what's the basic requirements for electronic assembly of lead-free solder
The basic requirements for electronic assembly of lead-free solder
The basic spec. of lead-free soldering assembly inclode:a)lead-free PCB assembly manufacture process b)The applying 96.5Sn/3.5Ag and 95.5Sn/4.0Ag/0.5Cu eutectic solder paste and near eutectic alloy systems.c)Application for wave soldering of Sn 99.3/0.7 Cu eutectic alloy system d)The applying used for manual welding system of 99.3Sn/0.7Cu .Alothough these are is viable technology, but there are also esists several large problm in the specific implementation.Such as the raw material cost is still higher than the standard Sn/Pb process, the limitation on the moisture increased, demand in the wave soldering process remain inert air (there should be adequate nitrogen) and are likely to be reflow temperature up to the limit temperature range (235 ~ 245 ℃) and improved heat resistance requirements for various components and so on.In terms of lead-free alternatives, and now there is no a set of widely recognized norms, after discussions with many professionals in this field, we obtained some technical and application requirements below：
Metal prices many pcb assembly manufature are required lead-free alloy price not higher than 63 sn / 37 pb, but unfortunately the existing all lead-free alternatives cost are all higher than 63 sn / 37 pb at least more than 35%. When choosing a lead-free solder and solder wire, metal cost is one of the most important factor; And when in the production of solder paste, due to technical costs in the proportion of the total manufacturing cost is relatively high, so for the price of the metal is not that sensitive.
Melting point most assembly manufacturer (not all) are minimum requirements solidus temperature is 150 ℃, in order to satisfy the requirement of the electronic equipment working temperature, liquid temperature is highest depends on the specific application. Crest welding with electrode: in order to successfully implement wave soldering, the liquid temperature should be below the furnace temperature is 260 ℃.
Hand/machine soldering with solder wire: liquid the temperature should be lower than soldering iron head working temperature 345 ℃.
Solder paste, liquid temperature reflow solder temperature should be lower than 250 ℃. For existing many reflow furnace, the practical temperature is the temperature limit. Many engineers require maximum reflow soldering temperature should be lower than 225 ~ 230 ℃, but now there isn't a feasible solution to satisfy this requirement. It is widely believed that alloy reflow soldering temperature is close to 220 ℃ the better, can avoid high reflow temperature is the most ideal, because it can make the damage rate of the components to a minimum, maximum limit reduce the requirement for special components, as well as minimize circuit board color and warp degree, and to avoid excessive welding plate and wire oxidation.