What are the test methods for reliability analysis of lead-free solder joints

Date：2022-04-29 14:23:36Views：808

Product inspection is an essential means of quality control in the production of modern electronic enterprises. It mainly plays the role of process control, quality control and judging the qualification of products. The product inspection shall implement the "three inspection" system combining self inspection, mutual inspection and full-time inspection. SMT packaging technology widely used in electronic packaging, new chip size packaging (CSP), solder ball array (BGA) and other packaging technologies require the direct realization of electrical and rigid mechanical connection between different materials through solder joints (mainly bearing shear strain). Its quality and reliability determine the quality of electronic products.

The failure of a solder joint may cause the failure of the whole device, so how to ensure the quality of solder joint is an important problem. Traditional lead tin solder contains lead, and lead and lead compounds are highly toxic substances. The long-term use of lead solder 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 great impact on the whole industry. Lead free solder has begun to gradually replace lead solder, but lead-free technology will inevitably bring new problems to the reliability of solder joints due to the difference of solder and the adjustment of welding process parameters. Therefore, the reliability of lead-free solder joints has been paid more and more attention.

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 failed products, find out the failure mode and analyze the failure cause. The purpose is to correct and improve the design process, structural parameters and welding process. The solder joint failure mode is very important for the prediction of cycle life and the basis of establishing its mathematical model. Three failure modes are described below.

1. Solder joint failure caused by welding process

Some adverse factors in the welding process and subsequent improper cleaning process may lead to solder joint failure. The reliability of SMT solder joint mainly comes from the production assembly process and service process. In the process of production and assembly, 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 welding, solder short circuit and Manhattan phenomenon.

On the other hand, in the process of use, the inevitable impact and vibration will also cause mechanical damage to the solder joint. For example, the rapid cold and heat change 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 assembly process of thick and thin film hybrid circuits (including chip capacitors), gold and silver are often corroded. This is because the tin in the solder forms a compound with the gold and silver in the gold-plated or silver-plated pins, resulting in the reduction of the reliability of the solder joint. Excessive ultrasonic cleaning may also affect the reliability of solder joints.

2. Invalidation caused by aging

When the molten solder is in contact with the clean substrate, intermetallic compounds will be formed at the interface. In 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 kinetics of IMC layer. Although 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 various materials constituting the solder joint, the solder joint will experience periodic strain in the service process, and failure will be caused when the deformation is large enough. The results show that the addition of trace rare earth element lanthanum to sn60 / pb40 soft solder alloy will 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 caused by thermal cycle

Under the service conditions of electronic devices, the periodic on-off of the circuit and the periodic change of ambient temperature will make the solder joint undergo the process of temperature cycle. The thermal expansion mismatch of packaging materials 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 of solder 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 and failed 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, 42% Ni Fe alloy (CTE = 5) has been developed × 10-6 ℃ - 1), cu-36% Ni Fe alloy (indium tile alloy), Cu Mo Cu and quartz fiber composite materials. Among them, the Cu indium tile Cu composite substrate changes the proportion of each component. The weldment welded with this substrate has undergone 1500 times of thermal shock test, and there is no solder joint failure. 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 practicability is limited.