Printed circuit board is widely used, but due to the cost and technology, there are a large number of failure problems in the production and application of PCB, and thus lead to a lot of quality disputes. In order to find out the cause of failure and find the solution to the problem and distinguish the responsibility, the failure case must be analyzed.
The sample is a PCBA board. After SMT, a small amount of solder pad is found to be soldering defects, and the failure rate of the sample is about 3/1000. The surface finished process of the PCB pad is immersion tin, and the PCB is double-sided surface mount technology , and all the pads with soldering defects are located on the second side.
2. Analysis explanation：
First of all, the appearance inspection was carried out. Through the microscopic magnification observation of the failed pad, there was no solder paste phenomenon on the pad, no obvious discoloration and other abnormal conditions were found on the surface of the pad. The result is shown in figure 1:
Figure 1 :soldering defects
Then the surface SEM observation and EDS composition analysis of NG pad, once over furnace pad and non furnace pad were carried out respectively. The surface of NG pad was well formed, and recrystallization appeared on the surface of once furnace pad and failure pad. No abnormal elements were found on the surface, and shown in figure 2 to 4 respectively:
Figure 2：SEM photos and EDS spectra of NG pad
Figure 3 EDS Spectral Diagram of EM Photo of Pad after a Wave Peak Welding
Figure 4 EDS Spectral Diagram of EM Photo of Pad
Then FIB technology is used to make the profile of the failure pad, the once over furnace pad and the unfired pad, and the electronic component line scanning on the surface of the section is carried out. It is found that the Cu element has already appeared in the surface layer of the NG pad, indicating that Cu has diffused to the tin layer surface. The Cu element appears in the surface layer of the overfurnace pad at the depth of about 0.3 μ m, which indicates that the thickness of pure tin layer is about 0.3 μ m after the primary pad. The thickness of pure tin layer is about 0.8 μ m, which indicates that the surface layer of unhearth pad is about 0.8 μ m deep, which indicates that the thickness of pure tin layer is about 0.8 μ m. In view of the low precision and relatively large error of EDS test, the surface composition of AES butt pad is further analyzed.
The result is shown in figure 5 to 7：
Figure 5:SEM photos and EDS spectra of NG pad profile
Figure 6:SEM Photographic EDS Spectral Diagram of the Pad profile after a Wave Peak Welding
Figure 7 :SEM Photographic EDS Spectral Diagram of the Pad profile
Finally, the composition of the electrode surface of NG pad and over-furnace primary pad is analyzed. The results show that the NG pad is mainly Sn,O in the depth range of 0~200nm, and in the range of 200~350nm depth, it is a copper-tin alloy, and there is almost no pure tin layer. The overburner pad is mainly tin layer in the 0~140nm depth range, followed by the element Cu (metal compound). The result is shown in figure 8 to 15:
Figure 8.NG pad test position
Figure 9.Composition analysis of NG pad electrode surface
Figure 10.Composition analysis of NG pad surface (about 50nm depth)
Figure 11.The composition Distribution Curve of 0~350nm depth
Figure 12.Schematic diagram of surface composition analysis of primary pad after wave peak welding
Figure 13.The composition Analysis of the Surface of the Primary Pad welded by the Wave Peak Welding
Figure 14.Composition analysis of primary pad surface (about 50nm depth) after wave peak welding
Figure 15.The composition Distribution Curve of the Surface depth of the Primary Pad (0~220nm) after the Wave Peak Welding
Conclusion: from the above analysis, it can be seen that the NG pad has been finished once before prototype pcb assembly mounting, during the over-furnace process, the surface tin will be oxidized, and at the same time, the high temperature will aggravate the diffusion between tin and copper, and form the copper-tin alloy, which will make the copper-tin alloy layer thicker. The tin layer is thinned. When the thickness of the tin layer is less than 0.2 μ m, the solderability of the pad will not be guaranteed, resulting in the failure of tinning.