Lead free welding knowledge: the difference between lead-free welding and solder joint

Date：2021-09-16 15:22:30Views：1084

Nowadays, in electronic assembly technology, people's awareness of environmental protection is becoming stronger and stronger. From the aspects of environmental protection, legislation, market competition and product reliability, lead-free is imperative. However, at present, lead-free is not mature in theory and application, and a relatively unified standard has not been formed.

The first challenge of lead-free technology to components is high temperature resistance. The influence of high temperature on component packaging should be considered. As the packaging materials of traditional surface mount components can meet the welding temperature of lead solder as long as they can withstand the high temperature of 240 ℃, while the welding temperature of complex products is as high as 260 ℃ during lead-free welding, whether the component packaging can withstand the high temperature must be considered.

In addition, the influence of high temperature on the internal connection of the device should be considered. The internal connection methods of IC include gold wire ball welding, ultrasonic pressure welding, flip chip welding and other methods, especially BGA, CSP, combined composite components, modules and other new components. For example, the solder paste for flip chip bumps of BGA and CSP is Sn-3.5Ag welding, with a melting point of 221 ℃. If such devices are used for lead-free welding, Then the solder joints inside the device and the solder joints assembled on the surface are melted and solidified almost at the same time, which is very harmful to the reliability of the device. Therefore, the internal connection materials of lead-free components should also meet the requirements of lead-free welding.

Most of the welding ends of lead-free components are sn / Pb coatings, while there are many kinds of coatings on the welding ends of lead-free components. For the requirements of lead-free process for flux, firstly, due to the chemical reaction between flux and alloy surface, different flux should be selected for different alloy components. Lead free flux must be specially prepared. With the deepening of lead-free process, the quality of lead-free solder paste has been greatly improved due to the efforts of solder manufacturers. At present, the appearance of lead-free solder joints has been improved compared with previous years.

The reliability of lead-free welding is of great concern to manufacturers and users. Especially at present, it is in a special stage of transition from lead-free welding to lead-free welding. There are no standards for lead-free materials, printed boards, components, etc. even when there are no standards for reliability test methods, the reliability is very worrying. At present, the lead-free process, especially in China, is still in a chaotic stage. Due to the mixed use of lead and lead-free, especially when the components at the lead-free welding end adopt lead solder and lead process, serious reliability problems will occur. These problems should be paid attention not only to the lead-free welding in the current transition stage, but also to the lead-free welding in the transition stage.

Requirements for the realization of lead-free solder:

1. The melting point of lead-free solder should be low, as close as possible to the eutectic temperature of 63 / 37 Sn Pb alloy of 183 ℃. The melting interval temperature difference of lead-free solder should be reduced as much as possible, that is, the temperature range between solidus and liquidus should be reduced as much as possible.

2. Lead free solder should have good wettability; Ensure that the solder can show good wettability within the above time range to ensure high-quality welding effect;

3. The conductivity and thermal conductivity after welding should be close to that of 63 / 37 Sn Pb alloy solder;

4. The tensile strength, toughness, ductility and creep resistance of solder joints are not much different from those of tin lead alloy;

5. Reduce the cost as much as possible; At present, it can be controlled at 1.5 ~ 2 times of tin lead alloy, which is an ideal price.

Because lead is relatively soft and easy to deform, the hardness of lead-free solder joint is higher than Sn / Pb, the strength of lead-free solder joint is also higher than Sn / Pb, and the deformation of lead-free solder joint is smaller than Sn / Pb solder joint, but these do not mean that the reliability of lead-free solder joint is good. Due to the poor wettability of lead-free solder, there are many welding defects such as voids, displacement and monument. In addition, due to the high melting point, if the activation temperature of flux can not match the high melting point, due to the high temperature and long time in the flux infiltration area, the welding surface will be re oxidized at high temperature without infiltration and diffusion, and a good interface alloy layer can not be formed, As a result, the bonding strength (tensile strength) of the solder joint interface is poor and the reliability is reduced.

Main characteristics of lead-free welding and solder joints

(1) Main features of lead-free welding

(A) The high temperature and melting point are about 34 ℃ higher than those of traditional lead eutectic solder.

(B) High surface tension and poor wettability.

(C) The process window is small and the quality control is difficult.

(2) Characteristics of lead-free solder joints

(A) Poor infiltration and expansibility.

(B) The appearance of lead-free solder joints is rough. Traditional inspection standards and AOI need to be upgraded.

(C) There are many air holes in lead-free solder joints, especially when the lead solder end is mixed with lead-free solder, the lead solder on the solder end (ball) melts first, covers the pad, and the flux cannot be discharged, resulting in air holes. However, the porosity does not affect the mechanical strength.

(D) Many defects - the self localization effect is weakened due to poor wettability.

Lead free solder joints have rough appearance, many pores, large wetting angle and no half moon shape. Because the appearance of lead-free solder joints is obviously different from that of lead solder joints, they can even be considered unqualified if measured by the original inspection standard with lead. With the deepening and development of lead-free technology, the improvement of flux and the progress of process, The rough appearance of lead-free solder joints has been improved.

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