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employing extreme protection measures such as full encapsulation/potting for electronic devices placed under the hood. Computer room environments represent the other extreme, with temperatures and humidity controlled to very tight tolerances. While servers, switches, hubs, routers, and other devices are housed in a controlled environment, their extremely large packages, coupled with power and mini cycles (discussed in the next chapter), create a set of loading conditions that are different from those found in automotive applications, but that are equally challenging. Worstcase loading conditions for a variety of end-use environments can be found in Table 58.1. Note the broad variation in temperature extremes, frequency of thermal cycles, and expected service life.
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TABLE 58.1 Examples of Worst-Case Environments for Different Use Categories3 Typical years of service 1 3 5 7 20 20 10 Approximated acceptable failure risk (percent) 1 0.1 0.01 0.001 0.1
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Use category Consumer Computers Telecom Commercial aircraft Industrial and automotive passenger compartment Military ground and ship Space Military avionics Automotive under hood
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Cycles per year 12 1460 365 365 20 185
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12 1 12 1 2 1 2
100 265 365 8760 365 40 1000
10 5 30 10 5
0.1 0.001 0.01 0.1
Tmin, minimum temperature; Tmax, maximum temperature; tD, dwell time at the operating temperature.
Prior to initiating any reliability assessment, it is crucial to understand the thermal, humidity, and mechanical loading conditions that a packaged device will be exposed to throughout its expected life time. Temperature maximums and minimums as well as the frequencies and rates at which the component cycles between extremes must be thoroughly understood. Complex thermal cycles (such as an outdoor product experiencing daily temperature changes coupled with a more local effect such as under-the-hood heating and cooling due to engine usage) must also be considered. Humidity and vibration loading conditions must also be evaluated as part of an overall package qualification process. End-use conditions can be estimated by attaching thermocouples, humidity, and shock/vibration sensors in the final product because it is used in field operating conditions.This data needs to be analyzed over time and different expected field conditions to get useful estimates of the end-use environment as illustrated in Table 58.1. Once this data is generated, it can be used in typical acceleration factor models relating accelerated stress tests to field data. For example, if it is determined that it takes x hours or cycles for an assembled package to fail in a laboratory test, the acceleration factor model can be used to extrapolate expected life of the device in typical field operating conditions. Details on acceleration factor models and sample calculations are provided in the next chapter.
COMPONENT-TO-PWB RELIABILITY
Although environmental impact has a large effect on interconnect reliability, there are some basic design variables that can have a positive or negative impact on solder joint reliability. These critical design variables and their impact on reliability are discussed in the following sections.
Double-Sided (Mirrored BGA) The high-density revolution has forced PWB designers to place increasingly complex surface-mount devices in PCB smaller and smaller spaces. This PWB density increase (where density refers to the number of components per unit area of PWB) may force designers to consider mirror BGA placement (see Figs. 58.7 and 58.8) or to place multiple devices on one side of a PWB. FIGURE 58.7 Double-sided BGA in The beginning of the density revolution occurred when mirror configuration. Note that the designers began implementing double-sided technologies PCA is an exact mirror about the cento allow for the placement of surface-mount components ter line of the PWB. In some instances the top- and bottom-side packages on both sides of a PWB. The continued push toward device share common vias. density has resulted in placement of BGA components in mirror and quasi-mirror configurations, shown in Figs. 58.7 and 58.8.
FIGURE 58.8 Double-sided BGA in quasi-mirror configuration. Note that although the packages are not right over each other, there is some overlap when looking from the top down or from the bottom up.
While the double-sided configuration certainly reduces signal delay, the primary disadvantage is a significant reduction in the thermomechanical reliability of the solder joint interconnects between the packages and PWB. Placing BGA packages in mirror or quasi-mirror configurations tends to rigidize the PWB assembly such that it does not bend as much during temperature cycling. The increased stiffness of the PWB due to double-sided mounting means the solder joints have to absorb more differential strain between the packages and PWB, resulting in as much as a 50 percent reduction in solder joint fatigue life (Figs. 58.9 and 58.10).4 In terms of solder joint strain, the effect is the same as having a thicker or stiffer PWB.The primary failure mode is cracking in the solder joints near the package/joint interface. Due to the dramatic decrease in assembly reliability when attempting mirrored configurations, these types of configurations should be avoided whenever possible.When it is impossible to avoid a mirror configuration, a quasi-mirror configuration without common through-hole vias is recommended (see Fig. 58.8). Common through-hole vias in this configuration tend to rigidize the assembly and contribute to further reduction in solder fatigue life. A standard rule of 50 percent reduction in fatigue life when employing a mirror configuration may be taken as
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