Poor microstructure with serious deterioration of solder joint performance

The mechanical properties of the solder joint depend on the microstructure of the solder joint, and the microstructure depends on the solder composition, the base metal to be welded, the structure of the solder joint and the process conditions during welding. In actual manufacturing, if the influence of solder joint structure and process conditions on the microstructure of solder joint is ignored, it may form seriously degraded mechanical properties of solder joint, and then form poor microstructure affecting the reliability of solder joint. This paper mainly introduces some common bad microstructure, so as to realize that the formation of high reliability solder joints can not be realized "casually", and it is not only necessary to see that the appearance meets the requirements of iopc-a-610. It is necessary to pay attention to the details of the process to ensure that the microstructure of solder joints meets the requirements.

The poor microstructures involved in this paper are mainly interfacial intermetallic compounds (IMC), which are very sensitive to over stress.

Interfacial intermetallic compounds usually have the following characteristics:

1) It is hard and brittle, which is a basic characteristic of intermetallic compounds.

2) The coefficient of thermal expansion is seriously compatible with the solder (e.g. 23 for Sn alloy) × 10-6 / ℃; ni3sn4 is 13.7 × 10-6/℃） 。

3) The microstructure often has defects, such as krkendall space, Ni3P crystal and black disk.

4) During service, intermetallic compounds at the interface will still grow continuously. If they grow excessively, the interface will weaken or even crack.

These characteristics have a great impact on the connection reliability of solder joints, especially when subjected to over stress and impact stress, solder joints are prone to cracking or fracture. For this reason, some papers or monographs focus on the influence of interfacial intermetallic compounds on solder joint reliability [1-2]

one Blocky imcgx

Blocky IMC is not a professional term. The author uses it to describe an ultra thick, ultra wide and intermittent IMC morphology (as shown in the slice diagram shown in Figure 1) - scallop shaped IMC tissue. The tissue is thick (w ≥ 5 µ m, H ≥ 5 µ m), the continuous layer is relatively thin, and even there is disconnection in some places.

Fig. 2 is the slice diagram of solder joint formed by high temperature and long-time reflow welding of BGA, showing a typical blocky IMC structure. BGA Solder ball is made of sac305 and OSP (organic solderability preserving) pad treatment process. SnPb solder paste is used for welding. The welding peak temperature is 235 ℃ and the time above 217 ℃ is 70s. The test shows that its shear strength is more than 20% lower than that of normal solder joints.