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66 SMT Magazine • January 2017 The difference among the components in the BGA (silicon die, the molding compound, substrate, etc.) causes thermal stresses due to the thermal expansion mismatch between the various components. A major reason for warp- ing in area array plastic package is this coeffi- cient of thermal expansion (CTE) mismatch. Using low-CTE advanced thermal materials, it is possible to tailor CTE, reducing this problem. Sometimes underfill is used to provide mechan- ical support and protection for the dieto-pack- age interconnects. This can minimize thermal stress on the die due to CTE mismatch with the substrate materials. As a result of the higher lead free process- ing temperatures in rework, device packages, initially constructed for lead bearing solders are subjected to greater thermal stresses and exhib- it a greater propensity to warp. Some research [1] has demonstrated that the impact of a high- er processing temperature, versus the molding temperature, of the package material is a direct causal link to the amount of device warpage. As the device packaging for lead free products have become more stable, this impact has been less- ened through material changes in typical BGA packages. The thinning of area array packages, due to the increasing demand to make end devices more portable, has brought the average mois- ture sensitivity of device packages up. This makes devices more susceptible to thermal dam- age based on a given heat exposure time. The purpose of the MSD standard is to identify the moisture sensitivity level at a fixed reflow tem- perature (Figure 4). The user can then proper- ly store and handle the devices, avoiding subse- REDUCING WARPAGE ON BGAS DURING REWORK Figure 2: Shadow Moiré output showing device warpage. (Courtesy: Akrometrix) Figure 3: Typical BGA package cross section. (Courtesy: Socionext)