Corrosion mechanism of tin-coated copper leads for electronic components Gao Su Zhang Qiyun School of Chemistry and Molecular Engineering, Peking University, Beijing 100871) The mechanism of wettability determination and other methods to judge the deterioration of solderability is: copper-tin intermetallic compound! Phase Eu6Sn5) The crystallites diffuse to the tin surface and form a microbattery pair with tin, which is caused by galvanic corrosion due to moisture and acid atmosphere in the air.
Hundreds of thousands of various electronic components on the printed circuit board, in order to complete the reliable soldering of the circuit on the board at a time on the welding line, the surface of the copper lead used in the production of electronic components is often hot-dipped or plated with tin in advance to ensure It has high solderability. Practice has shown that after such a tin-clad copper wire is stored for a period of time, sometimes less than half a year, its solderability is greatly reduced and causes the virtual soldering of components on the circuit board. There are several spot solder joints out of thousands of solder joints that are extremely difficult to detect, resulting in the scrapping of the entire circuit board or the emergence of future failures. For many years, it was generally believed that the decline in the solderability of the leads was an intermetallic compound generated by the reaction between the copper and tin interface! The phase composition is similar to C> 6Sn5) due to long transparent tin layer. Some papers do not bother to calculate the growth rate of this compound at different temperatures, think the compound! As long as the height does not extend through the tin layer during the storage period, the deterioration of the solderability of the lead will not occur. Therefore, it is recommended to increase the thickness of the tin layer of the lead to solve the problem. However, in fact, in a hot and humid environment, the solderability of the copper wire is still significantly reduced when the phase does not grow through the tin layer. What is the reason and what is the reaction mechanism? So far, no in-depth discussion and convincing conclusions have been made. This paper intends to study its mechanism by using metallographic, XPS and electrochemical and wetting force determination methods to discuss its solution.
1 is the thickness of the electroplated tin layer 100 ~ 120! . ) The result of 1553 16h aging. It can be seen that it is very different from before aging. On the copper-tin interface! Phases grow up with uneven protrusions, about 5 ~ 10! m. But more importantly, it is found that there is obvious diffusion of crystallites in the tin layer! As far as the surface of the tin layer. In addition, cracks were found on the smooth tinned surface. It can be seen that the long-term storage of tin coating has been noticed by various researchers, on the interface! In addition to the height of the phase, there is also the formation of diffusion! The phase crystallites exist and diffuse to the surface. Because most researchers in the past only entered Table 1 at different temperatures! (Cu6Sns)-Open circuit potential of Sn and Cu-Sn electrode pairs in +% NaCl solution Table 20! The low magnification observation of the thin tin clad layer around m to simulate the actual process conditions, the above facts are hardly noticed by all researchers.
XPS analysis on the surface of the tin coating layer In order to investigate the composition of the surface of the tin coating layer, the four samples before and after the hot dip and electroplated tin aging described in the previous section were subjected to XPS analysis. this.
Usually the wetting power is -30dyne as the practical minimum passing standard. The greater the value, the more difficult it is to wet. It can be seen from Table 2 that the wetting force of the tin-coated surface formed by hot-dip tin or electroplated tin after aging under 155O damp heat conditions is greatly reduced from the original state, although at this time the Cu-Sn interface! The thickness of the phase is far from overlying the tin layer.
2 Discussion From the above metallographic and XPS experiments, it can be confirmed that, except for the new table 2 wetting test result wetting time 3s) fresh electroplated tin layer, whether it is hot dip tin or electroplated tin layer after heat aging, The tin layer is free! Phase crystallites exist and diffuse all the way to the surface of the tin coating. Electrochemical measurements show that in 3% NaCl aqueous solution! The battery formed by the phase and the Sn phase has an open circuit voltage of 268mV at 25C and changes slightly with temperature. Due to the tin coating layer! The phase is closely intertwined with the Sn of the main body. When there is moisture and electrolyte in the surface air, it will undoubtedly form countless short-circuited micro batteries. At this time, accelerated oxidation of the negative electrode tin will inevitably occur and the oxidation products will accumulate on the surface of the tin-clad layer, which directly affects the wettability of the tin-clad copper wire by the solder. A study shows that pure tin does not cover the surface of copper) The oxidation rate is extremely small, the thickness of the oxide layer is only about 3nm under the aging conditions similar to the above, and this thickness does not increase with time. This degree of oxidation is completely insufficient to affect the wetting ability under the action of standard rosin flux. This shows that the tin surface without electrochemical corrosion is relatively stable. However, the experiment on the determination of the wettability of tin-coated copper wires described in Section 1.5 above fully illustrates the rapid deterioration of the surface state when the surface is electrochemically corroded.
In previous reports, almost all focused on the growth height and growth rate of intermetallic compounds on the interface, think! In fact, many researchers did not make pure! Phase) It is difficult to wet in the air. Only when it has penetrated through the tin coating layer will it affect the wettability of the tin coating layer. Therefore, it is believed that as long as the thickness of the tin coating layer is appropriately increased, the tin coating layer has 4 to 8 in the storage period! The margin of m will not affect the wettability of the tin coating. But that is not the case. Because these researchers focus more on thin tin coating (corrigendum)
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