Lead-free process application principles in circuit board assembly


Over the past eight years, the electronics assembly industry has been testing a variety of alloys, hoping to find several lead-free alternatives to conventional 63Sn/37Pb eutectic systems. There are many options that are feasible from a technical point of view, but do not take into account other factors such as cost, availability and processability. This paper aims to provide the industry with a practical lead-free process selection principle, including process requirements, conclusions based on requirements, and trial conditions under actual conditions. In the future, it is necessary to judge other more complex systems. The method provided here can also be used as an evaluation reference.

Basic requirements for electronic assembly for lead-free solder

The basic processes for lead-free soldering assembly include:
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 applications;
d. 99.3Sn/0.7Cu alloy system for hand soldering.
Although these are all feasible processes, there are still several major problems in implementation. For example, the raw material cost is still higher than the standard Sn/Pb process, the humidity limit is increased, and the inert air state is required to be maintained in the wave soldering process ( A sufficient amount of nitrogen is required) and the reflow temperature may be raised to the extreme temperature range (between 235 and 245 ° C) to increase the thermal requirements for various components and the like.

As far as lead-free alternatives are concerned, there is currently no universally accepted norm. After many discussions with many professionals in the field, we have the following technical and application requirements:


Metal Prices Many assemblers require that the price of lead-free alloys be no higher than 63Sn/37Pb, but unfortunately all existing lead-free alternatives cost at least 35% more than 63Sn/37Pb. Metal cost is the most important factor in the selection of lead-free electrodes and solder wire; in the production of solder paste, because the technical cost is relatively high 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 temperature of 150 °C to meet the operating temperature requirements of electronic equipment. The maximum liquidus temperature depends on the application.
Wave soldering electrode: In order to successfully perform wave soldering, the liquidus temperature should be lower than the furnace temperature of 260 °C.

Solder wire for manual/machine welding: The liquidus temperature should be lower than the tip temperature of 345 °C.

Solder paste: The liquidus temperature should be lower than the reflow temperature of 250 °C. For many existing reflow ovens, this temperature is the limit of the practical temperature. Many engineers require a maximum reflow temperature of less than 225 to 230 ° C, but there is currently no workable solution to meet this requirement. It is generally believed that the closer the alloy reflow temperature is to 220 °C, the better the effect is. It is ideal to avoid higher reflow temperatures, as this minimizes component damage and minimizes the need for special components. It also minimizes the discoloration and warpage of the board and prevents excessive oxidation of the pads and wires.

Good conductivity This is the basic requirement for electronic connections.

Good thermal conductivity In order to dissipate heat, the alloy must have a fast heat transfer capability.

The smaller solid-liquid coexisting temperature range of non-eutectic alloys will solidify in a temperature range between liquid phase temperature and solid phase temperature. Most metallurgists recommend that this temperature range be controlled within 10 ° C to form a good Solder joints to reduce defects. If the alloy has a wide solidification temperature range, cracking of the solder joint may occur and the device may be damaged prematurely.

Low-toxic alloys and their components must be non-toxic, so this requirement excludes cadmium, antimony and mercury from consideration; some also require that by-products derived from toxic substances cannot be used, and therefore cockroaches are excluded, since cockroaches are mainly A by-product derived from lead refining.

Good solderability This alloy should have sufficient wetting under existing equipment and no-clean flux conditions and can be used with conventional no-clean fluxes. Since the cost of inerting the peaks is not too high, it is acceptable to use wave soldering plus inert conditions; however, for SMT reflow soldering, the alloy preferably has the ability to reflow under air because of reflow The inertia of the furnace is costly.

Good physical properties (strength, elongation, fatigue, etc.) The alloy must be able to provide the mechanical strength and reliability that 63Sn/37Pb can achieve without protruding fillet welds on the through-hole device (especially An alloy with a large solid-liquid coexistence temperature range).

Production repeatability / melting point consistency Electronic assembly process is a high-volume manufacturing process that requires high levels of repeatability and consistency, if certain alloy components cannot be remanufactured under high-volume conditions, or The melting point is largely changed due to compositional changes during mass production and cannot be considered. Alloys composed of three or more components tend to undergo separation or compositional changes, so that the melting point cannot be kept stable, and the higher the complexity of the alloy, the greater the possibility of change.

Solder Joint Appearance Solder joints should look similar to tin/lead solder. Although this is not a technical requirement, it is a practical requirement to accept and implement alternatives.

