Polymer Crosslinking Densities Influence Die Attach Creep Rates
Higher crosslink density suppresses polymer die attach creep rates by restricting free volume and extending rubbery plateau modulus under continuous thermal load.
Steady-State Shear
Flow Characterization
A rheological testing condition where a fluid or polymer paste is subjected to a continuous and constant rate of shearing deformation measures the time-independent viscosity of the material. This flow regime, termed steady-state shear, is achieved when the internal stress in the fluid reaches a constant value under a specific shear rate. Analysis of this behavior determines whether the material behaves as a Newtonian or non-Newtonian fluid.
Viscosity Measurement
Rotational rheometers apply this test using a cone and plate or parallel plate geometry to measure the torque required to rotate the spindle at a specific speed. The material is sheared until the measured viscosity stabilizes, indicating that the internal structure has reached dynamic equilibrium. When testing thixotropic adhesive pastes, the shear rate is swept across several orders of magnitude to evaluate the degree of shear thinning.
This test provides a complete viscosity profile that helps engineers predict how the paste behaves during high-speed pumping and dispensing.
Instrument Calibration
Calibration protocols utilize certified viscosity standards, such as high molecular weight silicone oils, to verify the torque sensors and gap measurements of the rheometer. Temperature control remains critical during testing because viscosity is highly sensitive to temperature variations. Standard tests use Peltier plate systems with a feedback loop to maintain a stable temperature to within one tenth of a degree Celsius.
Dispensing Control
Knowledge of the viscosity under steady-state conditions enables the precise sizing of dispensing nozzles and pumping pressures on the assembly line. Materials that exhibit strong shear-thinning behavior can be dispensed at high speed with minimal tailing or stringing. Quality assurance procedures specify regular shear sweeps to verify that incoming adhesive batches match the flow characteristics required for the line.