Quantification of the leadframe roughness effect on adhesion properties

Numerical analysis of delamination at polymer-metal interfaces depends on phenomenological descriptions of interface adhesion. Therefore, extensive experimental characterization is needed. A physics-based multi-scale framework enables the derivation of interface properties by numerical analysis. Such a framework depends on both molecular and continuum analysis giving information on interfacial behavior at different scales. This work relates to the micro-scale analysis of leadframe/epoxy moulding compound (EMC) interfaces. At the micro-scale, intrinsic adhesion properties resulting from chemical and physical interactions at the molecular scale are used to analyse the geometric effect of leadframe roughness on the resulting macro-scale adhesion properties. It is well-known that a roughened surface enhances interfacial adhesion. Two main reasons for this effect are the increase of effective interfacial area and mechanical interlocking of the EMC within the roughness valleys of the leadframe. This enhancement is investigated by developing a micro-scale FE model consisting of two materials bonded solely by chemical and physical interactions. The topology of the interface follows from the conversion of statistical data of a measured roughness profile into a representative geometrical description. The intended FE model contains cohesive zone elements for both interface and bulk fracture to simulate the competition between adhesive and cohesive failure. The macroscopic traction-separation law is then determined from the micro-scale simulations by means of a homogenisation scheme. Preliminary results for mode I and mode II loadings are presented. These results indicate an enhancement of interface properties from 20% for mode I loading to over 2200% for mode II loading due to interfacial area increase and mechanical interlocking.