An Ultimate Guide: How to Identify Good Solder Joint​

The solder joint refers to the part that connects the pin of the electronic component to the pad on the circuit board by melting the solder. Good solder joint not only ensures good electrical connections, but also enables it to withstand the impact of environmental factors such as mechanical stress and temperature changes. So how to identify good solder joint? We can now dive into the quality insights of this helpful product and find the ways to choose the best solder joint to fulfill your needs.

Basic concepts of solder joint​

During the welding process, the solder (usually tin-lead alloy or lead-free solder) is heated to a certain temperature and melted, filling the gap between the component pin and the board pad, cooling and solidifying to form mechanical and electrical connections. The main role of the solder joint can be divided into two aspects: one is to provide mechanical support to ensure that the electronic components are firmly fixed on the circuit board and can withstand a certain external impact and vibration; The second is to achieve electrical conduction to ensure that the current can flow smoothly in the circuit, so that the electronic components work normally.

Here are two types of common welding. One is called wave soldering. This is a more traditional and widely used welding method. In this process, the melted solder forms an upward wave peak under the action of the pump, and the circuit board passes the wave peak at a specific angle and speed, so that the solder joint is formed between the component pin and the pad. Wave soldering is suitable for mass-produced circuit boards with regular component layout.

The other is named reflow welding. With the development of electronic products in the direction of miniaturization and high precision, reflow welding has gradually become mainstream. Reflow welding is to first print an appropriate amount of solder paste on the pad of the circuit board, and then accurately place the electronic components on the pad, and then send the circuit board into the reflow furnace. In the reflow furnace, the circuit board goes through the stages of preheating, heating, reflow and cooling. The solder in the solder paste melts and fills the gap in the reflow stage, and forms a solder joint after cooling. Reflow welding enables precise welding and is suitable for a variety of complex circuit board assemblies.

How to identify good solder joint​: Internal structure requirements

1. Uniform metallographic structure

The microstructure of solder joints can be observed by metallographic analysis. The metallographic structure of high-quality solder joints should be uniform, without obvious defects such as grain size, segregation or inclusion. The uniform metallographic structure means that the solder joints cool at a uniform rate during solidification and the atoms are arranged in an orderly manner, which helps to improve the mechanical properties and corrosion resistance of the solder joints. For example, electronic products used in high temperature environments, if the solder joint metallographic organization is not uniform, under the action of thermal cycling, the expansion and contraction of different areas are inconsistent, easy to lead to solder joint cracking, causing electrical failure.

2. Moderate intermetallic compound layer

At the interface between the solder joint and the base material (component pins and pads), an intermetallic compound (IMC) is formed. An appropriate amount of intermetallic compound layer is essential to ensure the strength of the connection between the solder joint and the base material. This layer of compounds can effectively transfer stress and current, but if the intermetallic compound layer is too thick or uneven growth, it will make the solder joint brittle and reduce its fatigue resistance. The thickness and quality of intermetallic compound layers are affected by many factors such as welding temperature, time and base material. For example, too high welding temperature and too long welding time will promote the excessive growth of intermetallic compounds, which will affect the quality of the solder joint.

3. No pores and cracks

Inside the solder joint, there should be no obvious porosity and cracks. The porosity is the void formed in the solder joint due to the failure of gas discharge in time during the welding process, which will reduce the effective bearing area of the solder joint and reduce the mechanical strength of the solder joint. Cracks are more serious defects, even small cracks, under the action of vibration, thermal stress, etc., may gradually expand, and eventually lead to solder joint failure. Detecting whether there are pores and cracks inside the solder joint usually requires non-destructive testing technology, such as X-ray testing, ultrasonic testing and so on.

How to identify good solder joint​: Appearance inspection

1. Surface gloss

The surface of a good solder joint should have a perfect gloss, showing a bright, uniform metallic texture, no obvious bumps or bubbles. This is because during the welding process, the appropriate welding temperature and time make the solder fully melt and integrate well with the base material, and the solder joint formed after cooling has a dense structure and uniform light reflection, so it looks shiny. For example, in some low-quality solder, the impurity content is high, and the solder joint is prone to gray and foggy phenomenon after welding.

