Optimization of high density interconnections (HDI) printed circuit boards robustness

This document provides a first "how-to" guide for modelling thermo-mechanical behaviour of an HDI board submitted to reflow process using finite element modelling (FEM). The parametric model constructed here includes microvias, buried vias and plated through holes (PTH). The HDI board is composed of a central core and two micro via layers on each external side. Firstly, equivalent material properties of the complete stack-up were determined according to three pre-preg materials (mat. A as ref., mat. B and mat. C) and core thickness (0.9, 1.1 and 1.3 mm as ref.) below and above their glass transition temperature, Tg. Several simulations were then done on the reference board in order to evaluate the impact of the temperature shock (above Tg) on the ageing of the HDI board. Finally, we focused on the behaviour of the buried via, which is a highly critic part of the HDI board. The objective was to optimize, for the different assembly processes (leaded and lead free), the choice of the FR4 material types as well as the design rules (core thickness and buried via diameter for example). Discussions on results in link with experimental data and calculation approximations are done in this study in order to better understand the failure roots and to determine way of simulation methodology improvements.