What reasons may affect the reliability of lead-free solder joints?

Date：2021-09-15 15:38:00Views：1152

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. Next, what reasons may affect the reliability of lead-free solder joints?

1. Performance requirements for lead-free solder

Traditional tin lead solder is a standard material for connecting components and printed circuit boards because of its low price, easy welding, beautiful forming and good physical, mechanical and metallurgical properties. It has formed a complete set of application technology and has been favored by electronic manufacturers for a long time. However, due to the adverse effects of lead and lead compounds on human health and living environment, the voice of restricting and prohibiting the use of lead containing solder is growing. Governments have formulated corresponding laws and regulations to restrict the use of electronic products and the treatment of waste. The environmental friendliness requirements of electronic packaging have become a global trend. Therefore, at present, the electronic industry is facing the requirements of lead-free, which has had a great impact on the whole industry. In recent years, lead-free solder has developed rapidly, and the most commonly used is Sn Ag Cu series.

2. Factors affecting the reliability of lead-free solder joints

Compared with the traditional lead containing process, lead-free welding will inevitably affect the reliability of solder joints due to the difference of solder and the adjustment of process parameters. Firstly, at present, the melting point of lead-free solder is high, which is generally about 217 ℃, while the melting point of traditional Sn Pb eutectic solder is 183 ℃, and the improvement of temperature curve will bring problems such as easy oxidation of solder and rapid growth of intermetallic compounds. Secondly, because the solder does not contain Pb and the wettability of the solder is poor, it is easy to cause that the self calibration ability, tensile strength and shear strength of the product solder joint can not meet the requirements. Taking a manufacturer as an example, the unqualified rate of solder joints in the original lead containing process is generally 50 × About 10-6 (0.05%), while the unqualified rate of lead-free process rises to 200 due to poor solder wettability × 10-6～500 × 10-6（0．2～0．5％）。

Since there are still many problems in the reliability of lead-free solder joints, it is necessary to analyze them. The reliability problems of lead-free solder joints mainly come from: shear fatigue and creep cracks of solder joints, electromigration, cracks formed by intermetallic compounds at the interface between solder and matrix, short circuit caused by Sn whisker growth, electrical corrosion and chemical corrosion, etc. In the following, we mainly introduce some factors affecting the reliability of lead-free solder joints from the perspective of design, material and process.

(1) Design: reasonable design of PCB. Such as unreasonable pad design, dense distribution of components with high calorific value, high-rise effect of adjacent tall components during reflow soldering, hot air impact, etc.

(2) Material: the choice of solder is extremely important. At present, Sn Ag Cu alloy series is mostly used, and the liquid phase temperature is 217 ℃ - 221 ℃, which requires high peak temperature for reflow welding. As mentioned above, it will lead to problems such as high-temperature oxidation of solder and conductor materials (such as Cu foil) and rapid growth of intermetallic compounds. Because in the welding process, when the molten solder contacts the welding substrate, a layer of intermetallic compound (IMC) will be formed at the interface due to high temperature. Its formation is not only controlled by reflow temperature and time, but also its thickness will increase with time in the later use process.

The results show that the intermetallic compound on the interface is a key factor affecting the reliability of solder joints. The existence of too thick intermetallic compound layer will reduce the fracture, toughness and low cycle fatigue resistance of solder joints, resulting in the reduction of the reliability of solder joints. Taking the most mature Sn Ag lead-free solder as an example, due to the higher melting point, the corresponding reflow soldering temperature will also increase. In addition, the Sn content in the lead-free solder is higher than that of Sn Pb solder, both of which increase the rate of metal compounds formed at the interface between the solder joint and the matrix, resulting in premature failure of the solder joint.

In addition, due to the different composition of lead-free solder and traditional Sn Pb solder, their reaction rate and reaction products with pad materials such as Cu, Ni and AgPd may be different, and the solder joints will also show different reliability. At the same time, the compatibility of solder and flux will also have a great impact on the reliability of solder joints. Studies have shown that the incompatibility between the components of solder and flux will reduce the adhesion. In addition, due to the mismatch of thermal expansion coefficient, the periodic fatigue failure of solder will be accelerated. Therefore, special attention should be paid to the selection of solder and flux with excellent compatibility in order to withstand the high temperature impact during lead-free reflow welding.

