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FIGURE 58.25 Distance from Neutral Point (DNP) for a BGA-type package. The DNP increases when moving away from the center (center line) of the package. Pitch is also indicated in this diagram. Pitch is defined as the center-to-center distance between two adjacent interconnects (in this example, between two adjacent solder balls).
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these experiments are failure distributions typically presented as Weibull plots (for details on Weibull distributions and failure life analysis, refer to Sec. 61.2.2 in the next chapter) that show failure percentage as a function of time.26 Note that if a package with a larger DNP than was qualified is considered, then there could be severe degradation in reliability and a full assessment should be conducted. However, if a package with a smaller DNP (10 to 20 percent smaller) than the package qualified (with all other material and design parameters being equal) is considered, then the proposed package should be at least as reliable as the qualified package. 58.3.13.2 Ball Pitch and Ball Size. Ball pitch is the center-to-center distance between two adjacent solder balls (see Fig. 58.25). In theory one could change the ball pitch and keep the same solder joint size. In practical applications though, decreases in ball pitch also require a reduction in solder ball diameter and pad size for physically accommodating the decrease in distance between adjacent balls. This decrease in ball size, with decreasing pitch, results in the formation of smaller balls with smaller stand-off between the package and the PWB. Smaller solder balls and decreased stand-off have been shown to decrease solder joint reliability (see Fig. 58.26). The dashed line in Fig. 58.26 represents the finite element analysis results for a 0.889mm (35 mil) ball. The solid line represents finite element analysis results for a 0.762 mm (30 mil) ball. Note that the 0.889 mm ball is 1.3 times more reliable than the 0.762 mm ball.Thus reduction in pitch can result in decreases in reliability due to the subsequent decreases in solder ball size necessary to accommodate adjacent balls and the reduced stand-off heights due to smaller solder balls. The PWB designer is therefore faced with a dilemma when considering a reduction in ball pitch. Not only is there a risk of reduced assembly reliability, but there is also a design challenge. A direct consequence of decreased pitch is added complexity in the ability to route all traces out from the package to the PWB. Designers are also sometimes forced to add additional layers to PWBs in order to accommodate reduced-pitch parts. This layer increase results in increased price and in some instances, drastic reductions in capacity due to the increased complexity associated with fabricating high-layer-count PWBs. 58.3.13.3 Ball Array (Full or Depopulated). Ball array is another variable that has an indirect impact on solder joint reliability. There is a coupled effect between the location of the edge of the packaged die, the solder ball array, and the state of stress in the solder joints. The silicon has a much lower CTE than plastic substrates. Thus the die can create a state of overconstraint, producing excessive stress in the solder joints under and near its edge.This overconstraint tends to be worst in joints near the edge of the die. Figure 58.27 contains schematics of one configuration where a full array is employed and the edge of the die falls over the solder joints. Fig. 58.28 illustrates an alternative design where a region near the perimeter of the die is depopulated of all solder balls (the depopulated configuration).
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99 90 80 70 60 50 40 30 20 Failure (%) 10 5 2 1 0.5 0.2 0.1 100 1000 N (cycles) Eta 4806 6231 Beta 2.6 2.6 r^2 ----n/s -/-/ID # Data Set 1 S3 30 Mil Ball 1.5 mm 2 S3 35 Mil Ball 1.5 mm 10000 100000
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FIGURE 58.26 Weibull plots for two different ball size/stand-off heights. The solid line indicates the finite element analysis (FEA) predicted failure distribution for an interconnect based on a 30-mil standoff height. The dashed line indicates the FEA predicted failure distribution for an interconnect based on a 35-mil stand-off height. Note that a reduction in stand-off height correlates with a decrease in fatigue life.
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Many plastic (laminate) package suppliers offer a depopulated option to enhance reliability by not placing solder joints in the worst-case regions under the perimeter of the die. Additionally, many package designers further reduce risk of CTE-induced joint failure by only placing redundant power and ground or nonfunctional thermal balls under the core. If redundant balls or thermal
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