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Electronic assembly of the basic requirements of lead-free solder

The basic process of lead-free solder assembly include:
. a lead-free PCB manufacturing process;
. b in the application of solder paste 96.5Sn / 3.5Ag and 95.5Sn / 4.0Ag / 0.5Cu eutectic and near-eutectic alloy systems;
. c for 99.3Sn / 0.7Cu eutectic soldering systems applications;
d. for manual welding 99.3Sn / 0.7Cu alloy systems.
Although these processes are possible, but there are still specific to implement several major issues, such as raw material costs are still higher than the standard Sn / Pb process, limiting the moisture increases, the state required to maintain an inert atmosphere in the wave soldering process (to have a sufficient amount of nitrogen) and may be raised to reflow temperature (between the 235 ~ 245 ℃) threshold temperature range and improving the thermal requirements of the various elements and the like.

Lead-free alternatives, it is now does not have a universally recognized norms, after many discussions with many professionals in the field, and we have come to the following technical and application requirements:

Metal prices of many assemblers require not be higher than the price of lead-free alloys 63Sn / 37Pb, but unfortunately all the existing lead-free alternatives cost than 63Sn / 37Pb for at least 35% higher. In the choice of electrode and lead-free solder wire, metal cost is the most important factor; and in the production of solder paste, due to the cost of the overall manufacturing cost technology relatively high proportion, so the metal is not so sensitive to the price .

Most melting assembly manufacturers (not all) require a minimum solidus temperature 150 ℃, in order to meet the requirements of an electronic device operating temperature, the maximum liquid temperature depends on the specific application.
Wave Rods: In order to successfully implement wave, liquid oven temperature should be below 260 ℃.

Hand / machine welding solder wire: liquid temperature should be below the tip working temperature 345 ℃.

Paste: liquid temperature reflow temperature should be below 250 ℃. Many of the existing terms of the reflow oven, the temperature is a practical temperature limit. Many engineers require maximum reflow temperature should be below 225 ~ 230 ℃, however, do not have a viable plan to meet this requirement. It is widely believed alloy reflow temperatures closer to 220 ℃, the better, to avoid the higher reflow temperature is ideal, however, because it allows components to minimize the extent of the damage, minimizing the special elements requirements while discoloration and the circuit board to minimize the degree of warpage, and to avoid excessive oxidation and wire pads.

Good conductivity which is the basic requirement of electronic connections.

Good thermal conductivity in order to be able to distribute the heat, the alloy must have rapid heat transfer capability.

Small solid-liquid coexistence temperature range of non-eutectic alloy in solidification temperature within a temperature range of liquid and solid phase temperature is between the majority of metallurgical experts recommend this be controlled within a temperature range of 10 ℃, in order to form a good solder joints, reduce defects. If the solidification temperature range is wide, the solder joint cracking may occur, so that the device prematurely.