Surface finishes Failure modes

Scanning electron microscopy is essentially a technique for the observation of surfaces and is thus especially applicable to the examination of adherends and failure surfaces, observations of the results of surface preparation of metals and polymers, investigations of failure modes (see Stress distribution mode of failure), and examinations of finishes on fibres are typical of work reported in the literature. 

See Table 32.11 for descriptions of failure modes of specific surface finishes... 

TABLE 32.11 Failure Modes Specific to PCB Surface Finishes... 

It is very important that the solderability test, including any stressing that may be imparted on the deposit, be representative of the failure mode that the surface finish finally succumbs to naturally over time.The use of steam to stress a deposit other than a lead-bearing one is not to be used the recommended stressing is eight hours of exposure to 72°C/85 percent R.H. 

The typical failure modes for the most common PCB surface finishes are listed next. It should be noted that these wetting curves are for PWB s that were not packaged and purposefully exposed to the environment for a worst case scenario. PWB s correctly packaged and stored win last a lot longer in the majority ... 

FIGURE 59.14 Data set comparing the force-to-failure value of Pb/Sn and Sn/Ag/Cu assemblies for two different surface finishes (ENIG and SOP) as a function of time after reflow. The histograms show a comparison of the different failure modes obtained in the bend tests. (Reprinted with permission from Kyocera SLC Technologies. /G [2005] IEEE.)... 

In addition to speed, a number of other factors could impact the fracture strength of solder joints, including surface finish, pad size, ball diameter, ball metallurgy, and pad construction. All these need to be taken into account when comparing ball shear data across different packages. Another important factor that could impact the force-to-failure and the failure modes induced is the time after reflow. It is preferable to perform the test as early as possible after reflow. ... 

Failure mode III Accelerated seal face wear. Its causes are excessive torque, misalignment, inadequate lubrication, shaft out-of-roundness, contaminants, excessive shaft end play, and surface finish deterioration. 

Generally, having a nonsolder surface finish to bond a conductive adhesive improves contact resistance [49]. For example, conductive adhesives perform well with palladium-based terminated components. Lead-free board finishes such as nickel/gold and copper/palladium have been evaluated. Failures at the interface between component terminations and conductive adhesives is a typical failure mode observed in durabiUty tests, and also the cause of an increase in electrical resistance. Oxidation and corrosion of Sn-Pb finishes take place at the interface [21]. 

On the RTV-SM assembly, component location and position had a strong effect on time-to-first-failure for PLCC-84 and LCCC-44 devices. While there was a wide range of performance exhibited across the eight solder alloys, a similarly wide variation in performance was observed among the various component sites on the same board. Furthermore, a wide variation in performance was also observed at all locations across the three replicate boards used to test each solder. It was concluded that this position effect, coupled with general data variability, overshadowed the effect attributable to the solder alloy alone, and masked the ability to distinguish one solder alloy from any other. None of the Pb-free alloys exhibited catastrophic failure or distinguishably poorer performance than the other Pb-free alloys or the eutectic Sn-Pb control. Component type determined the failure time and mode. There were no clear differences between solder alloys or PWB surface finishes under the vibration conditions studied. 

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