Supply Capabilities When trying to find a solution for the industry, it is important to consider whether the material has sufficient supply capacity. From a technical point of view, indium is a rather special material, but if you consider the availability of indium worldwide, people will soon be completely excluded from consideration.
In addition, the industry may prefer standard alloy systems rather than special systems. The standard alloys have wider access channels, so the price will be more competitive, and the supply channels of special alloys may be limited, so the price of materials will increase significantly.


Compatibility with Lead Since lead will not be fully converted to a lead-free system in the short term, lead may still be used on terminals or printed circuit board pads of certain components. Some lead-containing alloys have a very low melting point, which reduces the strength of the joint. For example, a certain bismuth/tin/lead alloy has a melting point of only 96 ° C, which greatly reduces the weld strength.
Metal and alloy selection

Among the various candidate lead-free alloys, tin (Sn) is used as the base metal because of its low cost, sufficient supply, and ideal physical properties such as electrical/thermal conductivity and wettability, and it is also 63Sn. Base metal of /37Pb alloy. Other metals commonly used in combination with tin include silver (Ag), indium (In), zinc (Zn), antimony (Sb), copper (Cu), and bismuth (Bi).

These materials were chosen because they generally reduce the melting point when alloyed with tin to achieve desirable mechanical, electrical and thermal properties. In addition, when investigating the supply capacity of materials, the results of the combined considerations will be more clear. For example, the consumption of 63Sn/37Pb in the electronics industry is about 45,000 tons per year, of which the amount in North America is about 16,000 tons, as long as 3% of assembly plants in North America use tin/indium lead-free alloys containing 20% indium, the indium consumption will exceed the global production capacity of the metal.

In the past five years, the industry has introduced a series of alloy composition recommendations, and these lead-free alternatives have been evaluated. The total number of options exceeds 75, but the main options can be grouped to less than 15. In the face of all candidate alloys, we use some technical specifications to reduce the choice to a smaller range for easy selection.

Indium Indium may be the most effective component for lowering the melting point of tin alloys, and it also has very good physical and wet properties, but indium is very rare, so large-scale applications are too expensive. For these reasons, indium containing alloys will be excluded from further consideration. Although indium alloys may be a good choice for some specific applications, they are not suitable for the entire industry. In addition, differential scanning calorimetry also shows that the melting point of 77.2Sn/20In/2.8Ag alloy is very low, only 114. °C, so it is not suitable for some applications.

Zinc Zinc is very cheap, almost the same price as lead, and readily available, and it is also very efficient at reducing the melting point of tin alloys. In the case of zinc, its main disadvantage is that it reacts rapidly with oxygen to form a stable oxide. In the wave soldering process, the result of this reaction is a large amount of tin slag, and more serious is the formation of stable oxidation. Things will cause the wettability to become very poor. These technical barriers may be overcome by inerting or special flux formulations, but it is now required to demonstrate zinc-containing solutions in a larger process range, so zinc alloys will be excluded in future considerations.

铋 作用 is more effective in reducing the solid phase temperature of tin alloys, but it has no such effect on liquid phase temperature, so it may cause a large solid-liquid coexistence temperature range, and a too large solidification temperature range will lead to the rise of the solder fillet. Tantalum has very good wetting properties and good physical properties, but the main problem with niobium is that the tin/bismuth alloy will have a lower melting point when it encounters lead, but at the component leads or printed circuit board pads. There will be lead on it, and the melting point of tin/lead/bismuth is only 96 °C, which easily causes the solder joint to break. In addition, the supply capacity of bismuth may be reduced due to limited lead production, because bismuth is mainly extracted from lead by-products. If lead is restricted, the production of strontium will be greatly reduced. Although we can also obtain plutonium through direct mining, the cost will be higher. For these reasons, niobium alloys have also been excluded.

Four and five component alloys

Alloys made up of four or five metals provide us with a range of alloy composition combinations, and the possibilities are endless. Most four or five metal alloys can significantly lower the solid phase temperature compared to bimetallic alloy systems, but may not do anything to lower the liquidus temperature because most of the four or five metal alloys are not eutectic, meaning Different metallographic forms are formed at different temperatures, with the result that the reflow temperature cannot be lower than that required for a simple bimetallic system.

Another problem is that the alloy composition often changes, so the melting point also changes, which is often encountered in four or five metal alloys. Alloys composed of three metals are difficult to achieve "same batch" and "batch-by-batch" in tin powder in solder paste, achieving the same consistency in four and five metal alloys. Bigger.

