Soldering Techniques and Solder Types

The term “joining technology” may remind you of bonding methods using wood glue or hot melt adhesives. This article introduces techniques for joining metals together using solder.

Three Techniques for Joining Metals

Soldering

Soldering is a technique in which metals are bonded together through melting using solder.

Soldering has been practiced since ancient times, and traces of soldering have been found in excavated objects from ancient Egypt. Soldering methods, materials, and tools are still evolving.

Lead and tin are commonly used as solder materials and can join materials together at relatively low temperatures (with melting points of 200-400℃). Bonding strength is not so high. Soldering is often used to assemble electronic components and precision equipment.

Soldering is essentially different from adhesion using glue and the like.

For example, when soldering on a copper board, an alloy layer (diffusion layer) of several micrometers is formed between the copper and the tin of the solder. This alloy layer tightly bonds the solder to the base copper (Figure 1).

Figure 1: Alloy layer between tin and copper of board
Source：https://toragi.cqpub.co.jp/tabid/693/Default.aspx

In the case of gold-plated PCBs, gold in the thin plating layer on the surface and tin in the solder form an alloy layer (AuSn2), and then nickel in the underlying plating layer and tin form an alloy layer for joining.

Brazing

Brazing is a metal joining technique utilizing the property of wettability that appears when a brazing material having a lower melting point than the base metal is melted. Brazing materials include silver, copper, and zinc. Brazed joints are stronger than soldering and can withstand higher temperatures, making brazing an effective method for assembling accessories and models, and for fabricating eyeglasses.

Welding

Welding is a joining technique that melts the base material itself at very high temperatures. It has higher joint strength than brazing, but may require appropriate tools.

Welding is an essential technique not only for joining large metal parts with sparks flying, but also for fabricating delicate electronic circuits. It is used to join electrodes (tabs) to battery terminals (spot welding) and to join the ends of two types of metal wires to make thermocouples.

Techniques for Making Electrical Connections

Crimping/press fitting

Mechanical pressure is applied to deform materials to bring them into contact with each other. This method is highly reliable, and is often used for connector terminals.

Thread fastening

Screws are used to mechanically connect the terminals. This method is used to connect wires to a terminal block or to attach a bus bar for high power transmission or for grounding.

Ultrasonic bonding

This method uses ultrasonic waves to generate friction between materials to cause heat and plastic deformation while removing oxides from the surface. It is used, for example, to bond fine aluminum or gold wires in order to draw signals from electrode pads on semiconductor chips to IC terminals, in a process called ultrasonic wire bonding.

Electrically conductive adhesive bonding

Electrically conductive adhesive is an epoxy adhesive mixed with conductive fillers such as silver and carbon nanotubes. Conductive adhesives are used where electrical connections are required because of their conductive properties. As the adhesive hardens, the epoxy resin shrinks and the filler particles make contact with each other to establish conduction.

This method is often used to bond semiconductor packages or electronic substrates that are difficult to solder or where there is a risk of heat damage from soldering.

Wire wrap

Wrapping wire is twisted around a terminal rod (post) to join the electrode and wire (Figure 2). It is more reliable than soldering and was once used in the aerospace industry, but is rarely used today.

Figure 2: Example of wire wrapping

Use of connectors

A component with a spring mechanism (connector) is used to provide mechanical contact between electrodes. Typical examples are USB connectors and power plugs.

Use of electrically conductive rubber

Electrically conductive rubber is used for electrical connections in products such as LCD panels. Conduction is established by applying pressure.

Eutectic Solder vs. Lead-Free Solder

Eutectic solder

Eutectic solder is generally made of tin and lead, with a melting point of 231.9℃ for tin and 327.5℃ for lead (Figure 3).

Adjusting the tin-lead ratio can lower the melting point to 183-184℃, making the solder easier to process.

Figure 3: Lead-Tin percentage and melting points of alloys
Source: FCT Solder’s website

The mixing ratio with the lowest melting point (eutectic condition) is 63% tin + 37% lead (Sn63-Pb37). This alloy is called a eutectic alloy.

There are also eutectic solders with mixing ratios outside the eutectic condition, such as 60% tin + 40% lead and 50% tin + 50% lead. The higher the lead content, the higher the melting point and the higher the strength. Solder with a higher lead content is used when joining heavy parts or parts to be subjected to high temperatures, or when “dripping” is to be avoided. On the other hand, such solder is not suitable for through-holes in PC boards due to its poor flow behavior.

There are also eutectic solders containing a small amount of copper (about 0.5%) to avoid copper erosion by tin in the solder. They are used for ultra-fine copper wires and copper patterns on PCBs. Similarly, solders with a small amount (~2%) of silver are available to prevent silver erosion.

The most commonly used solder mixture ratio for PCBs is 60% tin + 40% lead, with a melting point of 199℃.

Lead-free solder

Lead regulations

Lead is abundant in nature and is easy to handle, and is used in a variety of applications, including eutectic solder. However, lead is toxic and carcinogenic and has a large environmental impact. As a result, lead-related regulations have been initiated around the world since the 2000s.

The EU implemented the RoHS Directive in 2006 to regulate the sale of products containing lead in concentrations of 1,000 ppm or more. This directive covers lead, mercury, cadmium, and hexavalent chromium.

In addition, many Japanese appliances use lead-free solder due to environmental considerations.

Characteristics of lead-free solder

The word lead-free does not mean that a solder is completely free of lead, but rather that it is almost free of lead.

Lead-free solder includes the following types:

• 1. Tin (95% or more) + silver or copper (melting point 217℃)
• 2. Tin + zinc (melting point 200℃)
• 3. Tin + copper (melting point 227℃)

While lead-free solder is environmentally friendly, it has a higher melting point than eutectic solder and inferior wettability, making it less efficient to work with. In addition, such solders have a rough finish that lacks luster. Efforts are underway to improve the working efficiency of lead-free solder.

Low-melting point solder for desoldering

Low-melting point solder is used to desolder components soldered to the board (Figure 4).

Figure 4: Low-Melting Point Solder Kit
Source: https://shop.sunhayato.co.jp/en/products/smd-21

Low-melting point solders contain bismuth and other elements and have a low melting point of less than 100℃ (40-95℃). The solder does not harden immediately and remains in a liquid state for a long time, making it useful for removing multi-pin ICs. However, low-melting-point solders are not suitable for joining due to their low joint strength.

Summary

Solders require low melting points and high strength. The lower the melting point, the less damage to the board and components, and the higher the strength, the higher the reliability.

As electronic components become smaller, voids around the diffusion layer caused by soldering have become a growing problem. Many manufacturers and research groups are addressing this issue, while at the same time developing the next generation of lead-free solders.

References

Zenyoji, Kaoru. [ZEP Engineering]. “KiCad 6 Compatible, Complete Manual,” Super Introduction to KiCad: Printed Circuit Board Development with Video. [Video](Japanese only)

Zenyoji, Kaoru. [ZEP Engineering]. “KiCad 6 Compatible, Professional Finishing Techniques 101,” Super Introduction to KiCad: Printed Circuit Board Development with Video. [Video] (Japanese only)