2. The pin is in full contact with the pad

During welding, the pin of the electronic component should be in full contact with the pad on the circuit board and be fully wrapped with solder. If there is a gap between the pin and the pad or is not fully covered by the solder, it will seriously affect the electrical connection and mechanical strength of the solder joint.

3. Shape integrity

The ideal solder joint shape should be half-moon, full and continuous. Viewed from the side, the solder joint should cover the connection between the element pin and the pad and form a natural transition around the pin. The outline of the solder joint is clear, with no obvious notch, crack or bump. The complete solder joint shape provides sufficient mechanical strength and electrical connection area. When the shape of the solder joint is incomplete, such as virtual welding (there is a small gap between the solder joint and the pin or pad, and it is not really connected), it will lead to unstable electrical performance, and intermittent failure may occur during the use of the product, which seriously affects the quality of the product.

4. Good wettability

Wettability refers to the ability of solder to spread on the surface of the base material. The solder of good solder joints can well wet the component pins and circuit board pads, and the solder is evenly covered on the surface of the pins and pads, and there is no accumulation or contraction of the solder. Good wettability indicates that the interfacial tension between the solder and the base material is small and can form a strong bond. If the wettability is not good, it may be due to the oxidation layer, oil and other pollutants on the surface of the pin or pad, which hinder the spreading of the solder; It may also be that the welding flux is not selected properly or the amount is insufficient to effectively remove oxides and reduce surface tension.

How to identify good solder joint​: Electrical performance

For one part, a good solder joint should have a low resistance value to ensure that the current can pass smoothly. If the resistance of the solder joint is too high, it may be due to the failure of the solder to sufficiently wet the pad and pin, or the presence of cracks or pores in the solder joint. High resistance will not only affect the performance of the circuit, but also may cause problems such as overheating and burning.

In addition to ensuring electrical connections, a good solder joint should also have good insulation properties to prevent short circuits between adjacent circuits. If there are impurities, oil, or solder spills on the surface of the joint, it may reduce its insulation performance.

How to identify good solder joint​: Mechanical strength

Firstly, a good solder joint should have sufficient tensile strength to withstand the tension generated by electronic components during the working process. If the tensile strength of the solder joint is insufficient, it may cause the electronic components to fall off or the welded joint to break.

Secondly, in addition to tensile strength, solder joint should also have sufficient shear strength to withstand the impact of electronic components when subjected to shear forces. If the shear strength of the solder joint is insufficient, the circuit connection may fail.

Thirdly, in the daily working environment, a solder joint usually needs to withstand various vibrations and shocks. Good solder joint should have good vibration and shock resistance to ensure the stability and reliability of the circuit connection.

How to identify good solder joint​: Environmental adaptability

Electronic products may encounter harsh temperatures during operation. Good solder joint should be able to maintain stable electrical connections and mechanical strength under these extreme temperature conditions. If the solder joint is easy to melt or fall off at high temperatures, and becomes fragile and brittle at low temperatures, the reliability of electronic products will be seriously affected.

Moreover, under humid or corrosive environments, solder joints are vulnerable to erosion by moisture and harmful substances, resulting in electrical connection failure or decreased mechanical strength. Therefore, a good solder joint should have good moisture and corrosion resistance to ensure the long-term stable operation of the circuit.

Functional test for identifying good solder joint

The good solder joint judgment based on functional tests can be mainly divided into two types. First, electrical performance tests are frequently applied.

In addition to the electrical performance test, the environmental simulation test is also popular for identifying good solder joint.

 Final Verdict

As an important guarantee for the reliability of electronic products, the identification and control of solder joint are important. By means of appearance inspection, testing of electrical performance, mechanical strength and environmental adaptability, we can effectively identify good solder joint​. Hope this article is helpful for you.