Therefore, multi-alloys will be excluded from further consideration unless a certain multi-alloy component has better properties than a binary system. But for now, the industry has yet to find which four or five metal alloys are better than binary or ternary alternatives (both in terms of cost and performance).

Consider electrode (wave soldering) and wire bonding (manual and machine soldering) first.

Requirements for wave soldering electrodes include:
a. Continuous welding at a maximum temperature of 260 ° C tin furnace;
b. Less welding defects (leak welding, bridging, etc.);
c. The cost is as low as possible;
d. Does not produce too much slag.

As a result, all selected alloys meet the wave soldering requirements, but the 99.3Sn/0.7Cu and 95Sn/5Sb alloys can save more cost than other alternatives. In comparison, the liquidus temperature of 99.3Sn/0.7Cu is 13°C lower than that of Sn/Sb alloy, so 99.3Sn/0.7Cu is the best candidate for wave soldering.

The requirements for solder wire for hand soldering are very similar to those for the electrode above. Cost considerations are still a priority and are required to provide better wetting and soldering capabilities. The alloy for wire bonding must be able to be easily drawn into a wire, and it can be welded with a tip of 345 to 370 ° C. The 99.3Sn/0.7Cu alloy can meet these requirements.

Compared to electrodes and wire bonds, solder pastes are less costly to consider because metal costs account for a smaller proportion of the total cost of manufacturing processes using solder paste. The main requirement for solder paste alloys is to minimize reflow temperatures. The lowest liquidus temperature was found to be 95.5Sn/4.0Ag/0.5Cu (melting point 217-218 °C) and 96.5Sn/3.5Ag (melting point 221 °C).

Both alloys are more suitable and have their own characteristics. In contrast, the Sn/Ag/Cu alloy has a lower liquidus temperature (although only 4 ° C), while the Sn/Ag alloy shows a stronger consistency. Sexual and reproducible, and has been used in the electronics industry for many years, and has maintained good reliability. Some major multinational companies have chosen eutectic Sn/Ag alloys for evaluation as lead-free alternatives, and most large multinational companies have begun preliminary advanced testing of Sn/Ag/Cu alloys.

Measured evaluation result

Wave Soldering Evaluation The 99.3Sn/0.7Cu alloy was tested in a standard Electrovert Econopak Plus wave soldering machine equipped with a USI ultrasonic flux coating system, a Vectaheat convection preheating and an "A" wave CoN2tour inert system. The test was performed on two lead-free printed circuit boards: bare copper with OSP coating and bare copper with immersion silver polishing (Alpha standard), both boards with 2% solid state and no VOC-free cleaning aid Flux (NR300A2). In addition, as a control, the same board was soldered on the same equipment under the same conditions, except that the solder was made of a conventional 63Sn/37Pb alloy.

Through the experiment, the following conclusions can be drawn:


If 99.3Sn/0.7Cu alloy is used, it is necessary to inertize the wave soldering machine to ensure proper wetting, but no need to completely inertize the wave soldering machine or air duct, using CoN2tour's boundary inert welding system. That is enough.

The appearance of the board using 99.3Sn/0.7Cu soldering is no different from that of the board soldered with 63Sn/37Pb alloy. The brightness of solder joints, solder joint molding, pad wetting, and tin on the upper end of the via are also the same.

Compared with the Sn/Pb alloy, the Sn/Cu alloy has less bridging phenomenon, but due to the limited testing conditions, further research is needed on this point.

99.3Sn/0.7Cu alloy is very successful in welding at 260 °C, and there is no problem at 245 °C.

There was no change in the copper content in the weeks of using the Sn/Cu alloy. The reason for this concern is that the solubility of copper in tin is very low and is highly dependent on temperature. In high-volume production, the copper absorption of the board is the same as when using Sn/Pb alloy.
Print and Reflow Soldering Development A new flux has been developed for Sn/Ag and Sn/Ag/Cu alloys to achieve better wetting at higher reflow temperatures because of higher reflow temperatures ( It is 20 ° C higher than the conventional reflow temperature.) The active agent in the flux should have higher thermal stability. In addition, if working in the air, the reflow soldering temperature can also make the ordinary no-clean flux discolor, so the flux has a strong tolerance to high temperatures. At 95.5Sn/4.0Ag/0.5Cu


For manual/machine welding, the solder wire can be used with 99.3Sn/0.7Cu alloy.
Although the above solution has not yet reached the goal of the research of lead-free alternatives engineers, it is basically satisfactory. The maximum limitation of this solution is the reflow soldering temperature ratio Sn/Pb required for 96.5Sn/3.5Ag alloy. The alloy is 20 to 30 ° C higher, so the requirements for components for reflow soldering are also improved. Component suppliers should work closely with electronics assembly plants to address the issues associated with high temperature reflow soldering.

