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AUTHOR WITHDREW 8-29-11: Warpage and Stresses in Molded Package
Keywords: warpage, stress index, Stoney formula
AUTHOR WITHDREW 8-29-11: Molded plastic packages are the most commonly used packages in semiconductor industry. Examples include Plastic Ball Grid Array (PBGA) packages and Quad Flat No-leads (QFN) packages. The substrate materials of these packages are laminates or metal alloys. Warpage control is very important for plastic molded package because excessive warpage could cause difficulties in package strip processing, soldering etc. It could also cause elevated stress in the solder joints, and might be the indication of high internal stress in the package. A general form of Stoney formula is used to study package warpage. Even though this formula is for two layers structures, for packages with small or thin die, it provides quick and fairly accurate results, especially for warpage in the outskirt area of the package. Numerical studies are performed on wide range of material modulus ratio and layer thickness ratio between substrate and molding compound (EMC) materials. The discussions are focused on the change of thickness ratio and/or modulus ratio that could increase or decrease the warpage. For a given CTE difference, whether reducing EMC modulus may reduce package warpage depends on the modulus ratio, as well as the thickness ratio. Stress distributions based on Stoney formula are also discussed, and compared with empirical stress index formula, σ = (α(EMC)- α(subs))* E(EMC)*ΔT, where α(EMC) and α(subs) are coefficient of thermal expansion (CTE) of EMC and substrate respectively, E(EMC) is the modulus of EMC, and ΔT is the uniform temperature change. It should be pointed out that the mismatch in length due to CTE difference and temperature change is closed by tension and compression in the EMC and substrate layer, while empirical stress index attribute EMC material stress to the entire mismatch. Because the modulus of organic substrate and molding compound are in similar order, thickness and modulus of both materials should be considered for stresses estimation. In addition, warping induced bending also contributes significantly to the stresses in each layer. The stress distribution caused by CTE mismatch in dual layer package structure is neither simple compression/tension, nor simple bending as in mechanical loading.
Yuan Yuan, Mechanical Engineer
Freescale Semiconductor Inc.
Austin, TX

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