Epoxy Strain Transfer Efficiency Loss under Cyclic Thermal Loading
Cyclic thermal loading degrades epoxy shear modulus, causing strain transfer loss that demands strict bondline guard-banding in technical files.
Strain Signal Attenuation
Signal Loss
Reduction in the amplitude of measured strain data as it travels from the sensor to the acquisition system affects the accuracy of stress measurements. Strain signal attenuation occurs when cable impedance, poor adhesive bonding, or electronic filtering degrades the high-frequency components of the stress wave. This degradation can lead to an underestimation of the forces acting on the board.
Gauge Mounting
Adhesive selection and thickness represent the primary mechanical causes of this signal degradation at the sensor level. If the adhesive used to bond the strain gauge to the board is too thick or too soft, it absorbs some of the mechanical energy instead of transferring it to the foil grid. Under these conditions, strain signal attenuation increases because the elastic deformation is damped before it can change the electrical resistance of the strain gauge.
Structural Influence
Acquisition hardware and long signal cables introduce further electronic losses that must be minimized through proper calibration. In high-speed tests like board drop testing or in-circuit fixture actuation, the board deforms in milliseconds, generating rapid transient signals that require high bandwidth. Signal loss from improperly shielded cables can lead to false acceptance during qualification tests because the measured strain values appear to be below the maximum limits established by IPC standards.
Engineers must calculate the frequency response of the measurement chain to ensure that the reported values reflect the actual physical strain experienced by the board.