Introduction of plastic electroplating-tin-copper alloy electroplating technology.
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Tin-lead (Sn-Pb) alloy solder is widely used in the field of electronic components assembly because of its excellent performance. However, it is very regrettable that lead in Sn-Pb is harmful to the environment and human health, and the campaign to restrict the use of lead-containing electronic materials has been officially launched.
The third draft of the Electronic Machine discards Act has been submitted by the European Commission, which expressly stipulates that harmful substances such as lead Pb, cadmium Cd, mercury Hg and hexavalent chromium Cr are strictly prohibited in the waste in 2004. Japan in Asia has enacted the Home Appliances Recycling Act in 1998, and manufacturers have been performing recycling obligations for used waste household appliances since 2001. According to the bill, Japanese home appliance information machine manufacturers began to encourage activities to reduce the use of lead. Under this background, it is strongly required to develop lead-free welding technology and corresponding tin-copper Sn-Cu alloy electroplating technology.
Technical requirements for lead-free solder electroplating.
With regard to lead-free solder plating layers and electrolytes, apart from disallowing the use of lead-containing materials, it is more difficult to achieve the same valuable characteristics as Sn-Pb coatings that have been used in the past. The specific required properties are as follows: (1) environmental safety, which does not allow substances such as lead Pb that are harmful to human health and pollute the environment; (2) precipitation stability-obtaining uniform outer surface and uniform alloy ratio; (3) wettability of solder-when heat resistance test and high temperature and high humidity test are carried out, the wettability of solder allows only a small degree of deterioration; (4) inhibit the formation of metal whisker crystals. (5) Welding strength adhesion-joint reliability with solder materials; (6) flexibility-no fracture; (7) uncontaminated flow welding tank; (8) low cost; (9) good workability-mainly means that electrolysis is easy to manage; (10) long-term reliability-even if the electrolyte is used for a long time, it can also ensure the stability of the plating layer. (11) drainage treatment-without special chelating agent (Chelate), heavy metals can be removed by neutralization and coagulation precipitation.
When choosing lead-free solder plating technology, we should comprehensively analyze and weigh the above factors, choose lead-free solder plating technology with Sn-Pb plating performance, and choose Sn-Cu (alloy solder) electrolyte as lead-free solder plating technology. Many kinds of lead-free solder plating technology have been studied, such as trying to replace Sn-Pb plating with Sn-Zn, Sn-Bi, Sb-Ag and Sn-Cu electroplating. However, these lead-free electroplating technologies have their own advantages and disadvantages, and they are not perfect. For example, Sn electroplating has the advantage of low cost, and some electronic components do use the force method of electroplating tin, because it is a single metal tin, of course, there is no problem of the management of electroplating alloy ratio. However, the disadvantages of Sn plating are prominent, such as the formation of metal whisker (Whisker) and the deterioration of solder wettability over time. The advantage of Sn-Zn electroplating lies in its low cost and low melting point. The disadvantage is that it is difficult to weld in the atmosphere and must be welded in nitrogen. The advantages of Sn-Bi plating are low melting point and excellent wettability of solder, and its disadvantages are numerous: because Bi is a brittle metal, the Sn-Bi coating containing Bi is prone to crack, and the assembled device lead and circuit board welding interface stripping (Liftoff), what is more troublesome is the replacement deposition of Bi3+ ions in the electrolyte on the Sn-Bi alloy anode or electrocoating. The advantage of Sn-Ag plating is that the bonding strength and thermal fatigue resistance are very good, but the disadvantage is the high cost, and there is also the phenomenon of Ag replacement deposition on Sn-Ag anode and Sn-Ag coating.
The above-mentioned lead-free plating technologies all have excellent characteristics, at the same time, there are many topics to be further studied, and it is too early to be practical. For this reason, Japan's Uemura Industrial Co., Ltd. believes that Sn-Cu plating is the most promising alternative to Sn-Pb plating and can be developed into a practical technology, so it decided to develop Sn-Cu electrolyte. With regard to the characteristics of Sn-Cu coating, it has good wettability except that its melting point is slightly higher (Sn-Cu eutectic temperature is 227C). The utility model has the advantages of low cost, no pollution of convection welding groove, and can inhibit the formation of metal whisker crystal.
The standard electrode potential of Sn/Sn2+ for the development of Sn-Cu alloy solder is-0.136Vvs.SHE (25 ℃), but Cu/Cu2+ is + 0.33V, and the potential difference between them is relatively large. Copper Cu is easy to precipitate preferentially in pure salt electrolyte.
Moreover, when using a soluble Sn anode or an Sn-Cu alloy anode, the product of precipitating table 1 standard electrolyte and operating conditions (the case product of obtaining sn-lwt%Cu coating) is produced due to the replacement reaction between the Cu2+ ion in the electrolyte and the Sn of the anode. Therefore, in order to make the precipitation potential of Sn2+ and Cu2+ in the electrolyte close, it is necessary to have a complexing agent to inhibit the excellent precipitation of copper Cu. Through the study of various complexing agents, the formula of Sn-Cu electrolyte was finally found, which can make Sn and Cu form alloys and inhibit the replacement deposition on the copper Cu anode.
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