Stress effects of epoxy adhesives on ceramic substrates and magnetics

Conventional, high T g adhesives can result in cracking of magnetics (transformers and inductors) and ceramic substrates after cure or after thermal cycling and extended stress screens. Cracking in substrates may result in electrical “opens” in underlying circuitry. Cracks in large ( 1.8×3.650 in. 2 ) substrates under magnetics (ferrite cores/inductors and wire-wound transformers) have been observed after screen testing. Cracks in the substrates are most likely due to stress transfer in centrifuge in combination with “off-axis” stresses in thermal cycling and environmental stress screening. Cracks are also observed in the epoxy fillet. Stress modeling is often difficult or impossible due to complex routines that are required and the lack of material characteristics such as Young’s modulus, Poisson’s ratio, and stress/strain curves for materials. Direct comparison of materials is often not possible due to missing vendor data or data from different vendors performed under different cure conditions using varying test methods. A series of materials with varying room-temperature modulus characteristics ranging from over 1×10 6 to approximately 19,000 psi were evaluated for material characteristics and reliability in screen testing using sequential thermal cycling and centrifuge. Test vehicles were constructed using four sets of magnetics in packaged low-temperature co-fired ceramic (LTCC) substrates. The specific objectives of this work were to select, evaluate, and recommend a new adhesive for core-and-coil attachment. Evaluations performed for the materials selected include (1) material testing and preliminary, first-order stress calculations, (2) testing and evaluation of actual parts, and (3) correlation of results using NASTRAN (NASA structural analysis) mechanical modeling. This material was, in part, previously presented at the 1999 International Microelectronics and Packaging Society (IMAPS), is copyright 1999 by IMAPS, and is reprinted with permission from the Proceedings of the 1999 International Symposium on Microelectronics, pp. 896–902, 26–28 October, 1999.