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Mold compound, substrate core, solder resist, underfill and die attach materials have been used for semiconductor packages for a long time. Recently, 3D packages (such as package-on-packages, stacked-die-packages) were introduced in the electronics industry. Moreover, pursing ultimate performance requires package body downsizing more and more. 3D packages are achieved by stacking laminate substrate packages in vertical direction and interconnecting them with solder balls. However, Package on Package (POP) usually shows large warpage for the complex structural mismatch, and also this makes more difficult to do surface mount. Especially, heavy package warpage for POP leads to solder joint failures between two packages. Package warpage is one of the important factors for the surface mount yield. To improve package flatness, a laminate-based substrate package structure and material property were evaluated with a Shadow Moire equipment, DMA (dynamic mechanical analysis), Nano-indentation equipment and finite element-based modeling. Finite-element modeling was utilized to improve package flatness after observing the correlation between models and measured data. For building up high accuracy warpage modeling, we studied viscoelastic parameter for mold compound, substrate core, solder resist, underfill and die attach materials, and created material database. However, die attach materials and solder resists are very thin material compared to other materials. Therefore, it is very difficult to measure viscoelastic material using DMA. To solve the problem, we studied the measurement method using a Nano-indentation. This paper describes material property effects on package flatness at the peak temperature during the reflow process. In addition, we verified experimental result and finite element-based modeling result.