What are the factors that affect the reliability of lead-free solder joints?

Lead-free soldering does present some solder joint reliability challenges relative to traditional lead (Pb) containing processes. This stems mainly from the differences between Pb-free brazing materials and traditional brazing materials, as well as the adjustment of process parameters. So what are the specific factors that affect the reliability of lead-free solder joints? The following is the relevant expertise compiled by Fitech:

First, lead-free brazing material characteristics on the reliability of the solder joints higher melting point: lead-free brazing material melting point is generally around 217 ℃, than the traditional SnPb eutectic brazing material (melting point of 183 ℃) about 35 ℃ higher. The increased temperature profile results in a solder that is prone to oxidation and rapid growth of intermetallic compounds.

Poor wettability: Pb-free brazing materials have poorer wettability. May lead to product solder joints tensile strength, shear strength, etc. can not meet the requirements.

Second, the reliability of lead-free solder joints Shear fatigue and creep cracks: solder joints in the shear force is easy to fatigue and creep, resulting in cracks.

Electromigration: under the action of the current, the metal elements in the solder joints may occur ionic migration, affecting the stability of the solder joints.

Intermetallic compounds form cracks: Intermetallic compounds formed at the interface between the brazing material and the substrate may cause cracks.

Short circuits due to whisker growth: Whiskers may form in lead-free brazing materials, causing short circuits.

Galvanic and chemical corrosion problems: soldered joints may be affected by galvanic and chemical corrosion, reducing reliability.

Third, the brazing material selection and process requirements brazing material selection: most of the current SnAgCu alloy series, the liquid phase temperature is 217 ~ 221 ℃.

Process requirements: the requirements of reflow soldering has a high peak temperature, but will bring the brazing material and device materials are easy to high-temperature oxidation, rapid growth of intermetallic compounds and other issues.

Fourth, other factors affecting the quality of parts vary: different manufacturers of interconnect soldering parts of different quality, such as device pads, device feet can be soldered bad, etc.,

 lead-free process has a greater impact on the reliability of the solder joints.

V. Conclusion and Prospect

Importance of solder joints: Solder joints are the connection channel between the chip and external devices, and play multiple roles in the microelectronics packaging industry, and the relevant design and process should be given due attention.

Optimisation of soldering process: It is of great significance to the electronic packaging industry to actively optimise the soldering process, find out the failure mode, analyse the failure mechanism, and improve the quality and reliability of products.

Unresolved issues and research direction: there are still some outstanding issues in the process of lead-free, such as shear fatigue of solder joints, creep problems, false soldering phenomenon, etc., are to be further studied. At the same time, new reliability issues brought about by lead-free electronic assembly also need to be explored。