Failure mode analysis: what are the lead-free reliability test items for welding?

Date：2021-11-12 11:26:00 Views：1292

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. 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.

Reliability test items for SMT lead-free solder joints include:

1. Mechanical vibration test

2. Mechanical shock test

3. Thermal shock test

4. High accelerated aging test (halt test)

5. Thermal and humidity test

Basically, after the above tests, the reliability of lead-free solder joints can be verified.

At present, the reliability of lead-free solder joints has been paid more and more attention. The failure mode of solder joints and the factors affecting the reliability of lead-free solder joints are introduced as follows:

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 joints 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, gold and silver are often etched in the assembly process of thick and thin film hybrid circuits (including chip capacitors). 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. Aging induced failure

When the molten solder contacts 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 grow continuously. 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 during service, 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 temperature cycle process. The thermal expansion mismatch of packaging materials will produce stress and strain in the solder joint. 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 cracks and leads to failure 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 & Metals; 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 no solder joint failure after 1500 thermal shock tests. 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.

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