Failure mode analysis of solder joint quality in lead-free solder joint reliability test

Date：2022-05-31 15:30:41Views：718

With the rapid development of the electronic information industry, micro spacing devices have developed with higher and higher assembly density, and new SMT and MCM technologies have been born. The solder joints in microelectronic devices are becoming smaller and smaller, but the mechanical, electrical and thermodynamic loads they carry are becoming heavier and heavier, and the requirements for reliability are increasing. SMT packaging technology, new chip size packaging (CSP), solder ball array (BGA) and other packaging technologies widely used in electronic packaging require that electrical and rigid mechanical connections between different materials (mainly bearing shear strain) be directly realized through solder joints. Its quality and reliability determine the quality of electronic products.

The failure of a solder joint may lead to the failure of the whole device, so how to ensure the quality of the solder joint is an important issue. Traditional lead tin solders contain lead, and lead and lead compounds are highly toxic substances. The long-term use of lead containing solders will bring serious harm to human health and living environment.

At present, the demand for lead-free soldering in the electronic industry is becoming more and more urgent, which has had a huge impact on the whole industry. Lead free solder has begun to replace lead solder gradually, but lead-free technology will inevitably bring new problems to solder joint reliability due to solder differences and adjustment of welding process parameters. Therefore, the reliability of lead-free solder joints has been paid more and more attention. This paper describes the failure modes of solder joints and the factors affecting the reliability of lead-free solder joints, and introduces the reliability test methods of lead-free solder joints.

Failure mode of solder joint

The reliability experiment of solder joint includes reliability experiment and analysis. On the one hand, its purpose is to evaluate and identify the reliability level of integrated circuit devices and provide parameters for the reliability design of the whole machine; On the other hand, it is necessary to improve the reliability of solder joints. This requires the necessary analysis of the failed products, finding out the failure mode and analyzing the failure cause. The purpose is to correct and improve the design process, structural parameters, welding process, etc. the failure mode of the solder joint is very important for the prediction of the cycle life and is the basis for the establishment of its mathematical model.Three failure modes are described below.

1. Solder joint failure caused by welding process

Some unfavourable factors in the welding process and the subsequent improper cleaning process may lead to solder joint failure. SMT solder joint reliability mainly comes from the production assembly process and service process. During the production and assembly process, due to the limitations of equipment conditions such as pre welding preparation, welding process and post welding inspection, as well as human errors in the selection of welding specifications, welding faults are often caused, such as false soldering, solder short circuit and Manhattan phenomenon.

On the other hand, in the process of use, the inevitable impact, vibration, etc. will also cause mechanical damage to the welding joint. For example, the rapid cold and heat changes in the wave soldering process will cause a temporary temperature difference to the element, making the element bear thermal mechanical stress. When the temperature difference is too large, the ceramic and glass parts of the element will produce stress cracks. Stress crack is an unfavorable factor affecting the long-term reliability of solder joints.

At the same time, in the process of assembling thick and thin film hybrid circuits (including chip capacitors), gold and silver are often corroded. This is because tin in the solder and gold or silver in the gold-plated or silver-plated pins form a compound, which reduces the reliability of the solder joint. Excessive ultrasonic cleaning may also affect the reliability of the solder joint.

2. Invalidation due to aging

When the molten solder contacts the clean substrate, intermetalliccompounds will be formed at the interface. During the aging process, the microstructure of the solder joint will coarsen, and the IMC at the interface will continue to grow. The failure of solder joint depends partly on the growth dynamics of IMC layer. Although the intermetallic compound at the interface is a sign of good welding, with the increase of its thickness during service, it will cause microcrack initiation and even fracture in the solder joint.

When its thickness exceeds a certain critical value, the intermetallic compound will show brittleness. Due to the thermal expansion mismatch between the various materials that make up the solder joint, the solder joint will experience periodic strain during service. When the deformation is large enough, it will lead to failure. The results show that the addition of lanthanum to sn60 / pb40 soft solder alloy can reduce the thickness of metal compounds, increase the thermal fatigue life of solder joints by two times, and significantly improve the reliability of surface assembled solder joints.

3. Failure due to thermal cycling

Under the service condition of electronic devices, the periodic on-off of the circuit and the periodic change of the ambient temperature will make the solder joint undergo the temperature cycle process. The thermal expansion mismatch of the packaging material will produce stress and strain in the solder joint. For example, in SMT, the coefficient of thermal expansion (CTE) of chip carrier material A1203 ceramic is 6*10-6 ℃ -1, while the CTE of epoxy resin / glass fiber substrate is 15*10-6 ℃ -1. When the temperature changes, the solder joint will bear certain stress and strain. Generally, the strain borne by the welding joint is 1% ~ 20%. In THT process, the flexible pin of the device will absorb most of the strain caused by thermal mismatch, and the strain borne by the solder joint is very small. In SMT, the strain is basically borne by the solder joint, which will lead to the initiation and propagation of cracks in the solder joint and eventually failure.

Because the solder joint is cracked due to the thermal stress caused by the mismatch of thermal expansion coefficient, improving the thermal matching between leadless components and substrate materials is the first concern. At present, new materials such as 42%ni Fe alloy (cte=5*10-6 ℃ -1), cu-36%ni Fe alloy (indium tile alloy), Cu Mo Cu and quartz fiber composites have been developed. The Cu indium tile Cu composite substrate changes the proportion of each component. The weldment welded with this substrate has undergone 1500 times of hot impact tests, and no solder joint has failed. In addition, the technology of compounding a stress absorbing layer with large elasticity on the printed board to absorb the stress caused by thermal mismatch has also been developed, and good results have been achieved. However, the process of the new substrate material is complex and the price is relatively expensive, so its practicality is limited.