What Does a Solderer Do?
Solder is an important industrial raw material for connecting electronic components in soldering circuits. It is a solder with a lower melting point, which mainly refers to solder made of tin-based alloys. The manufacturing method of the solder is to first make an ingot by the melting method, and then press-process it into a material.
- Solder materials are essential in the production and maintenance of the electronics industry. Generally speaking, commonly used solder materials are tin-lead alloy solder, antimony-added solder, cadmium-added solder, silver-added solder, and copper-added solder.
- The wire solder used in standard soldering operations is called
- Tin (Sn) is a common low melting point metal, non-toxic, good ductility, physical and chemical properties are quite stable at room temperature, can resist organic acid corrosion, and form compounds with many metals. Traditional solders mostly use tin-lead as the main component. This material has become a classic product with its low melting point, good soldering performance and favorable price. However, the heavy use of lead will bring great harm to human beings and the environment. Japan, the European Union, the United States, China and other countries and regions are actively legislating to limit the use of lead.
- The research of solder powder at home and abroad mainly focuses on the particle size and size distribution, oxidation resistance, different alloying ratios of various alloys, and the effect of adding trace elements. For the flux, it mainly focuses on the composition and its ratio, expansion rate, viscosity, and corrosion.
- 1.High temperature lead-free solder paste
- 1) Sn-Ag (-Cu) system
- The Sn-Ag eutectic component is Sn3.5Ag, and the eutectic point is 221 ° C. The most mature of this series is the Sn-Ag-Cu (SAC) series. Sn3Ag0.5Cu (SAC305) is a classic product of this series. This alloy has excellent physical properties and high temperature stability, and its joint strength after soldering is the same as or even higher than that of traditional tin-lead eutectic solder. When starting lead-free, most manufacturers will choose SAC305. As the price of Ag continues to rise, institutions are working to study low-silver solder pastes containing Ag below 1%.
- In 2004, SAC solder containing 0% to 8% Ag and 0% to 5% Cu. This solder can meet the reflow temperature but cannot eliminate the monument effect. In 2006, Sn-Ag-Cu-P solder with Ag of 0.3% to 0.4%, its statement has the same solderability, conductivity, mechanical properties as SAC305, and can reduce the toxicity problem caused by the reaction between Ag and acid-base . In 2010, a Sn-Ag-based solder with an Ag content of 0.2% to 1.0% was added with trace amounts of Sb, Cu or Ni, Co, Fe, Mn, Cr, Mo or P, Ga, Ge. Adding these elements can increase its mechanical strength, but if the amount is too high, the liquidus temperature of the alloy will also increase.
- The composition of low silver solder paste developed by Koki is Sn0.1Ag0.7Cu0.03Co, and the melting point is 217 ~ 227 . The reduction in the amount of silver added reduces the volatility of the product market. Cobalt can prevent the tissue change caused by thermal cycling, can keep the tissue dense, and inhibit the segregation and aggregation of aging intermetallic compounds. The cost is reduced by 10% to 20% compared with SAC305. Genma's low-cost silver-free solder has a composition of Sn0.7Cu0.03Ni0.01Co0.005Ge and a melting temperature of 226 ~ 228 ° C. Adding nickel and cobalt can improve the strength and reliability of the solder, and its cost is 54% lower than SAC305 .
- 2) Sn-Cu series
- The eutectic composition of Sn-Cu is Sn0.7Cu, and the eutectic temperature is as high as 227 ° C. Cu and Sn alloys are dispersed in Sn in the form of two intermetallic compounds Cu 6 Sn 5 () and Cu 3 Sn (). Cu 6 Sn 5 is a benign alloy layer with spherical crystals and high strength, which is the fundamental guarantee for the electrical contact performance and strength of the solder joint. Cu 3 Sn is a poor alloy layer, which is located between the copper layer and Cu 6 Sn 5 Bone needle-like crystals, brittleness, directly affect the electrical contact and strength properties of the solder joint, and cause non-wetting.
