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Application of environmentally friendly coating layer
 
Application of environmentally friendly coating layer

The application of environmentally friendly coating is the world's environmental requirements, as centuries of human existence and development, production and use of a lot of damage to the natural substances that are the result of the rapid development of industrial production, due to the lack of human nature knowledge and understanding of the ecological balance of nature, suffered great damage, leading to global climate anomalies, human beings are often faced with difficulties. With the progress of human science to explore the natural law of development, gradually deepening understanding of nature, full use of high-tech means, for the systematic destruction of nature to improve the ecological balance, so that the face of natural reproduction. Is that bore the brunt of the industrial printed circuit harmful substances used in industry, in particular surface treatment in hot air leveling materials contain lead solder dip. According to lead-free materials, electronic products tend to the needs, we must reform the current lead technology, Application of environmentally friendly coating layer. From the theoretical analysis of metal, it is best to use tin instead of chemical HASL process, a large number of technology test proved to be feasible. Lead-free components of plating (coating), many different types, such as OSP (organic solder protective film to help), palladium plating, electroless nickel / immersion gold, electroless nickel, palladium / immersion gold, immersion tin. Through research and technology test results indicate that tin plating process in which cheap, easy preparation, and production process is stable and reliable characteristics of thin wires which have obvious advantages in applied, compared with development potential.

One. Tin tin chemical reaction principle of chemical reaction mechanism is the chemical replacement reaction, in essence, the electrochemical reaction, it is replaced by divalent copper tin process, so that the copper in solution dissociate into divalent copper ions, while the release of two electrons, bivalent tin is reduced by electrons of tin metal deposition on the surface of the substrate to form copper tin coating, the reaction is as follows:
Sn2 + + Cuo → Sno + Cu2 +
Cuo → Cu2 + +2 e-
Sn2 + +2 e-→ Sno

II. Chemical bath dip tin types and the types of chemical reducing agents are more basic, there are three or methyl sulfonic acid tin plating, tin sulfate and tin fluoroborate. According to technology test proved that tin-series solution of methanesulfonate more applications, but also with environmental requirements together, using the reducing agent is thiourea [(NH2) 2CS] and role of its main features, is to reduce the tin bath potential increase the reaction rate, soluble in cold water, sulfur, ammonium cyanide and alcohol, melting point 180-182 ℃, and the time at 150-160 ℃ in vacuum to sublimate, accounting for 1.406, shiny white crystals. Stannous sulfate bath in which the PH value of less than 1, the temperature from room temperature to 40 ℃, dipping time 1-2 minutes.

III. Characteristics of the coating application can be the greatest advantage of the dip coating to ensure the smoothness of the surface for welding many times, to ensure the accuracy and stability of the device size of the installation role. Especially for high-density, high-precision, thin wires, narrow pad and the small distance between the surface treatment of printed circuit boards. Therefore, many products such as surface mount with plates, multi-chip module with a board surface treatment requirements is to ensure that micro-pad surface roughness and reflow reliability and stability, chemical tinning is the most short-tailed bird of choice.

IV. Reliability testing of the treated substrate surface tin layer graphics, subject to weldability, adhesion with the solder mask layer and the flux of the applicable performance require quantitative and qualitative tests. According to MIL-STD-883 soldering performance testing part, IPC-SM-840C solder layer adhesion test part, IPC/J-STD-004 flux part.
(1) The solder float test methods:
IPC norms in the welding of tin layer there are five methods for measuring the performance of which edge wetting test, swing wetting test, solder float test, wave soldering and soldering test test scales. Provides detailed test procedures and test methods and requirements specific to the technical data and requirements.
Tin test usually drift (Solder Float Test) test performance of the tin layer more welding, the ultimate goal of testing is to ensure that when the reflow soldering electronic equipment reliability and stability. In particular parts of the device solder solder solder foot capacity is especially important. For this reason, must be welded in the factory performance test, the majority of manufacturers have this type of equipment, and develop test procedures, test methods of use of such product or test sample products tested in order to provide the device after installation welding process parameters (temperature, velocity and time) to set a reference.
(2) expansion of molten tin test (Solder Spread Test)
According to the different types of surface treatment, the composition of solder balls were also different (Sn / Pb, Sn / Ag / Cu etc) expansion rates are different, the chemical surface treatment of the expansion rate of tin requires 5 more before the solder has superior ability to .
(3) welded ball shear test method The test method is to printed circuit board through heat treatment, uniform coating layer of flux, and then place the ball further welding, after washing and drying the substrate after the fusion ball shear test.
When the ball weld after welding, bonding in the bottom of the full treatment by chemical tin pad surface, the sidewall and the solder layer surface  S / M) phase, and then after washing and drying the shear test to the number of residual solder determine its class, proved the greater the volume of the surface residues that chemical tin solder layer of solder ability to handle the better.

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