Joint Life Prediction Research Articles

Creep life estimation of a service‐exposed P91 cross‐weld joint based on a primary‐secondary creep deformation model

AbstractCreep deformation and rupture behavior of welded joints from service‐exposed P91 steel were investigated by creep tests and microstructure examination. A series of miniature tensile creep tests were conducted at 569°C under applied stresses ranging from 182.5 to 286 MPa for the heat‐affected zone (HAZ), base metal, and weld metal. Creep deformation behavior was modeled using the experimentally measured creep strain data as the primary creep‐secondary creep (PC‐SC) model and fitting closely with the measured creep curves of each metal. Power‐law type creep equations were employed to describe the primary and secondary creep behavior. The creep rupture life of the cross‐weld specimen was predicted using the PC‐SC model with HAZ properties. An accurate prediction was possible if the stress in estimation was increased to 1.14 times of the actual applied stress. The creep test results with the cross‐weld specimens were used for verification. Applying the Monkman–Grant relationship and the creep damage tolerance factors to the cross‐weld joint life prediction are also discussed.

An evaluation of dwell time and mean cyclic temperature parameters in the Engelmaier model

The Engelmaier model is a strain-range-based fatigue model for evaluating the life expectancy of solder joints under power and temperature cycling. The model relates cycles to failure to a strain range metric using a power law relationship. In Engelmaier’s original formulation in 1983, the exponent term in the power law relationship was defined to be a function of mean cyclic temperature and cyclic frequency. Engelmaier replaced the cyclic frequency with dwell time in 1988 without explanation.This paper provides a rationale and the implications for changing the formulation of the exponent term of the Engelmaier model. Using non-linear regression, model constants are derived for Engelmaier model formulations with cyclic frequency and dwell time parameters. A t-statistic measure is estimated to quantify the influence of cyclic frequency and dwell time parameters on the cycles to failure. In tin-based lead-free solders, the absolute values of the t-statistic for cyclic frequency are approximately 3–10% higher than that for dwell time. The higher absolute value of a t-statistic identifies a stronger dependence of cycles to failure on cyclic frequency than on dwell time. Then, the feasibility of replacing the mean cyclic temperature parameter in the exponent term with time-averaged cyclic temperature using t-statistic measures is explored. Based on the evaluation of t-statistic measures, the authors recommend the Engelmaier model with cyclic frequency and mean cyclic temperature parameters to be used for solder joint life prediction.

Comparison of LCC Solder Joint Life Predictions With Experimental Data

The ability of solder joint life-prediction algorithms to predict the failure of solder joints due to temperature-cycling induced creep-fatigue has been investigated using representative leadless chip carriers (LCCs) as the test vehicle. Four different algorithms are assessed: the classic Coffin-Manson algorithm, a modified Coffin-Manson algorithm with dependency on peak stress, and two strain-energy based algorithms. JPL’s special purpose nonlinear finite element computer program was used to dynamically simulate the solder joint response to the standard NASA temperature cycling environment, which ranges from −55°C to +100°C with a 4-hour period. The computed stress-strain history provided the inputs needed by each of the failure algorithms. To test the accuracy of the analytical predictions, three different sizes of LCCs (68 pins, 28 pins, and 20 pins) were subjected to an experimental test program using the same 4-hour temperature cycle as used in the analytical predictions. The three different sized ceramic packages, each with a 50-mil pitch, provided a range of cyclic strain ranges and solder fillet geometries so as to test the algorithms against realistic electronic packaging variables. The study highlights limitations in the historical Coffin-Manson relationship, and points up possible improvements associated with incorporating a stress modifier into the Coffin-Manson equation. This modification is also somewhat simpler and more accurate than the energy-density based algorithms, which also performed quite well.

A comprehensive surface mount reliability model covering several generations of packaging and assembly technology

An extensive set of surface mount reliability data gathered between 1984 and 1992 has been integrated in a single design-for-reliability tool, the comprehensive surface mount reliability (CSMR) model. This analytical/empirical model fits 28 accelerated test datasets with a correlation coefficient of 0.97, a significant improvement over a correlation of 0.40 over the same database for the existing figures of merit. The new solder joint life prediction capability spans test conditions with a wide range of frequencies and temperature limits and covers three orders of magnitude in cyclic life. The validity of the model has been verified for evolving families of components, substrates and assembly technology, including 0.5-mm pitch Alloy 42 thin small-outline packages (TSOPs) and large, 25-mil pitch devices. The innovative features of the CSMR methodology are presented, and its application is is illustrated for several design examples. Important benefits of the model are more accurate surface mount (SM) reliability predictions and less conservative design decisions than from previous models.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>