As new technologies evolve, more and better alternatives will be introduced in the future. The greatest value of the systems discussed here is that other complex systems can be referenced against the standards they provide. Before looking at a more complex system, ask the following questions that can be answered quantitatively:

Solder paste

1. Can the new alloy system reduce the reflow temperature to about the same level as the Sn/Ag alloy (the minimum reflow temperature of the Sn/Ag alloy is 240 °C, and the minimum reflow temperature of the 95.5Sn/4.0Ag/0.5Cu alloy) Then 235 ° C)?

2. What is the cost of metal and solder paste compared to Sn/Ag or Sn/Ag/Cu alloys?

3. Are there any technical parameters for each material in the alloy? What is the change in solid and liquid temperature of each material as it varies within the technical parameters?

welding rod

1. What is the cost of the alloy? Which one is more expensive than Sn/Cu alloy?

2. Does the alloy have the advantages that Sn/Cu alloys do not have?

Why use a lead-free solution?

When talking about health and safety issues, people generally consider two factors - hazards and dangers. Hazard refers to the toxicity of a substance and its effects on the human body after ingestion, inhalation or absorption. The hazard refers more to the safety of the material or the possibility of harm after taking corrective measures.

Lead is a toxic substance. Excessive amounts of lead in the body can cause lead poisoning. Ingestion of low doses of lead may affect human intelligence, nervous system and reproductive system. When assessing the risk of lead, it is usually by investigating the possibility of eating or breathing. As far as the electronics industry is concerned, under normal conditions, lead does not reach the temperature at which it is vaporized, so the danger in this area cannot be quantitatively determined and can only be determined by monitoring.

Some simple precautions can be taken, including wearing a mask when servicing the wave soldering furnace and cleaning up debris, and ban smoking in leaded areas to minimize the possibility of lead in the work area. In addition, after touching lead-containing objects such as solder paste, welding rods, and wire bonding, be sure to wash your hands before eating, drinking, and smoking to completely eliminate possible exposure to lead.

In general, the risk of lead ingestion is higher than that of respiratory inhalation from the source of lead absorption, so the importance of developing good hygiene practices should be emphasized. Eating and smoking are strictly prohibited in areas containing lead to minimize the risk. It turns out that using lead in the workplace is relatively safe as long as the correct precautions are taken.

Since the danger of using lead in the electronics industry is so small, why are people considering completely eliminating the use of lead? In fact, the main concern is the disposal of lead-containing materials, such as printed circuit boards. The reason for this concern is that the waste printed circuit boards that are disposed of as garbage can be leached from the circuit board into the groundwater after being buried in the ground, and then flow into our drinking water.

There are some technical debates that focus on the possibility of lead leaching from the PCB and the acceptable lead content of drinking water (if it does contain lead).

Is lead-free legislation coming soon?

The legislative process is very policy-oriented, so it is difficult to predict. For example, the US Congress has not yet adopted active legislative measures to limit the use of lead in the electronics industry. Since the lead used in the electronics industry accounts for a very small share of the total use of human lead, and the share of the electronics industry in the gross national product is very important and growing, it is very difficult to limit lead in the electronics industry. Great resistance. In addition, all lead-free solder replacements will increase the cost of electronics manufacturing. In some areas, research on lead-free solders has not yet begun, so in a sense this legislation may weaken the competitiveness of the country's electronics manufacturing industry.

Europe has adopted a more positive attitude in promoting lead-free legislation. The EU plans to conduct a vote to prohibit the use of lead in all electronic products except vehicles and certain special occasions before January 2004. The Netherlands and Switzerland have already implemented decrees on electronic waste.

Countries in Asia have issued a statement on the recycling of electronic waste. The Japan Electronics Industry Development Association and the Japan Electronics Packaging Institute established the research principles and plans for lead-free solutions in January 1998. Some OEMs have successfully developed recycling methods, such as Sony, Toshiba, Matsushita, Hitachi and NEC. It has also pledged to comply with Japanese laws and achieve lead-free in certain electronic products in 2001.

Is the electronics industry implementing a lead-free process?

Many multinational electronics companies have begun launching lead-free technology projects to evaluate lead-free alternatives in preparation for implementation of lead-free systems after the entry into force of restrictive laws.