SMT007 Magazine

SMT-July2016

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72 SMT Magazine • July 2016 such behavior. Eslampour 5 , lists many measure- ment tools that can be used to measure the dy- namic warpage of a package. The most common tool made available for this study was the ther- mal shadow moiré tool. The ability to measure warpage at elevated temperature provides bet- ter risk assessment for the formation of compo- nent board assembly joints. The common con- vention used to define the warpage direction is based on "+" and "–" magnitude which repre- sent convex and concave direction. However, there are shapes that are hard to determine just using these two signs. The measurement was conducted based on the availability of the sample and perceived risk level. There were three preconditioning con- siderations: "as is," "bake" and "MET" (manu- facturing exposure time), listed in Table 2. The purpose of these considerations is to mimic po- tential conditions prior to board assembly. 'As is' mimics the potential condition where packages are directly mounted to the board af- ter taken out of sealed bags without much stag- ing time. 'Bake' mimics the condition where the package is baked after being staged for unknown condition prior to board assembly. The baking process potentially alters the stress state of the package and removes any diffused moisture. MET nine days mimics the condition where the package is being staged in the factory floor for nine days, exposed to 30°C and 60%RH prior to component board assembly process. The typical MSL 3 calls for a maximum seven days of staging, but the work here extended to nine days to take into account any unforeseen circumstances. These three precondition environments may potentially demonstrate different pack- age warpage behavior and board assembly yield depending on the packaging technology used. Due to uneven samples acquired, some package types listed here were not subjected to all these preconditions. PoP Bottom and Memory Package Warpage Characteristics The 'as is' dynamic warpage behavior for these PoP bottom packages is shown in Fig- ure 2. It can be observed that the majority of the dynamic warpage behavior started from Figure 2: Dynamic warpage of PoP bottom package. Table 2: Precondition. PACKAGE-ON-PACKAGE WARPAGE CHARACTERISTICS AND REQUIREMENTS

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