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|The Case of Failure Analysis of the PCBA Wire Corrosion under High Reliability Requirements|
|Keywords: No-clean, Ion residue, Corrosion Failure|
|The soldering of Printed Circuit Board Assembly (PCBA) is an indispensable process in the assembly. There is an obviously tendency about the environmentally friendly (such as RosH, etc.) requirements, the cost-cut pressure, and the tendency of the simplification towards the manufacture process. Taken those into consideration, more and more companies are increasingly inclined to use halogen-free flux of electronic components and to achieve a no-cleaning process. In order to enjoy a good soldering result, higher activity flux are often used, that is, by adding various chemicals to remove oxides. However, while utilizing the strong de-oxidation capability of lead-free flux, the side effects require critical attention that may be caused by their own characteristics, such as corrosion of copper, corrosion migration of solder residues, etc. In this paper, the break-failure case of a server backplane is included, the causes and the relevant suggestion of corrosion failure of PCBA after soldering are discussed. 1 The cause of PCBA wire corrosion PCBA wire corrosion failure generally occurs in the aging selection or normal service stage, which is the later failure. Once corrosion failure occurs, there is extremely rare possibility for rescue. Depending on the function of the PCBA, the signal cannot be transmitted or feedback or even the system could be breakdown. For industries with high reliability requirements, such as transportation, aerospace, etc., such failure is almost zero tolerance. The PCBA-failure analysis of the author's team for many years, with the statistics of the corrosion failure analysis of the wire, found that the occurrence of corrosion is basically related to the following aspects: (a) The PCBA is energized working for a long time, and there is a definite potential difference between the wires; (b) There is a certain amount of corrosive ion residue on the surface of PCBA; (c) The PCBA surface protection layer insufficiently against the external moisture and ion infiltration. The above three predisposing factors interacts with each other. For example, if the surface protection ability is so weak, even if the ion residue is low, that the wire corrosion may occur after the PCBA works for a certain period of time; another instance, the surface ion residue is high and the protection ability is extremely fragile, but it has not been energized, such as, in the storage process, wire corrosion does not occur, it is highly probable that corrosion failure will occur in a short time when it starts to service. 2 Results and Discussion of a Wire corrosion failure case Visual inspection of the failure samples, which is speculated that corrosion failure may be related to wave soldering; cross-section of corrosion failure locations found that solder resist density is acceptable, and the main cause of corrosion failure is not caused by inadequate solder mask protection; the results of EDS and ion chromatographic analysis confirmed that the abnormal element Br was mainly derived from the flux; the ion chromatography analysis of the failure position revealed that the high content of ions on the surface of the board was not left in the cleanliness of the PCB before shipment, but was introduced during wave soldering or later application. The high content of ionic residues provides a medium for the electrochemical reaction of corrosion, causing the corrosion reaction that result in a circuit break-failure within 15 days. Depending on the background of the case, the manufacturer AAA uses the no-cleaning process, and the theoretical use of the line flux should also be a no-cleaning flux. In fact, the line flux does not match this process, and the flux residue contains a large amount of ions on the surface of the board, which gives a rise for the occurrence of a corrosion reaction. In addition, in order to meet RoHS requirements, the AAA promised that the line flux is a halogen-free flux. Although the flux content is not directly detected, the halogen content is found to be as high as 5439 mg/kg in the total halogen content analysis, and with this flux the sample bearing the simulated wave soldering is also detected bromide ion in the test. When halogen is added to the flux, there are two forms, one is the ionic state and the other is the combined state. Ionic halogen has a side-effect on the surface of soldering (wire) of the PCB, and the halogen of the combined state may undergo a chemical reaction converting into an ionic state after high temperature of soldering. This is the reason why the ionic halogen is negatively demonstrated in the flux, however, there is a positive result after the wave soldering simulation using the flux. Therefore, no halogen ion in the original flux doesn’t equal halogen-free. The manufacturer needs to strengthen the understanding of the halogen-free conception. Based on the analysis results of this case, in order to reduce or even avoid similar corrosion failure, it is recommended that enterprises should emphasize the control in the following two aspects. First, it is essential to strengthen the risk awareness of the residue on the board surface, and have a more scientific and reasonable understanding of|
|Jie Zheng, Senior Engineer