| Objectives | Background | Approach |
To develop a methodology to assess the durability of surface
mount interconnects due to PWB flexural deformations.
A surface mount electronic assembly sees a host of stresses in its field usage due to mechanical and/or thermal loads. The mechanical field loads may be due to vibration, impact, or quasi-static flexural deformations caused by handling. Interconnects fail due to a combination of the shear and normal stresses developed during the field usage, because of bending and twisting flexural deformations.
Considerable work has been done at CALCE on the durability of surface mount interconnects caused by PWB flexure because of vibration loads (C95-13, C96-13, C97-13, C98-41, C99-22, C00-52) and quasi-static bending loads (C99-31, C00-06). The loading was carefully selected to generate unidirectional curvature on the test specimens, and stress assessment models were developed for such situations. The approach and methodology developed in this project has laid the ground work for any further work involving PWB flexure in multiple directions. Examples include multi-directional bending, vibration induced flexure and twisting deformations.
A combined experimental-analytical study will be conducted to study the influence of quasi-static flexural deformations on the durability of surface mount interconnects. The test specimen will consist of a specially designed PWB consisting of a variety of daisy-chained area-array and perimeter array surface mount components. In particular, the interconnect durability of a micro-lead-frame (MLF) package (which is a plastic package with lead-less interconnects along the perimeter) will be investigated for quasistatic flexure. Twist-controlled mechanical fatigue tests will be conducted and the cycles-to-failure will be monitored with an event detector. The amount of flexure under critical components will be monitored with strain gages. Failure analysis will be conducted to verify the failure mode. Simple mechanical models of the assembly will be generated using finite element analysis. Global models of the entire assembly will be generated to assess the twist under each component. The stiffening effect of each component will be approximately included by using a shear deformable laminated plate finite element model, developed in CALCE (C00-52). A local finite element model of selected components will be generated, which will provide detailed information of solder strains and energy densities for different magnitudes of applied twist. Damage analysis and life prediction will be carried out using these solder strain and energy values. Virtual qualification methodologies will thus be developed for twist loads. Virtual testing simulation will be used to assess acceleration factors for the test. Field durability will thus be estimated.