounds" similar to alloys, and can write the molecular formula. In a narrow sense, the welding field refers to the * * * compound between copper tin, Suk Kim, nickel tin and silver tin. Among them, benign cu6sn 5(η phase) and malignant Cu3Sn(ε phase) between copper and tin are the most common, which have the greatest influence on solderability and solder joint reliability (i.e. solder joint strength). The essence of many papers is specially arranged to explain.

I. Definition

Metals that can be welded with tin-lead alloy solder (or solder solder solder), such as copper, nickel, gold, silver, etc. At high temperature, a thin layer of compound similar to "tin alloy" will quickly form between the solder and the welded metal. This substance originated from the interaction, infiltration, migration and diffusion of tin atoms and welding metal atoms. After cooling and solidification, a thin layer of "* * * compound" appeared immediately, and then it gradually grew and became thicker. The aging degree of this kind of materials is affected by the mutual penetration between tin atoms and bottom metal atoms, which can be divided into several grades. This intermetallic compound formed between solder and its welding metal interface is called IMC for short. This paper only discusses IMC containing tin, and other IMCs will not be involved in depth.

Second, the general nature

Because IMC used to be a quasi-compound whose molecular formula can be written, its properties are very different from those of the original metal, and it also has different degrees of influence on the overall solder joint strength. First of all, its characteristics are briefly described as follows:

◎ IMC only occurs when PCB is welded at high temperature or tin-lead remelting (that is, tinplate is melted or sprayed with tin). IMC has a certain composition and crystal structure, and its growth rate is directly proportional to temperature, and its growth rate is slow at room temperature. It will not stop until an all-lead barrier appears (see Figure 6).

◎ IMC itself is not brittle, which will damage the mechanical strength and life of solder joints, especially the fatigue strength, and its melting point is higher than that of metal.

◎ Because tin atoms in the solder near the interface will gradually move away and form IMC with the metal to be soldered, the amount of tin there will decrease and the proportion of lead will increase relatively, which will lead to the increase of ductility of solder joints, the decrease of fixing strength and even the relaxation of the whole solder body over time.

◎ Once the original molten tin layer or sprayed tin layer of the solder pad has a "thicker" IMC with a too small distance from the bottom copper, it will greatly hinder the welding of the solder pad in the future; That is, solderability or wettability may deteriorate.

◎ Due to the infiltration of tin-copper crystals or tin-silver crystals in solder joints, the hardness of solder itself will also increase, and there will be trouble of embrittlement after a long time.