OCTOBER 2004 What's up with primary and tertiary creep? Understanding the mechanical behavior of materials is key to the success of any project. After close examination of hundreds of creep curves for lead-free solders, it appears that primary and tertiary creep are significant and cannot be disregarded. It also appears that, under some stress and temperature conditions, the steady-state or secondary creep stage is almost non-existant. This has huge implication for the design of accelerated tests and the interpretation of test results. Assuming that secondary creep is the dominant stage under-estimates solder deformations and neglects damage accumulation due to softening of solders under load. Evidence of the above, and a method on how to handle both primary and tertiary creep in lead-free solder constitutive models, will be presented in our upcoming workshops. See links to upcoming events in the lower left section of our home page.
SEPTEMBER 2004 1) Jean-Paul Clech will give a presentation on lead-free reliability trends and solder properties during a workshop hosted by the Santa Clara Valley Chapter of IEEE-CPMT, on October 5, 2004 in Cupertino, CA: "RoHS and Pb-free Reliability Symposium: Exploring the transition’s impact on product reliability".
Lead-free reliability trends and challenges will be discussed in the first part of the presentation. The second part will demonstrate the importance of primary and tertiary creep behavior in many lead-free solders. For further info and registration to the workshop:, visit the IEEE-CPMT Santa Clara Chapter web site.
2) NEW SIMULATION SERVICES 2.a) Thermal cycling hysteresis loops: we have developed an in-house capability to simulate thermal cycling hysteresis loops (stress/strain history and cycles) for any specified thermal cycling profile, including typical profiles with linear ramps and dwell periods, and for both standard Sn-Pb and lead-free solders.
These simulations provide insight into the stress/strain history experienced by solder joints. They are of use, for example, to figure out the effect of ramp rates, and of possibly different dwell times on the cold and hot side of the cycle, on accelerated testing programs. If you find that your test programs take too long, you may want to consider these thermal cycling simulations to optimize your test profile and reduce test duration.
2.b) Creep curve simulations: we have developed a new solder constitutive model that handles both primary and tertiary creep of lead-free solders and accurately captures the entire creep curve (strain vs. time at a given stress and temperature). A side benefit of the model is that it provides accurate estimates of creep rupture times (typically within 10-25% of actual failure time) without any empirical correlation of creep rupture data.
These simulations provide insight into the importance of primary and tertiary creep for lead-free solders. Primary and tertiary creep can also be included in the thermal cycling simulation of solder joint hysteresis loops. These simulations are also of use if you have to deal with long-term creep deformations and creep rupture of solders.
If you would like us to run the above type of simulations for a particular application of yours, please contact Jean-Paul Clech , tel: +1 (973)746-3796.
AUGUST 2004 Paper: "An obstacle-controlled creep model for SnPb and Sn-based lead-free solders", by J-P. Clech, to be presented at SMTA International 2004, Chicago, IL, Sept 26-30, 2004. View abstract...
Properties of Sn-Ag-Cu solder joints in shear: see 2004 Micromaterials and Nanomaterials paper under publications page.
MARCH 2004 APEX 2004 paper and presentation available for download. Please quote full reference when referring to paper:
"Lead-free and mixed assembly solder joint reliability trends", by J-P. Clech, in Proceedings (CD-ROM), IPC / SMEMA Council APEX 2004 Conference, Anaheim, CA, Feb. 23-26, 2004, pp. S28-3-1 through S28-3-14 (Copyright EPSI Inc., 2004).
Download (note: files and contents are for personal use only, not to be distributed, posted on the web or sold in any form or shape without prior authorization in writing by EPSI Inc.):
2) APEX 2004 Presentation (238 kbytes PowerPoint file). Slides # 8 and # 9 show comparisons of SnPb and lead-free isothermal creep rate data at temperatures from -55C to 125C. Figures shows the cross-over of SnPb and lead-free creep curves for common lead-free alloys.
FEBRUARY 2004 Lead-free solder reliability trends:
1) While most of the popular lead-free solders are more creep resistant than SnPb under low to medium stress, many of them creep as fast as SnPb under high enough stress conditions (see next article)
2) Examples of failures we have experienced on lead-free product boards: a. Fatigue failure of resistor solder joints. b. Ceramic capacitor cracking. This is an old problem that has resurfaced. The mechanics behind it is that higher forces are transmitted to components when the lead-free solder is more creep-resistant (i.e. stronger) than SnPb. This is both a solder joint design and a component-related issue.
3) Under accelerated thermal cycling conditions, and with long enough dwell periods at the temperature extremes, lead-free solder life in test can be shorter than that of SnPb joints. Extrapolation of test results to use conditions will tell if your own product reliability requirements are still met.
The above are examples of issues to be addressed at the design and prototyping stage.
How does lead-free solder compare to standard SnPb? As far as reliability is concerned, the jury is still out and blanket statements should be taken with a grain of salt. Reliability - which is defined as the ability to survive the planned design life -- is application-dependent. However, lead-free reliability trends are showing as discussed in our upcoming APEX 2004 paper (see abstract in 2003 archives). Evidence is mounting that there is a cross-over point beyond which the relative reliability of lead-free vs. SnPb assemblies is reversed. The reliability trend reversal is one way or the other depending on the solder alloy composition.
In parallel with reliability trends, we have been looking at the creep behavior of a variety of lead-free solders from SnBi to SnCu and popular SnAgCu (SAC) alloys of varied compositions. It is now clear that under high stress conditions, and regardless of temperature (in the range -55C to 125C), many of the popular lead-free solders creep at similar rates, some of them faster than SnPb. The transition stress, when SAC solder creeps as fast as SnPb, is in the approximate range 10-25MPa, depending on the test temperature. Those conditions arise in accelerated thermal cycling. They may also be encountered during rapid transients in service as in automotive underhood applications, aerospace applications (e.g., rapid climbing or descent of fighter jets) and others. Implications in terms of field reliability, but also in terms of acceleration testing and how to extrapolate test results to service life, are under investigation.