In addition, all interconnect welding parts come from different manufacturers, so the quality of parts is inevitably uneven, such as poor solderability of device pins, which has a great impact on the reliability of lead-free solder joints. A typical example is the quality problem of PCB pad. Due to the shortcomings of the previous hot air leveling (HASL) pad coating process, the organic solderable protective layer (OSP) and Ni / Au coating process are widely used by OEM manufacturers.

There are two kinds of Ni / Au coating: Gold dipping method and gold plating method. Gold dipping method is favored by domestic manufacturers because of its simple process, but this method is difficult to control the thickness of Au layer. Insufficient thickness of Au layer often leads to the oxidation of Ni layer under it, which affects the performance of solder joint during reflow welding. In this case, manufacturers can generally use Auger electron spectrometer (AES) to accurately measure whether the Au layer thickness of PCB pad meets the specification.

(3) Process: during the manufacturing process of SMT and MCM, problems such as improper solder storage temperature, insufficient solder on the pad, improper setting of reflow soldering temperature curve and so on are usually encountered. For lead-free welding, the optimization of reflow welding process temperature curve is very important. Excellent process can not only ensure the formation of high reliability welding, but also maintain the peak temperature as low as possible.

Therefore, at present, except Japan, consumer electronics companies in other countries seem to accept the SN Ag Cu alloy series. The proportion of silver in the alloy is 3.0% ~ 4.7% and that of copper is 0.5% ~ 3.0%. The melting points of the alloys with different compositions have little difference, basically between 217 ℃ - 221 ℃, while the liquid phase temperature of tin lead alloy (63% tin and 37% lead) is 183 ℃, with a difference of 34 ℃.

Therefore, the key variables in the reflow process, such as peak temperature, time higher than the liquid phase temperature, immersion time, immersion temperature and ramp rate due to the selection of flux and solder paste, are closely monitored to ensure that the reflow process maintains CPK of 1.33 or higher. Another point to note is the use of lead-free solder containing Bi. It is found that Sn Pb Bi eutectic alloy will be formed after reflow welding when Bi containing solder is in contact with Sn Pb coated devices. The melting point is only 99.6 ℃, which is very easy to lead to cracking of the welding part. Therefore, for the use of Bi containing lead-free solder, attention should be paid to whether the device coating is Sn Pb coating.

In addition, the problem of voids in lead-free welding process. Cavity is a common defect in reflow soldering of interconnect solder joints, especially in BGA / CSP devices. Due to the large differences in the size, location, proportion and measurement of holes, the safety evaluation of hole level has not been unified so far. Experienced engineers are used to attribute the voids without large voids (the sum of the volume of small voids does not exceed 0.5% of the volume of the solder joint), the void proportion is less than 15% ~ 20%, and the voids not concentrated at the connection to a common defect in reflow welding, and consider it acceptable;

On the other hand, according to the research results of Motorola, the diameter is 3 μ m～5 μ M cavity can actually improve the long-term reliability of solder joints, because it can prevent the propagation of cracks in solder joints to a certain extent. However, it is generally believed that large cavities, or when the cavity area reaches a certain proportion, will adversely affect the reliability.

Therefore, cavity is still a problem that must be paid attention to in lead-free welding. In the molten state, the surface tension of Sn / Ag / Cu alloy is greater than that of Sn Pb alloy. The increase of surface tension is bound to make the gas overflow more difficult in the cooling stage and increase the void ratio. This was confirmed during the development of lead-free solder paste. The results showed that the number of holes in solder joints using lead-free solder paste was more than that using tin lead solder paste.Large voids and some small spherical voids are caused by the volatilization of flux. The ratio of flux in solder paste is the most direct factor affecting solder joint voids. Therefore, there is still much room for improvement in lead-free solder paste.