PCB News - pcb factory: SMT solder joint fatigue failure mechanism analysis

PCB factory: SMT solder joint fatigue failure mechanism analysis

As the assembly density of electronic products becomes higher and higher, the size of the solder joints that undertake the mechanical and electrical connection functions is getting smaller and smaller, and the failure of any solder joint may cause the overall failure of the device and even the system. Therefore, the reliability of solder joints is one of the keys to the reliability of electronic products. In practice, the failure of solder joints is usually caused by the interaction of various complex factors. Different use environments have different failure mechanisms. The main failure mechanisms of solder joints include thermal failure, mechanical failure and electrochemical failure.

Thermally induced failures are mainly fatigue failures caused by thermal cycling and thermal shock, and failures caused by high temperatures are also included. Due to the mismatch of thermal expansion coefficient between surface mount components, PCB and solder, when the ambient temperature changes or when the power of the component itself heats up, because the thermal expansion coefficients of the components and the substrate are inconsistent, the solder joints will generate thermal stress and stress. The periodic changes of the solder joints lead to thermal fatigue failure. The main deformation mechanism of thermal fatigue failure is creep. When the temperature exceeds half of the furnace temperature, creep becomes an important deformation mechanism. For tin-lead solder joints, even at room temperature, it has exceeded half of the melting point temperature, so Creeping becomes the main thermal deformation fatigue failure mechanism during thermal cycling.

Compared with the thermal cycle, the failure caused by thermal shock is caused by the large additional stress brought by the different temperature rise rate and cooling rate to the component. During thermal cycling, it can be considered that the temperature of each part of the component is exactly the same; and under thermal shock conditions, due to various factors such as specific heat, quality, structure and heating method, the temperature of each part of the component is different, resulting in additional thermal stress. . Thermal shock can cause many reliability problems, such as sweat spot fatigue during overload, and cracks in the coating that lead to corrosion failure and component failure. Thermal shock may also cause failure modes that did not occur during the slow thermal cycle.

Mechanical failure mainly refers to overload and impact aging caused by mechanical shock, and mechanical fatigue failure caused by mechanical vibration. When the printed circuit assembly is subjected to bending, shaking or other stress, it may cause the solder joint to fail. When the printed circuit assembly is subjected to bending, shaking or other stress, it may cause the solder joint to fail. Generally speaking, smaller and smaller solder joints are the weakest link in the component. However, when it connects components with flexible structures such as pins to the PCB, since the pins can absorb part of the stress, the solder joints will not bear much stress. However, when assembling leadless components, especially for large-area BGA devices, when the components are subjected to mechanical shocks, such as dropping and PCBs undergoing greater impact and bending in the subsequent equipment and testing procedures, the components themselves The rigidity is relatively strong, and the solder joints will bear greater stress.

Especially for portable electronic products with lead-free soldering, due to their small size, light weight and easy to slip, they are more likely to collide and fall during use, and lead-free solder is higher than traditional lead-tin solder. Elastic modulus and other different physical and mechanical characteristics make the resistance of lead-free solder joints to reduce mechanical impact. Therefore, attention should be paid to the reliability of lead-free portable electronic products and the drop impact reliability. When the welding part is subjected to repeated mechanical stress caused by vibration, it will cause the fatigue failure of the solder joint. Even when this stress is much lower than the yield stress level, it may cause metal material fatigue. After a large number of small amplitude, high frequency vibration cycles, vibration fatigue failure will occur. Although the damage to the solder joints is small for each vibration cycle, cracks will occur at the solder joints after many cycles. Over time, cracks will spread with the increase in the number of cycles. This phenomenon is more serious for solder joints of leadless components.

Electrochemical failure refers to failure caused by electrochemical reaction under certain temperature, humidity and bias conditions. The main forms of electrochemical failure are: bridging caused by conductive ion pollutants, dendrite growth, conductive anode filament growth and tin whiskers. Ion residues and water vapor are the core elements of electrochemical failure. The conductive ionic contaminants remaining on the PCB may cause bridging between solder joints, especially in a humid environment. Ionic residues can cross metal and insulating surfaces. Move to form a short circuit. Ionic pollutants can be produced in many ways, including printed circuit board manufacturing process solder paste and flux residues, manual handling pollution and atmospheric pollutants. Under the combined influence of water vapor and low-current DC bias, metal migration from one conductor to another due to electrolysis causes metal dendrites that look like branches and ferns to grow. The migration of silver is the most common. Copper, tin, and lead are also susceptible to dendrite growth, but they are slower than silver dendrite growth. Like the growth of other metals, this failure mechanism can cause short circuits, leakage and Other electrical failures. The growth of conductive anode filaments is a special case of dendrite growth. The transport of ions across the insulator and between several conductors causes the growth of metal filaments on the surface of the insulator, which can cause short circuits in adjacent conductive lines. Tin whisker refers to the fact that some whisker-like tin single crystals will grow on the surface of the tin plating layer under the influence of machinery, humidity and environment during long-term storage and use of the device, the main component of which is tin. Because tin whisker has caused several typical major accidents such as aerospace and aviation, it has received widespread attention.