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Plastic vs. Ceramic Package. The global mismatch between the component body and substrate is minimized for most printed circuit boards if the package is plastic rather than ceramic. Most electronic ceramics have CTEs in the neighborhood of 4 to 10 ppm. Since the printed circuit board CTE is 14 to 18 ppm in-plane below Tg, the match to plastic packages which usually have average CTEs of 20 to 25 is better. The overall CTE of a plastic package can be significantly below that of the plastic if the die is large compared to the total package body. For example, TSOP components can have overall CTEs as low as 5.5 ppm. It is also worth recalling that component-level reliability must be considered; plastic packages suffer from other disadvantages versus ceramic packages, such as moisture absorption. Leaded vs. Leadless Surface-Mount Components. Leadless surface-mount components with peripheral solder joints (e.g., leadless ceramic chip carriers, LCCCs) are more susceptible to solder joint failures due to thermal and mechanical stresses than leaded components because there is no compliance in the system (see Fig. 57.21). A compliant lead can take up relative displacement between the component body and the substrate during mechanical or thermal stressing. In doing so, it minimizes the stress and strain imposed on the solder joint, thus reducing the likelihood of failures. Large leadless components should be avoided whenever possible. If they must be used, the substrate must have as close a CTE mismatch as possible and be protected from mechanical stresses. A conformal coating should be considered.
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Shear stress Heating Stress relaxation at Tmax Heating Cooling Stress relaxation at Tmin Cooling Solder yield stress
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FIGURE 57.21 Schematic illustration of the strain in the solder during a thermal cycle for leaded and leadless surface-mount components. (After W. Engelmaier) (IPCTP-797, Surface Mount Solder Joint Long-term Reliability: Design, Testing, Prediction.)
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Ball grid arrays (BGAs) are a new style of leadless SMT component with an areal array of solder joints. The reliability of these components has come under intensive study. Plastic BGAs are less susceptible to solder joint fatigue failures than ceramic BGAs because the laminate and plastic body match the CTE of the PCB much better.At this time, it seems that there will be size and power limitations to ensure solder joint reliability. Lead Compliance. As described, leadless components cause more reliability problems than leaded components because the entire displacement is imposed on the solder joint; however, there are also large differences in compliancy among leaded surface-mount components (see Fig. 57.22). Body height plays an important role because it determines the length of the
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RELIABILITY OF PRINTED CIRCUIT ASSEMBLIES
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2.58 0.61 TSOP
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FIGURE 57.22 Schematic illustration of different surface-mount component lead types with widely differing compliance: (left) J-lead, (right) gullwing leads.
compliant beam. Other important lead characteristics that affect compliance are lead shape (e.g., J-lead vs. gullwing) and lead thickness (stiffness is proportional to thickness cubed). Lead frame material also plays a role in determining solder joint life, although it is not as important as lead frame geometry. Common lead frame materials are Cu and Alloy 42 (Fe42Ni); Alloy 42 has a better CTE match to silicon (and a greater mismatch to the solder), but it is much stiffer than Cu (see Table 57.3) Thin small-outline packages (TSOPs), which are becoming increasingly common memory packages, pose the greatest solder joint reliability risks of any packages commonly used today. These packages generally have Alloy 42 lead frames and very low standoff from the PCB, resulting in a very stiff lead that transfers most of the relative displacement between the component and the substrate to the solder joint. The situation is exaggerated for this component because the overall CTE of the package is quite low. While adequate solder joint reliability can be achieved with TSOPs in many situations, some vendors have opted to encapsulate the solder joints with a filled epoxy to better distribute the stresses.40 57.5.3.2 Component Selection for Cleanliness. If fluid cleaning is used to remove flux residues after assembly, then a minimum component standoff (distance between the component body and the seating plane of the leads) is critical to ensuring proper cleaning and drying, and, therefore, resistance to corrosion and moisture-related failures. Industry standards for low standoff components permit the components to have 0 0.25 mm standoff from the board. These low standoffs permit corrosive residues and the cleaning fluid to be trapped
TABLE 57.3 CTE and Elastic Modulus at Room Temperature of Some Important Packaging Materials Cu CTE, ppm/ C E, GPa 17 130 Alloy 42 (Fe-42Ni) 5 145 63Sn-37Pb Solder 25 35 Si 3.5 113
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