Creep testing addresses the measurement of dimensional changes over time when a material is statically loaded. Creep measurements are important for solders, polymers, and adhesives because they experience large strains over time, and the strain rate is very sensitive to temperature.
Software can be used to perform shear creep tests using shear cells. While these cartridges can be used to perform high temperature shear tests, they are not recommended for prolonged use.
Creep testing can be performed on individual solder balls, test structures, or components soldered or bonded to a substrate. In the absence of a defined test structure, such measurements are largely comparative, but always provide valuable feedback on the effects of material changes.
The main parameters are test load and end point conditions.
Test load
The load that will be applied to the shear tool during the test; active feedback ensures that this load is maintained as the sample deforms.
End condition
Completion of the test can be time-based or dependent on sample deformation. The endpoint conditions are as follows:
Stop after Duration: The system will hold the test load for the specified duration for the sample
Stop after displacement: If the XY table moves the specified displacement value, the test ends
Stop after load detection: If the load cell measurement is satisfied, by using the internal sensor (external sensor can also be used), the test will stop. This is measured in %.
Displacement can be measured using a platform encoder (internal sensor) or an external sensor (external sensor). If an external sensor is used, it should be positioned to measure the relative motion between the tool tip and the sample or workpiece holder.
Why Use High Temperatures for Creep Testing?
Creep in metals is governed by the thermally activated motion of dislocations. At very low temperatures where creep does not occur, the metal work hardens as more and more dislocations are blocked by other dislocations, grain boundaries and precipitates. At higher temperatures, dislocations are able to bypass these obstacles by climbing onto new glide planes, so the material can continue to deform.
At what temperature should I perform the creep test?
As a rule of thumb, creep is important for temperatures above half the absolute melting point of the material in Kelvin. For solder, room temperature (20°C = 293K) is more than half the melting point (188°C = 461K for 60/40 Sn/Pb). The temperature (homogeneous temperature) is usually expressed as a fraction of the absolute melting point of the material. 60/40 solder has a homogenous temperature of 0.64 at room temperature.
