Domestic lead-free solder technology development overview
Lead-free soldering electronics manufacturing has become an irreversible trend in technology. Since 2005, the domestic lead-free process enters the implementation phase.

Lead-free solder is very important to select the appropriate

Electronics industry with 60/40, 63/37 solder more than 50 years of history, has become a very mature technology, so to replace lead solder must meet some sufficient and necessary conditions.

First, from the starting electronic welding process requirements, in order not to destroy the components of the basic characteristics, the use of lead-free solders have a melting point close to the tin-lead eutectic solder melting point, can not use a high melting point solder. Secondly, from the viewpoint of weldability, to the printed circuit board with electronic components and plating copper, nickel, silver, has a good wetting. Starting from the reliability of electronic products, in order to form a good metallurgical bond joints, mechanical strength of the solder itself is important. Special requirements solder having thermal fatigue resistance, which is due in the course of electronic products will inevitably produce heat and thermal expansion phenomena, and the temperature drop when not in use will produce contraction, and so forth cycle took place in the solder heat fatigue phenomenon.

The actual operation from the welding point of view, little hope welding defect is very important. Especially the bridge, pull the tip and other undesirable defects are closely related and solder wettability. In reflow soldering, due to surface oxidation of the base material, in order to increase the flux to oxidation, must be used to remove the active flux, but this will result in residues arising from corrosion and electromigration phenomenon. Meanwhile, in the mobile welding, due to wave soldering tin oxide slag generated is also a problem, not only cause bad spot rate, while also increasing costs. Besides the preservation of solder paste is a necessary condition for good printability. Since the flux paste during storage reacts with the alloy is deteriorated, resulting in increased viscosity and poor printing. These are lead-free solder must be considered

Problem.

From the point of view to select the appropriate lead-free solder is very important. SnAgCu have become the mainstream of the current practical lead-free solder.

Lead-free solder technology has improved significantly

In recent years, more practical standard alloy lead-free tin-silver-copper generally has become a consensus basis, but due to the melting point of the alloy is still high, even heat-resistant components has increased, but multi-layer, thin-shaped PCB Heat resistance is still a problem, so the tin-silver alloy based on the addition of bismuth, indium decrease the melting point and the development of lead-free solder tin zinc will become the future direction of development.

SnAgCu lead-free solder has a relatively good mechanical properties, with a tin-lead solder 1.5-2.0 times the tensile strength and excellent thermal fatigue resistance, but on the other poor wettability of copper, from the diffusion rate of Look, tin-diffusion rate of over 90%, while the tin-silver-copper solder at about 80%, tin-zinc solders under atmospheric diffusion rate is very poor. However, due to technological advances, the current SnAgCu lead-free solder wettability has achieved significantly improved, almost reached the level of tin-lead solder paste.

Lead-free soldering processes to meet the challenges

Reflow process technology, from tin-tin-silver-copper solder to the actual operation will change a lot.

First, the current operating temperature of the heat resistant temperature rise and the gap between components significantly reduced, it is necessary to have a more accurate than the conventional process temperature management. In addition, since the diversity of PCB, the heat capacity of the components will have to be different in temperature 10 ℃, so the preheating temperature and time must be increased. Reflow equipment must be multi-zone heating to reduce the temperature error, to be an effective measure. Since the melting point rise, welding technology and equipment will undergo major changes, for the implementation of SnAgCu lead-free solder, reducing its melting point will become an issue of concern.

Secondly, generally considered SnAgCu solder wettability than low, the diffusion rate of 75% -80%, compared with tin-lead decreased by 15 percent. In order to improve the weldability increase in the flux-active agents is necessary, but will cause increased viscosity and other undesirable phenomena. In addition, as lead-free solder surface tension higher than lead solder, wetting the same conditions will deteriorate.

Third, since the high melting point of lead-free solder, and the peak temperature must be considered part of the heat resistant temperature adaptability, so warm end temperature must be high, so that there is difference in heat capacity can achieve uniform temperature components. Furthermore, since the base metal oxide component, solder balls prone to loss of activity, when using tin-lead solder paste flux SnAgCu solder paste is used, must be increased preheating temperature and preheating time.

Fourth, the printing process, since the solder powder within the flux and reaction of the powder surface with an organic metal compound and an organic metal salt precipitate, resulting in decreased mobility, increased viscosity to affect printing performance, a poor welding causes.

In summary, the theory and practice of lead-free solder soldering technology belong to the field, only leaded to lead-free process, the current standard system of lead-free solder, tin-silver-copper has become a consensus, but its still high melting point of indium and tin-silver-bismuth-tin-bismuth, tin-zinc solders will become the focus of attention and direction.

As lead-free solder liquidus temperature and peak temperature temperature range Lead temperature range is small compared with, so the temperature management has become an important lead-free soldering. Further, since the high melting point lead-free solder, it will components, PCB characteristics lead to higher requirements.

Lead-free is a long process, is still in the lead with a coexistence period, during which the issue of lead contamination is a difficult problem. As unleaded and leaded solder coexistence peeling problems arising from lead-free process is the most difficult.撤消修改Alpha