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Warpage versus Max Diameter 10 9 8 7 Warpage (mils) 6 5 4 3 2 1 0 28 29 30 31 32 33 34 35 36 37 38 39 40 41 Max Diameter (mils)
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FIGURE 58.13 Warpage versus Max Diameter for Solder joint bridging ASME [2005].11
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would be easier to assemble on a relatively thin PWB, but with a 50 mm organic package, warpage is much more difficult to control, and it is a challenge to assemble it on a thick ( 125 mil, 20 + layer) PWB. Tolerances are also difficult to control on large-panel, thick PWBs. Typical tolerances on high end PWBs are 4 mils over the package site. Ensuring tolerances less than this is cost prohibitive. Current JEDEC guidelines10 require 8 mil coplanarity across the package. However, there are no specifications on the maximum acceptable warpage of the package at or above the reflow temperature. The typical relationship between maximum diameter and warpage is shown in Fig. 58.13 and 58.14.
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FIGURE 58.14 Bridging of solder joints wireframe (unwarped), contour (warped) ASME [2005].
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The joint parameters used in the analysis are: Paste volume (m3) Solder ball volume (m3) Upper pad diameter (mm) Lower pad diameter (mm) Viscosity of solder (Pa-sec) Surface tension (Nm 1) (Tin-lead) 4.909576e-011 1.130972e-010 0.60 0.55 2e-3 0.386
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For a 1 mm pitch joint array, if the maximum diameter exceeds 40 mils (1.016 mm), bridging will occur. Based on the analysis results, if the effective warpage exceeds ~9 mils (0.23 mm) over a 7 mm distance along the package, the joints will bridge (Figs 58.13 and 58.14). Due to misregistration, bridging could occur at lower warpage levels. But, for the sake of clarity, the base of the solder joints has been assumed to have no warpage.Thus the predicted value (9 mils) is the effective warpage of the assembly. In practice, if the board has a concave warpage of 5 mils and the package has a convex warpage of 4 mils over 7 mm (above solder melting temperature), it could result in bridging.11 Given these values, there are a few critical points to note: a. Simply maintaining the JEDEC recommended 8 mils coplanarity could be insufficient in some applications (for example, large body size packages on thick PWBs). b. It is important to characterize the effective warpage between the package and PWB in addition to the warpage of the free standing package alone. c. This warpage characterization should be performed over the entire reflow temperature range, as opposed to at room temperature. Of critical value is the warpage at or above the melting temperature. The data should be recorded in accordance with the JEDEC high temperature package warpage measurement specification.12 d. Where possible, the characterization should be performed on PWBs that are representative of actual PWBs used in volume production. One useful way to mitigate solder bridging is to depopulate the corner-most balls of the package. In large body-size organic packages, a significant amount of warpage occurs at package corners, so depopulating the 6 corner-most balls at each corner could help absorb about 2 mils of effective warpage (Fig. 58.15). This helps minimize the likelihood of solder bridging/opens. Many package suppliers already depopulate the four corner balls to improve long term reliability of the solder joints.
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FIGURE 58.15
Corner Depopulated Package
Depopulated corners to improve warpage-related assembly yield.
58.3.8.1 Effect of Lead-Free Conversion. It is important to note that the data presented in Figs. 58.13 and 58.14 is based on the surface tension of tin-lead solder. The surface tension13 and contact angle of lead-free solders is higher than for tin-lead solders, but analytically, for the solder joint parameters analyzed, the warpage versus ball diameter curve is not significantly different from the data presented in Figs. 58.13 and 58.14. This is because the wetting area is presumed to be fixed in the analysis. For other solder geometries (gullwing, J-lead, QFN, and so on), the behavior of lead-free solders can be different.
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