Experimental design for tensile tests on PCB copper traces for board level packaging

Abstract

This paper presents an approach to adequately design a test setup and specimen in order to perform tensile tests on PCB copper traces for board level packaging. The difficulty to conduct tensile tests on thin pcb copper traces is caused by the requirements of representative specimens. Most of the failure sites on copper interconnect structures in electronic assemblies are characterised by a high aspect ratio between the thickness and width. While the width is on the range of some hundred micrometres to a few millimetres, typical thicknesses are between 1 to 100 micrometres. Therefore specimens that adequately represent the properties of pcb copper are difficult to test in a standard mechanical test setup, which is usually made for compact specimens. The experimental design for determining the deformation behaviour of the PCB copper trace material encompasses two aspects: (1) experimental setup design and (2) specimen design. The two aspects contribute to the accuracy of the later material model for the use in FEM simulation. In order to conduct tests on thin copper film specimens, a test setup was designed, which is characterized by higher compliance of its frame. This way the test setup is able to compensate for misaligned angles of the sample to the load axis of the setup. An optical measurement is used in order provide accurate strain measurements on the sample. The role of specimen design of thin copper foil specimens is crucial for the accuracy of the test. FEM simulations of stress distributions have been carried out on classical dog bone specimens and on stripe specimens. The paper will discuss the inhomogeneous stress distributions of the gauge length of the stripe specimen compared to the dog bone specimen. Another aspect is the sample preparation. The paper will report the difficulties that exist, when the specimen is prepared by milling a copper sheet. The paper will discuss the connection between test setup design and specimen design with respect to the effects on the results of a tensile test on thin copper specimens.