Lead-free welding and assembling the basic process includes: A. lead-free PCB manufacturing process; and approximate eutectic alloy system B. in the solder paste application 96.5sn/3.5ag and 95.5Sn/4.0Ag/0.5Cu eutectic; C. for wave soldering applications 99.3Sn/0.7Cu eutectic alloy systems. D. for manual welding 99.3Sn /0.7Cu alloy system. Although the technology is feasible, but specific to the implementation of several major problems still exist, such as raw material cost is still higher than the standard of Sn / Pb process, of moist degree increased, the requirements in the wave soldering process keep inert air state (to have a sufficient amount of nitrogen) and likely will return to the welding temperature rise to limit (235 ~ 245 DEG C temperature range between) and improve of various components of the heat requirement and so on. As far as lead - free alternatives are concerned, there is not a set of universally accepted norms that have been discussed by many professionals in the field:
Metal prices of many assembly manufacturers have asked the price of lead-free alloy can not be higher than 63Sn/37Pb, but unfortunately all the existing lead-free alternatives cost is higher than the 63Sn/37Pb at least 35% or more. In the choice of lead-free solder and solder wire, metal cost is one of the most important factors; and in the production of solder paste, due to the cost of technology in the overall manufacturing cost in the proportion of relatively high, so the price of metal is not so sensitive.
Melting point of most assembly manufacturers (not all) are required to be a minimum of 150 degrees Celsius temperature, in order to meet the working temperature of electronic equipment, the maximum liquid temperature is depending on the specific application.
Electrode soldering: in order to successfully carry out the wave soldering, liquid phase temperature should be lower than the temperature of 260 DEG C.
With the solder wire welding machine: manual / liquid temperature should be lower than the tip temperature 345.
Solder paste: liquid phase temperature should be lower than reflow soldering temperature 250. For many reflow ovens, the temperature is the limit value of the practical temperature. Many engineers require the highest reflow soldering temperature should be below 225 to 230 degrees Celsius, but there is no feasible solution to meet this requirement. People generally believe that alloy solder reflow temperature closer to 220 DEG C effect is better, can avoid appear higher reflow temperature is the best, but, because this can make the components the extent of the damage to a minimum, maximum reduce the requirements of particular components, but also will be a circuit board change color and to minimize the warpage degree, and avoid pads and wire over oxidation.
Electrical conductivity is good this is the basic requirements of electronic connection.
Thermal conductivity is good in order to distribute heat energy, the alloy must have the ability of rapid heat transfer.
Smaller solid-liquid coexistence temperature range of non co amorphous alloy will be in between liquid phase between the temperature and the temperature of the solid phase in a certain temperature range of solidification, the majority of metallurgy experts suggest the temperature control in the range 10 DEG C to form a good solder joints and reducing the defects. If the alloy has a wide range of solidification temperature, the solder joint cracking may occur, so that the equipment is damaged too early.
Low alloy and its components must be non-toxic, so the requirements of cadmium, thallium and Mercury will be excluded from consideration; some people also asked not to use toxic substances are refining by-products. Thus, it will bismuth excluded, as a by-product of bismuth, mainly from lead extraction.
Has good welding under the existing equipment and cleaning free soldering flux conditions the alloy should have sufficient moisture and can be used in conjunction with conventional non cleaning flux. Due to the inertia of the peak processing cost is not too high, so you can accept the conditions for the use of wave soldering with inert environment; but SMT reflow soldering is concerned, alloy is the best to have reflow soldering ability in the air, because of reflow oven were inert processing cost is high.
Good physical properties (strength, tensile strength, fatigue, etc.) alloy must be able to provide / 37Pb can achieve mechanical strength and reliability, but not in through hole devices appear projection welds (especially on the solid-liquid coexistence a wide range of temperature of the alloy).
Production can be repeated / melting consistency electronic assembly process is a kind of batch manufacturing process, its reproducibility and consistency requirements are maintained at high level, if certain alloy components in under the condition of large quantities of repetitive manufacturing, or its melting point in mass production due to changes and great changes cannot be given consideration. 3 or more components of the composition of the alloy will often occur separation or composition changes, so that the melting point can not be kept stable, the higher the complexity of the alloy, the greater the possibility of its occurrence and change.
Appearance of solder joint appearance should be close to that of tin / lead solder, although this is not a technical requirement, but it is the actual need to accept and implement alternative solutions.
Supply capacity when trying to find a solution for the industry, we must consider whether the material has sufficient supply capacity. From a technical point of view, indium is a very special material, but if you consider the global scope of indium supply capacity, people will soon be completely excluded from the scope of consideration.
In addition the industry may prefer the standard alloy system rather than choose special system, alloy standard access to relatively wide, so the price will be more competitive, and special alloy supply channels may limit, so material prices will greatly improved.