The phenomenon of tin not soldering onto the pad of a desoldered PCB

Failure samples for double-sided mounting PCBA, found that the second soldering individual pads do not appear on the tin phenomenon, as shown in Figure 1-1.

Figure 1-1 The phenomenon of tin not soldering onto the pad

In the second soldering process of double-sided mounting PCBA, it is found that individual pads appear tin not soldering onto the pad phenomenon.Observation by Appearance，The position of the pad where tin does not solder is not fixed.There is no obvious discolouration on the surface.Further observation by SEM (Scanning Electron Microscope) revealed that the surface layer of immersed tin on the unovened pads was well formed, while tiny raised particles existed on the surface of the once-ovened and failed pads, which were confirmed to be Cu6Sn5 CuSn compounds by EDS (Energy Scattering Spectroscopy) analysis.As shown in Figure 1-2.

Figure 1-2 Surface morphology of immersed tin in different states

The profiles of the unoverfurnished pads, overfurnished pads and failed pads are made by FIB, and then the profiles are scanned by EDS.The results show that Cu elements have appeared in the surface layer of the failed pads, in the depth of 0.3μm for the overfurnished pads, and in the depth of 0.8μm for the unoverfurnished pads, which indicates that the thickness of the pure tin layer is about 0.8μm.

Since the detection depth of EDS exceeds the thickness of the immersion tin layer, AES (Auger Electron Spectroscopy, with an analysis depth of about 5nm) is used again to analyse the depth of composition of the failed pads and the once-overfurnished pads.As shown in Figure 1-3 for the failure of the pad surface in the depth range of 0 ~ 350nm composition distribution curve.As can be seen from the figure, in the depth range of 0~200nm, the composition is mainly Sn and O; in the depth of 200nm, Cu elements appear; in the depth range of 200~350nm, mainly Cu-Tin compounds.The composition analysis indicates that there is no pure tin layer on the surface of the failed pad.This case proves that the immersion tin layer is not resistant to soldering.

Figure 1-3 Composition distribution curve in the depth range of 0~350nm on the surface of the failed pad.、

Recommendations for improvement

Optimise the immersion tin process:

Adjust the composition and concentration of the immersion tin solution to improve the quality and solder resistance of the immersion tin layer.

Control the immersion time and temperature to ensure that the thickness of the immersion layer is uniform and moderate, to avoid over-thickness or over-thinness resulting in non-resistance to soldering.

Improve the solderability of the base metal:

Appropriate pretreatment of the base metal, such as pickling, polishing, etc., to remove surface oxides and impurities, to improve the solderability of the base metal.

Selection of high quality and low impurity base metal materials.

Eliminate impurities and outgassing sources of the base metal:

Strictly control the quality of raw materials of base metals and avoid using materials containing excessive impurities and outgassing sources.

Adequate cleaning and drying of the base metal prior to welding to minimise outgassing during the welding process.

Use inert or reducing reflow gases:

Use inert (e.g., nitrogen) or reducing gases as protective gases for reflow soldering to reduce oxidation reactions during the soldering process and to protect the immersion tin layer from damage.

Use a suitable reflow soldering temperature profile:

Optimise the temperature profile of reflow soldering to ensure that the temperature is moderate and uniform during the soldering process to avoid damage to the immersed tin layer caused by too high a temperature and too long a holding time.

Regular maintenance and calibration of soldering equipment to ensure the accuracy and stability of soldering parameters.

Strengthen quality control and process monitoring:

Establish a perfect quality control system to strictly monitor and record the immersion tin process and welding process.

Regular sampling and analysis of welding quality, timely detection and resolution of problems.

Through the optimisation of tin sinking process, improve the solderability of the base metal, eliminate impurities and outgassing sources, the use of appropriate reflow gas and temperature profile and strengthen the quality control and process monitoring and other measures, can effectively solve the double-sided mounting PCBA in the second soldering pads do not appear on the tin problem.