- At present, the modification of this alloy system is mainly reflected in the addition of a small amount of Ni, Ag, Bi, etc., to increase the fluidity of the solder and its mechanical properties. The advantage of this solder is that it is cheap and can inhibit the leaching of the Cu layer of the PCB pad during solder work.
- 3) Sn-Sb series
- Sn-Sb alloys have a narrow melting range of 240 ~ 250 ° C. The mainstream alloy ratios are: Sn 5 Sb and Sn 10 Sb. Sn 5 Sb is considered as a possible alternative to Sn 40 Pb. Its wetting angle is 35 ° ~ 55 °, which is wider than Sn 40 Pb's 20 ° ~ 35 ° range, and has good shear strength at 100 ° C. The structure of Sn 5 Sb at room temperature is composed of -Sn with body-centered tetragonal structure and -Sn-Sb with face-centered cubic structure. With the increase of Sb content, the Sn-Sb phase particles precipitated in the Sn matrix, and its mechanical properties also improved.
- 2.Medium temperature lead-free solder
- 1) Sn-Bi-Ag (Cu) system
- The eutectic composition of the Sn-Bi-Ag alloy is Sn58Bi0.3Ag. Hiroshi Ohtani et al. Found that the melting point is close to that of Sn-Bi alloy, and when the Bi content is close to 50%, the temperature interval between the solid-liquid phase region is less than 10 ; when the Ag content is greater than 2%, the intermetallic compound Ag 3 Sn is formed. Sebo et al. Studied Sn100-x Bi10Agx (x = 3% -10%) solder and found that the Cu 3 Sn layer exists only at the Cu-based interface, Cu 6 Sn 5 exists at the matrix interface and inside the solder, and Ag only exists at Ag 3 Sn. The change of Ag content will not significantly change the microstructure of the joints, but the size of Ag 3 Sn will grow with the increase of Ag; the thickness of the Cu 3 Sn layer will decrease with the increase of Ag. Since Ag is a precious metal, a small amount of Cu is added instead of Ag.
- 2) Sn-Zn series
- The Sn-Zn eutectic (Sn 9 Zn) point is 199 ° C, 20 ° C lower than the SAC eutectic temperature (217 ° C), 15 ° C higher than the Sn-Pb (183 ° C), and the melting point is closest to the melting point of Sn-Pb. alloy. Under normal circumstances, copper is used as the under bump metal (UBM) of the welding part, and the use of Sn-Zn alloy will generate a metal compound of zinc and copper. This metal compound will soften under high temperature and high humidity environment, and the connection strength will be Weakened, but as long as nickel or gold plating is used on the metal under the bump, such problems can be solved. In addition, due to the shortcomings of high Zn activity, easy oxidation, and difficult to control process conditions, it is currently not widely used in China, and has a bright future application prospect.
- 3. Low-temperature lead-free solder
- Sn-Bi system
- The eutectic point of Sn-Bi alloy (Sn 58 Bi) is 139 , and the mounting temperature using this solder can be lowered below 200 . Sn-Bi lead-free solder has the advantages of low melting point and good wettability. Sn 58 Bi eutectic alloy has been used in motherboard packaging for more than 20 years. However, due to its easy segregation and easy peeling of welded joints, its application scope is limited.
- 4.Flux
- The flux is mainly composed of an active agent (rosin, organic halide, etc.), a surfactant (mainly an efficient surfactant such as alkane or fluorocarbon) and a solvent. In addition to the above components, fluxes often add different additives according to specific requirements, such as film-forming agents, antioxidants, corrosion inhibitors, matting agents, flame retardants, thixotropic agents, etc.
- At present, the flux used in the production of SMT soldering process is divided into three types: solvent cleaning type, water cleaning type and no-cleaning type according to its subsequent cleaning process. [2-3]
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