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FIGURE 58.27 Full array with solder joints under the die perimeter. Solder ball locations are indicated by the array of circles. The die perimeter is indicated by the black dashed rectangular outline. Note that the die edge falls directly over solder joints.
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FIGURE 58.28 Depopulated array with no joints under the perimeter of the die. Solder ball locations are indicated by the array of circles. The die perimeter is indicated by the dashed rectangular outline. Note that the die edge falls in an area of depopulated solder balls.
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balls do crack, the impact on device performance is negligible. Finally, in large laminate packages, the corner-most balls could also fail early.As a result, many package suppliers depopulate the corner balls of the package to enhance solder fatigue life. 58.3.13.4 Heat Spreader/Stiffener. A cross-sectional schematic of a flip-chip assembly, with a heat slug and stiffener, is shown in Fig. 58.29. The primary reason for employing a heat spreader lid is for thermal management. A secondary benefit is that the stiffener and heat slug reduce the potential for out-of-plane warpage occurring during reflow and thermal cycling. An additional benefit of incorporating a heat slug and stiffener into a laminate package design is improved ability to handle the package due to increased robustness of the packaged die.
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FIGURE 58.29 Flip-chip BGA with stiffener and heat slug. The stiffener provides added rigidity to the package and also provides mechanical support for the heat sink.
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A variety of heat spreader materials have been used: copper, aluminum, steel, and aluminum silicon carbide (AlSiC). Copper provides the best thermal conductivity, but it has a high CTE. AlSiC has a slightly higher modulus than copper, but it has a lower CTE. As a result, in several applications, AlSiC is gaining more prevalence. Its thermal conductivity is lower than copper and comparable to aluminum (180 to 200 Wm 1K). For cost reasons, several package suppliers also use an alternative to the heat spreader/stiffener design, namely a single piece lid design to act as a rigid support and provide heat transfer at the same time.This design is easier to manufacture and also reduces assembly steps (See Fig. 58.30 for more details). The thickness of a single piece lid is typically less than that of the heat spreader/stiffener combination. Changing the stiffness of the package could alter its out-of-plane warpage, which
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COMPONENT-TO-PWB RELIABILITY
Single Piece Heat Slug
Silicon Package Substrate Stiffener
FIGURE 58.30 Single piece lid design.
in turn could impact the reliability of the solder joints. Therefore the effect on the overall stiffness should be characterized when using a single piece lid. One characterization method is to perform Moir measurements on the free standing package, as a function of temperature. Another variation of the standard heat spreader/stiffener design is the use of just a stiffener alone. This is usually for bare die applications where the wattage of the chip is so high that the thermal resistance between the chip and the heat spreader is unacceptable. In such cases, the stiffener is retained for stiffness and robustness, but the heat spreader is removed. 58.3.13.5 Mold Compound Selection. The overmold compound used in plastic ball grid array (PBGA) packages could significantly impact the assembly yield and reliability of solder joints. If the glass transition temperature (Tg) of the mold compound is significantly below the peak reflow temperature and if its modulus and CTE drastically change above its Tg, the package could warp significantly as it is heated. As a result, when the package reaches liquidus, the effective warpage between the package and PWB could be such that solder joint opens/bridges could occur. In large PBGA packages (>40 mm), it would be more effective to use an integrated heat spreader in the molded package to rigidize it and minimize excessive warpage. Integrated heat spreaders are primarily used to improve thermal performance, but they also help rigidize the package for improved assembly yield. With the conversion to lead-free materials, the mold compound selected needs to be such that it can withstand the higher reflow temperatures of lead-free assembly. If the mechanical material properties of the mold compound (elastic modulus, CTE, and Tg) are not appropriately selected, the package could warp significantly at reflow temperatures and result in solder joint opens. 58.3.13.6 Underfill Material Selection. The underfill material selected in flip chip BGA (FCBGA) packages could significantly impact the reliability of the board mounted package. If the underfill material is too stiff, it would rigidize the assembly and thus transfer more strain to the silicon die. This could result in delamination at any of the critical interfaces: underfill/ passivation, passivation/die, underfill/substrate, or within low-k layers. On the other hand, if the underfill material is too soft, it could result in first level interconnect failure. A less stiff underfill could also help reduce board level assembly warpage, which in turn helps the second-level interconnects. Thus, the stiffness of the underfill selected should be optimized such that it does not lead to delamination or first-level interconnect failures.9 In addition to stiffness, CTE, and Tg, there are several other factors that dictate the underfill material selected, such as thixotropy, viscosity, shrinkage, curing temperature, and curing time. The choice of underfill material is also influenced by the conversion to lead-free. For leadfree packages, even though there is a European exemption on the first level interconnects,27 many package suppliers are looking at using lead-free first level interconnects. As a result, the underfill material selected has to be such that it can withstand the higher reflow temperatures, and has low shrinkage for the increased temperature range.
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