Welding process requirements basic requirements for lead-free solder in electronic assembly

Date：2022-05-09 15:48:45 Views：823

The basic requirement of electronic assembly for lead-free solder is a control requirement of electronic equipment for solder. The basic process of lead-free solder assembly includes: A. lead-free PCB manufacturing process; b. 96.5sn/3.5ag and 95.5sn/4.0ag/0.5cu eutectic and near eutectic alloy systems used in solder paste; c. 99.3sn/0.7cu eutectic alloy system for wave soldering application; d. 99.3sn/0.7cu alloy system for manual welding. Although these are feasible processes, there are still several major problems in the specific implementation, such as the raw material cost is still higher than the standard Sn / Pb process, the limit on humidity is increased, the requirement to maintain inert air state in wave soldering process (sufficient nitrogen) and the possibility to raise the reflow temperature to the limit temperature range (235 ~ 245 ℃), which improves the thermal requirements of various components, etc.

Basic technology

The basic process of lead-free welding and assembly includes: A. lead-free PCB manufacturing process; b. 96.5sn/3.5ag and 95.5sn/4.0ag/0.5cu eutectic and near eutectic alloy systems used in solder paste; c. 99.3sn/0.7cu eutectic alloy system for wave soldering application; d. 99.3sn/0.7cu alloy system for manual welding. Although these are feasible processes, there are still several major problems in the specific implementation, such as the raw material cost is still higher than the standard Sn / Pb process, the limit on humidity is increased, the requirement to maintain inert air state in wave soldering process (sufficient nitrogen) and the possibility to raise the reflow temperature to the limit temperature range (235 ~ 245 ℃), which improves the thermal requirements of various components, etc. As far as lead-free substitutes are concerned, there is no set of generally recognized specifications. After many discussions with many professionals in this field, we come to the following technical and application requirements:

Metal price many assembly manufacturers require that the price of lead-free alloy should not be higher than 63Sn / 37Pb, but unfortunately, the cost of all existing lead-free alternatives is at least 35 higher than 63Sn / 37Pb. When choosing lead-free electrode and solder wire, metal cost is the most important factor; When making solder paste, due to the relatively high proportion of technical cost in the overall manufacturing cost, it is not so sensitive to the price of metal.

Melting point most assembly manufacturers (not all) require a minimum solid-phase temperature of 150 ℃ to meet the working temperature requirements of electronic equipment, and the maximum liquid-phase temperature depends on the specific application.

Electrode for wave soldering: in order to successfully implement wave soldering, the liquid phase temperature should be 260 ℃ lower than the furnace temperature.

Solder wire for manual / machine welding: the liquid phase temperature shall be 345 ℃ lower than the working temperature of the soldering iron head.

Solder paste: the liquid phase temperature shall be 250 ℃ lower than the reflow temperature. For many existing reflow furnaces, this temperature is the limit value of practical temperature. Many engineers require that the maximum reflow temperature should be lower than 225 ~ 230 ℃, but there is no feasible scheme to meet this requirement. It is generally believed that the closer the alloy reflow temperature is to 220 ℃, the better the effect. It is ideal to avoid higher reflow temperature, because it can minimize the damage of components, minimize the requirements for special components, minimize the discoloration and warpage of circuit board, and avoid excessive oxidation of pads and wires.

Conductivity

Good conductivity is the basic requirement of electronic connection.

Good thermal conductivity. In order to dissipate heat energy, the alloy must have the ability of rapid heat transfer.

In a small temperature range of solid-liquid coexistence, non eutectic alloys will solidify in a certain temperature range between liquid phase temperature and solid phase temperature. Most metallurgical experts recommend that this temperature range be controlled within 10 ℃ in order to form good solder joints and reduce defects. If the solidification temperature range of the alloy is wide, the solder joint cracking may occur and the equipment may be damaged prematurely.

Low toxicity alloys and their components must be non-toxic, so cadmium, thallium and mercury are excluded from this requirement; Some people also require that by-products extracted from toxic substances cannot be used, so bismuth is excluded, because bismuth mainly comes from by-products extracted from lead.

It has good weldability. Under the condition of existing equipment and cleaning free flux, the alloy should have sufficient moisture and can be used with conventional cleaning free flux. Since the cost of inert treatment of wave crest is not too high, the service conditions of wave crest welding and inert environment can be accepted; However, for SMT reflow welding, it is better for the alloy to have the ability of reflow welding under air, because the inert treatment cost of reflow furnace is high.

The alloy with good physical properties (strength, tensile strength, fatigue, etc.) must be able to provide the mechanical strength and reliability that 63Sn / 37Pb can achieve, and there will be no protruding fillet weld on the through-hole device (especially for the alloy with large temperature range of solid-liquid coexistence).

Production repeatability / melting point consistency electronic assembly process is a mass manufacturing process, which is required to maintain a high level of repeatability and consistency. If the composition of some alloys cannot be manufactured repeatedly under mass production conditions, or their melting point changes greatly due to component changes during mass production, it cannot be considered. Alloys composed of more than three components often have separation or component change, so that the melting point cannot be kept stable. The higher the complexity of the alloy, the greater the possibility of change.

Solder joint appearance the appearance of solder joints should be close to that of tin / lead solder. Although this is not a technical requirement, it is the actual need to accept and implement alternatives.

Supply capacity when trying to find a solution for the industry, we must consider whether the materials have sufficient supply capacity. From a technical point of view, indium is a very special material, but if we consider the global supply capacity of indium, people will soon completely exclude it from consideration.

In addition, the industry may prefer the standard alloy system to the special system. The acquisition channel of standard alloy is relatively wide, so the price will be more competitive, while the supply channel of special alloy may be limited, so the material price will increase significantly.

Compatibility with lead because it will not be fully transformed into a lead-free system immediately in the short term, lead may still be used on the terminals of some components or printed circuit board pads. Some lead containing alloys have a very low melting point, which will reduce the strength of the connection. For example, the melting point of a bismuth / TiN / lead alloy is only 96 ℃, which greatly reduces the welding strength.

The above is the basic requirements of electronic assembly for lead-free solder. I hope it can help you. We will bring more wonderful contents in the later stage. The company's testing services cover: electronic component testing and verification, IC authenticity identification, product design and material selection, failure analysis, function testing, factory incoming material inspection, tape braiding and other testing items. Welcome to call Chuangxin testing. We will serve you wholeheartedly.