J Fail. Anal. and Preven. (2013)13: 194-201 Fig. 1 The role of surface finish in PCB Resin Substrate sooderobum(pes Copp SoodeOOooo( Surface finish Resin Substrate as 3D stereo microscopy, scanning electron micros pe One well-wetted pad was also analyzed by EDs for con- (SEM), and energy dispersive spectroscopy(EDS) trast and comparison adopted. The un-wetting mechanism and the root cause of he failure were illustrated. Counter strategies and sug- gestions were given in the end Results and discussion Sample and Experiment A section of failed pad indicated by the red square Fig. 2a was chosen for SEM inspection. The reason for Figure 2a is the plane configuration of the failed PCB choosing this section is that it contains both the well-wetted which has a dimension of 1.2 cm x 2.4 cm. The sample and de-wetted pads, thus the contrast is distinct was treated with ENIG surface finish, then desmeared, and As Fig 3a shows, although the normal and failed pads subsequently went through hot air solder leveling. The little all appear gray under SEM, there is still observable dif- squares on sample are pads where tin solder should be ference between the two parts. The normal(well-wetted) attached on. Under optical inspection, as Fig. 2a shows, pad displays a bulgy surface topography, while the failed some pads are emitting shiny silver light and these are the ( de-wetted) pad displays completely fatness. This is ones soldered appropriately. While some of the pads just because tin solder will form a bump on pad under the effect appear golden, and these are the ones not properly wetted of gravity and capillary force, rendering the normal pad a by tin solder, thus letting the underlying golden layer bulgy profile. In a similar way, since the de-wetted pad has our naked eye. Figure 2b is the corresponding schematic the final finish of ENIG which accounts for its flatness exposed to the environment, displaying a color of golden to no solder attached on it, its outermost surface is actuall diagram of the failed sample which clearly shows that there The failed pad which is indicated by the red circle are 8 out of 129 pad un-wetted which is not endurable. Fig 3a was further magnified. As it can be seen in Fig. 3b. In order to dig out the primary cause of the failure, the grains of the immersion gold on the failed pad are cle modern analytic instruments and characterization methods and it appears coarse under magnification of 2,000x. This have been adopted. The de-wetting pad was observed by feature became more obvious when the spot was magnified SEM, and its chemical composition was analyzed by EDS. to 4,000X(Fig. 3c). When magnified to 16,000X, besides Springas 3D stereo microscopy, scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS) were adopted. The un-wetting mechanism and the root cause of the failure were illustrated. Counter strategies and sug￾gestions were given in the end. Sample and Experiment Figure 2a is the plane configuration of the failed PCB which has a dimension of 1.2 cm 9 2.4 cm. The sample was treated with ENIG surface finish, then desmeared, and subsequently went through hot air solder leveling. The little squares on sample are pads where tin solder should be attached on. Under optical inspection, as Fig. 2a shows, some pads are emitting shiny silver light and these are the ones soldered appropriately. While some of the pads just appear golden, and these are the ones not properly wetted by tin solder, thus letting the underlying golden layer exposed to the environment, displaying a color of golden to our naked eye. Figure 2b is the corresponding schematic diagram of the failed sample which clearly shows that there are 8 out of 129 pad un-wetted which is not endurable. In order to dig out the primary cause of the failure, modern analytic instruments and characterization methods have been adopted. The de-wetting pad was observed by SEM, and its chemical composition was analyzed by EDS. One well-wetted pad was also analyzed by EDS for con￾trast and comparison. Results and Discussion A section of failed pad indicated by the red square in Fig. 2a was chosen for SEM inspection. The reason for choosing this section is that it contains both the well-wetted and de-wetted pads, thus the contrast is distinct. As Fig. 3a shows, although the normal and failed pads all appear gray under SEM, there is still observable dif￾ference between the two parts. The normal (well-wetted) pad displays a bulgy surface topography, while the failed (de-wetted) pad displays completely flatness. This is because tin solder will form a bump on pad under the effect of gravity and capillary force, rendering the normal pad a bulgy profile. In a similar way, since the de-wetted pad has no solder attached on it, its outermost surface is actually the final finish of ENIG which accounts for its flatness. The failed pad which is indicated by the red circle in Fig. 3a was further magnified. As it can be seen in Fig. 3b, the grains of the immersion gold on the failed pad are clear and it appears coarse under magnification of 2,0009. This feature became more obvious when the spot was magnified to 4,0009 (Fig. 3c). When magnified to 16,0009, besides Fig. 1 The role of surface finish in PCB J Fail. Anal. and Preven. (2013) 13:194–201 195 123