Copper Trace Thermomechanical Reliability Analysis of Ball Grid Array Package

Abstract

In semiconductor substrate fabrication process, copper traces are electroplated on prepreg material to play an important role of communicating signals between functional ICs. In recent years, larger size and high I/O number packages such as HFCBGA, 2.5D and 3D IC were developed, finer copper line width and space (L/S) trace substrate design become more and more important. However, the copper trace broken risk under several thermal manufacturing processes and reliability testing also increasing. A temperature cycling test (TCT) is one of the key experimental testing items for package reliability performance evaluation. The accumulated thermal stress will lead to trace broken risk during test, then the discontinue signal cause function failure. For specific application such as Automotive (ATV), function fail is lethal. Thus, consider risk assessment in the primary structure design stage is essential.In this work, a three-dimensional finite element model is used to evaluate structure design and material selection. In generally, the bilinear casting bulk model was used to evaluate copper trace lifetime performance under thermal cycling test condition. However, this bilinear constitutive model is difficult to simulate long-term creep behavior. Therefore, a new experimental technique was developed to measure the copper trace material properties. The mechanical tensile strength of the thin copper foil was tested with a micro-tester to examine the tensile stress–strain relationship. The copper foil is fabricated to be 18 micron thickness which similar to substrate copper trace thickness. Four kinds of tensile speed (1x100 ~ 1x10-3 mm/minute) at four temperature environments (25rC ~ 150°C) were applied and Stress-Strain (S-S) curves were measured by micro force test system. The tensile result of electroplated copper film shows lower Young’s modulus and higher tensile strength than casting bulk copper.The test vehicle of 11.8 x 11.4 mm2 BGA package which has substrate copper trace crack issue after TCT 1000 cycles (-65°C ~ 150°C) was chosen. Besides, the creep behavior of the copper was taken into account in the simulation model to verify the numerical model and copper’s constitutive model. The copper trace broken risk from the simulation was verified against the experimental data. Additional critical data, such as creep strain energy density (CSED) of the copper trace in the organic substrate and the maximum CSED level location in the substrate layer can be predicted. From the result, reduce the package thermal stress can bring lower CSED on trace, and in this study changed the lower Young’s modulus substrate material to control the thermal stress. The experimental and numerical methods presented here can be used as useful performance evaluation tools to support the choice of suitable package geometry and bill of material (BOM) selection.