It might be hard to swallow, but solder is an essential part of the U.S. nuclear arsenal. Each nuclear weapon contains hundreds of thousands of solder joints, and each is a potential point of failure. To help ensure they don’t fail, researchers at Sandia National Laboratories have developed and refined computer models to predict the performance and reliability of soldered joints.
“Computational modeling of solder joint fatigue has become critical in the current nuclear weapons life extension programs, even before production assembly at the Kansas City National Security Campus,” says materials scientist Paul Vianco, who works with material modeler Mike Neilsen. “Sandia uses the computational model to solve manufacturing issues, as well as assess the effect of design changes on solder joint reliability.”
Anything with circuit boards requires countless solder joints, and miniaturized electronics have vastly increased the number present in printed wiring assemblies. Vianco notes two examples among a host of weapons’ wiring assemblies: one with more than 900 solder joints (400 on a single component), and the other with about 300 joints.
Also, according to Vianco, Sandia has advanced computational modeling to the point where it can help guide component design decisions and assembly processes in Kansas City, establish qualification and acceptance test definitions, and provide long-term reliability of solder interconnections in the stockpile.
“In the early stages of model development, we could sit with designers and give them a broad reliability window for solder joints. It was a case of saying, ‘Well, you’re not going to get into a lot of trouble because we know what’s going to happen here and here,’’’ he recalls. “But what was happening could not be predicted with any confidence that let engineers use the models to guide designs of electronic assemblies.”
The current solder model is based on years of research and collaborations with universities and others. Sandia has modeled solder performance for more than 30 years, increasing models’ match with reality using increased knowledge of material properties and experiments to develop and validate the models.
The key, however, is that today’s models provide quantitative as well as qualitative data.
Qualitative engineering judgments are based on experience and comparing outcomes. Quantitative predictions are explicitly based on the physical behavior of a material in a design rather than a comparison with another material. One could say, for example, that A is a better design than B because it will last X number of years longer.
That saves enormous amounts of money by eliminating the need to fabricate samples and the time required for equipment operations and data analysis. “Modeling can provide answers in one to two weeks instead of the months needed for experiments,” says Vianco.
Neilsen said earlier Sandia researchers had pushed for early investment into computer models that could be used to predict when thermal mechanical fatigue cracks would start and grow in solder joints. It was also recognized that there was less margin for error as Sandia moved from plated through-hole interconnects to newer, more complex, surface mount assemblies.
Plated through-hole solder interconnects refer to drilling holes through printed circuit boards, plating the holes with copper, pushing component leads through the holes and soldering them in place. Surface mount components are soldered to pads on the top or bottom of printed circuit boards.
Vianco and colleagues started experimental characterization work in the 1990s that provided data for a unified creep plasticity model—one that captures both creep of solder at low stress levels and plastic deformation at higher stress levels. The model accurately describes the mechanical response of solder to slow loading typically generated during thermal cycling. Recent experimental material characterization helped refine the model to capture the mechanical response of solder under fast loading rates, such as those generated by shock and vibration.
Sandia validates models by accelerated aging, which are thermal cycling tests that quantify the statistical reliability of solder joints. Vianco’s team designs and builds printed wiring assembly mockups scaled to the size of real assemblies, then tests them in research ovens that cycle temperatures between set maximums and minimums to record electrical failures in solder joints. The team analyzes the failure to confirm it was due to the expected solder fatigue and not some other, unexpected cause.
Researchers validated solder fatigue crack initiation and growth predictions by comparing the model’s predictions with experimental results for different components, including surface mount resistors and capacitors, leadless ceramic chip carriers, and plastic ball-grid arrays.
The model also has generated some surprising results. “For example, voids in solder look bad, but a uniform distribution of voids generally has little effect on fatigue life,” Neilsen says, then jokes, “Maybe we should be using solder foam — if we could just figure out how to make it.”