A study on the effect of mold compound moisture related properties and leadframe dimension on the reliability of IC packages using an integrated mechanical modeling approach

Abstract

Due to production cost conscious, high density (HD) packaging is gaining importance with high volume production but less material usage. However, significantly extended periods of wire bond operations in HD packaging can render aggressive leadframe oxidation. Since epoxy's bond to copper oxide is often stronger than the bond between copper oxide and the leadframe base metal [17], oxide to leadframe delamination likely to occur under stress condition and inevitably causing subsequent popcorn package cracking. Hence it is desirable that the package is designed to have strong resistance to this delamination. This paper aims at finding out suitable polymeric material and leadframe dimension that can mitigate the hygro-thermo vapor pressure induced stresses and hence delamination growth at the EMC to top die pad interface of IC package for HD packaging. A two-dimensional Non Fickian moisture absorption and desorption FEA model is firstly applied to quantify the wetness at the interface post Moisture Sensitivity Level (MSL) 1 under 85°C/85%RH and at peak IR reflow temperature 260°C. The critical interface is found to be far from the saturation after MSL 1. After exposed to solder reflow, the wetness along the interface is noted to remain intact as compared to post MSL 1 while the package exterior witnessing a substantial drop. Evaluation was conducted on four different EMCs. The variation of the residual moisture at the interface between various EMCs is found to be considerable small. For instance, MC-D exhibits 47–65% saturation along the interface against 54–70% saturation for MC-B at 260°C. An analytical 2D model with thermo-hygro-vapor pressure mechanical coupling effect is then used to numerically predict the peeling and shear stresses along the critical interface for various investigated EMCs. Although the assumption of uniform saturated moisture distribution made in the constitutive coupling model does not reflective of the real case of moisture gradient and hence vapor pressure variety along the interface, it does not much affect the qualitative prediction of the improved EMC due to minor variation of the wetness at the interface between EMCs as determined by the preceding diffusion model. Analysis addresses EMC formulated with low modulus, low coefficient of thermal expansion (CTE), low saturated moisture concentration (Csat) and low coefficient of moisture expansion (CME) at peak reflow temperature exhibits enhanced performance. These predicted results match well with the C-SAM results. Apart from these, both interfacial shearing and peeling stresses were found to play comparable role for delamination initiation at top die pad corner. Conversely, interfacial peeling stress is identified as a major cause of delamination that initiated from the region close to die attach. Also covered is the die pad size optimization analysis. Indeed, it has been a lump sum of analyses focusing on the evaluation of possible adhesion improvement through chemical treatment of the copper-based leadframe such as Ni plating, surface roughness modification, etc. Nonetheless, it is scarce to observe any analyses that reveal the effect of die pad size on the propensity to interfacial delamination. This drive towards the additional exploration for identification of its effect in this paper.