Warpage minimization of the HVQFN map mould

Abstract

A FEM model for the calculation of the HVQFN 100 warpage after moulding and post mould curing (PMC) was created. A viscoelastic model for the compound is required, since time and temperature history play a major role. In the production process the HVQFN 100 is forced to remain flat during PMC by applying of a dead weight. The good agreement between warpage measurments and FEM warpage results shows that fully cured viscoelastic compound properties can be used. Also adding curing shrinkage had only little effect on the end warpage. However in case of no dead weight, i.e. map mould free to warp, the curing shrinkage had a large effect on the end warpage and then curing effects can absolutely not be neglected. After validation (with dead weight) the model was used to minimize the warpage. To do so first the viscoelastic 1 Hz curve of the compound needs to be characterized by some parameters. Therefore the curve is described by three lines from which a viscoelastic spectrum can be derived. The input variables were die thickness and the compound properties: thermal expansion, glass transition temperature and stiffness. A design of experiments was created and used as input for the FEM model. The results were described by a so called response surface model, which showed that there is a lot of room to further reduce the warpage by choosing a compound with different properties and/or by variation of the die thickness. The response surface model was implemented in a spread sheet tool so that the effect of compound selection on warpage can be calculated in a split second.