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FIGURE 58.12 High-density placement of BGAs on top side of a PWB. This diagram illustrates the convex-type curvature of a PWB assembly where multiple BGA devices are all placed on one side of a board.
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58.3.4.1 Effect of Lead-Free Conversion. Typical lead-free reflow temperatures (235 to 260 C) are higher than tin-lead reflow temperatures (220 C). As a result, the residual stresses built into the assembly after reflow could be higher in lead-free assemblies. These higher residual stresses are also a function of the cooling rate. Thus, it is important to not only control the heating side of the reflow profile, but the cooling side as well.The use of multiple zones for cooling is preferable, as it ensures better control. Moreover, forced convection ovens would be preferable over IR ovens for better control. Due to significant variations in the thermal mass of typical boards, it is difficult to provide universally applicable recommendations on reflow profiles.
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Warpage Issues The impact of warpage on a PWB assembly is multifaceted. First, warpage must typically be overcome when a PWB is placed in a card cage. Card cage placement of warped boards results in a rapid change in the loading of the solder joints that may lead to micro-crack formation or, in severe instances, solder joint fracture. Severe PWB warpage may create a card that physically cannot be placed in a card cage, resulting in a non-manufacturable product. Warped boards should be clamped in a controlled fashion to reduce the stress imparted to solder joints upon installation in a card cage. 58.3.5.1 Effect of Lead-Free Conversion. With the conversion to lead-free, it is imperative to ensure that the base material used in the PWB is compatible with the higher reflow temperatures. While more industry wide research is needed, the use of materials such as Dicyandiamide (DICY) cured FR-4 appears to be limited to applications with less than 240 C peak reflow temperature, whereas Phenolic cured PWBs are preferred for applications higher
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than 240 C. This is because of the better resistance to delamination observed for the phenolics after multiple reflows.7, 8 The thermal mass of the components and the amount of copper in large panel boards could make it difficult to maintain temperatures below 240 C. Detailed temperature measurements and reflow profiling should be performed on large boards to ensure that the temperature rise across the PWB is compatible with the PWB material. 58.3.6 Die and Package Stress Issues A second issue associated with board warpage is die and package stress. The package experiences an amount of stress as it constrains the PWB from contracting during cooling after reflow. If this stress is severe enough, the silicon may crack. Also, severe stress may create multiple types of package delamination and/or cracking. The extreme danger here is that a device could pass full first-level qualification (first-level qualification typically consists of thermal cycling, humidity and temperature exposure, extreme-temperature storage, and shock and vibration experiments performed on packaged devices not attached to a PWB) yet suffer from delamination/die cracking due to stresses imparted by a warped PWB. This is one of the primary reasons that many packaged IC suppliers and consumers require that secondlevel qualification test vehicles include daisy chains at the first-level interconnect. Inclusion of first-level joints or wire bonds allows for the impact of second-level attach on first-level reliability to be assessed simultaneously with second-level solder joint reliability during a second-level qualification.9 58.3.6.1 Effect of Lead-Free Conversion. Lead-free conversion exacerbates these potential silicon-level failures. Since the assembly is subjected to a higher temperature range than with tin-lead assemblies, there is a higher propensity for early failures due to residual stresses during cool down. In addition, there are potential concerns over the effect on low dielectric constant (k) materials in the silicon during second level temperature cycling. This is discussed in more detail in Sec. 58.3.13.9. The effect of mold compound and underfill material selection on first-level interconnects is also discussed in more detail in Secs. 58.3.13.5 and 58.3.13.6 respectively. 58.3.7 Back-Side Component Issues A third challenge related to warped PWBs concerns the back-side components in double-sided designs. During cool down after reflow, the top of the PWB is restrained from contraction due to the smaller effective CTE of the packaged device.This results in the bottom side of the PWB over contracting during cool down. The state of stress on the bottom solder joints due to warpage may be severe enough to fail solder joints or crack the die in bottom-side components. Additionally, the back-side components may be subject to residual stresses imparted after the second reflow (when the top-side components are attached via SMT) that may impact the long-term reliability of the devices on both sides and their second-level interconnects. As a result, it is important to characterize the thermomechanical reliability of the components that are mounted on the back-side by performing temperature cycling qualification.
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Solder Joint Geometry Issues Another consequence of severe warpage is the shape of the post-assembly solder joints. Joints can become excessively deformed, such that the cross section is too small to withstand fatigue. Joints can also become compressed or elongated, causing shorts or opens in some cases. Moreover, with significant increases in body sizes of packages, there is a higher propensity for opens or bridges during reflow. For example, a 20 mm body-size package (1mm pitch)
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