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JULY 2019 I SMT007 MAGAZINE 17 in-pillow (HIP). In the case of Type 2 and 3 separation, the result is a defect known as a non-wet open (NWO). Because the design of the layers of the pack- age and the selection of materials is determined by many complex considerations that cannot be compromised by a need to minimize warp- age at the peak process temperature, ways of reducing process temperature have had to be sought. For many years, the electronics industry has been aware of—and made limited use of— solders based around the Sn-57Bi eutectic that has a melting point of 139°C. With that melt - ing point, the peak process temperature could be kept below 200°C, at which the component warpage is low enough that the risk of separa- tion defects is significantly reduced. However, this alloy is brittle and cannot deliver the reli- ability required in joints to area array packages. The reliability of Sn-Bi alloys can be improved by reducing the bismuth content and adding small amounts of elements, such as silver or lead, but the alloy still cannot match the reliability of the higher melting point alloys. And as the compo - sition moves further away from the Sn-Bi eutec- tic, the liquidus temperature increases, necessi- tating high process temperature. A solution that the industry is now evaluat- ing is to continue to use a conventional SAC alloy for the BGA ball but to reflow solder the package to the substrate with a low-melting- point alloy based around the Sn-57Bi eutectic. With this technique, it is possible to securely attach a BGA package to a substrate with a peak process temperature under 200°C. The result of this process is a joint such as that in Figure 2 [2] . Reliability testing of these mixed alloy or hybrid joints indicates that as long as a sub- Figure 1: Defects caused by dynamic warpage of FCBGA components during reflow soldering [1] . Figure 2: Typical joint formed by reflow of an Sn-Ag-Cu solder ball with an Sn-Bi low-melting-point solder.

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