LITTELFUSE, INC. USES ALGOR'S ACCUPAK/VE TO DESIGN BETTER FUSES FOR TODAY'S WIRED WORLD

Littelfuse, Inc., one of the leading producers of circuit protection products worldwide, is improving mechanical performance in the next generation of their fuses with ALGOR's Accupak/VE Mechanical Event Simulation (MES) software with linear and nonlinear material models. The fuses shown here protect electrical circuits from overload on the printed circuit boards of computers. Littelfuse's first experience with MES was to study the "drawbridging" phenomenon, a form of component misalignment that can possibly result from forces acting on a surface-mounted fuse as it is soldered onto a computer's printed circuit board.

We live in a wired world - full of computers, cell phones, personal organizers, electronic games and other electronic gadgets. Although you can not see them, fuses are protecting electrical circuits from overload in the electronic systems of these devices and others, including streetlights, cars, trucks, factories, heating and cooling systems and even satellites. In the quest to build smaller, faster, safer circuit protection, fuse developers are extending their analytical methods to encompass the geometric and mechanical aspects of fuse design, thus supplementing their established electrical performance modeling and analysis methods. 

Littelfuse, Inc., one of the leading producers of circuit protection product worldwide, is improving the mechanical performance of their fuses with ALGOR's Accupak/VE Mechanical Event Simulation (MES) software with linear and nonlinear material models. "MES is changing the way we work," attests Tom Hall, Ph.D., Senior Scientist at Littelfuse. "Physics-based Accupak/VE enables us to predict and study mechanical behavior, for example in manufacturing and assembly, with greater assurance of the precision of our results than we can obtain through trial and error in the laboratory."

Littelfuse's first experience with MES was to study the "drawbridging" phenomenon, a form of component misalignment that results from forces acting on a surface-mounted fuse as it is soldered onto a computer's printed circuit (PC) board. Based on the MES results, Hall was able to suggest alterations to the fuse geometry that would practically eliminate the drawbridging phenomenon. This type of analysis and development could be used to alleviate this phenomenon for many other types of PC board components.

A Rare but Problematic Phenomenon
About 1,000 components are surface-mounted onto each PC board. In the surface-mounting process, solder paste is first applied to the board and components are placed onto it. Then, the PC board travels on a conveyor belt through an oven that heats the assembly so that the patches of solder paste liquefy, thus forming joints between the PC board and its components. After only a few seconds, the PC board moves into a cooling zone and the solder changes back into a solid form. In the case of components such as fuses, which are placed parallel to the board and are soldered on each end, the drawbridging phenomenon can sometime occur when one end of the component is drawn up, thus misaligning the fuse at a small angle to the surface. This phenomenon occurs because the solder joint at one end of the component forms and cools before the other. (Drawbridging sometimes referred to as "tombstoning" because in a worst case scenario, the component draws up so far that it is almost perpendicular to the surface.) 

Misalignment of the fuse can prevent it from functioning properly. Although drawbridging may occur in only a small fraction of a percent of cases, PC board manufacturer are constantly looking for ways to reduce the likelihood of drawbridging because of the time and cost involved in manually adjusting misaligned components. "Some of the OEMs to which we supply fuses are producing as many as 10,000 PC boards a week," explains Hall. "Finding solutions for drawbridging is especially important when it affects high-volume, mass production of PC boards. Even if the misalignment of components occurs only in a fraction of a percent of cases, that can add up to be an unacceptable number of components that need manually adjusted."

Although there are many variables PC board manufacturers can change to reduce drawbridging, Littelfuse was asked by one of its OEMs to look into how their fuses could be altered to mitigate the problem and produce results in just 6 to 8 weeks. 

Forging a New Design Direction
As Senior Scientist at Littelfuse, Hall studies the origins of problems in order to recommend new design directions. Because Hall needed to find a reproducible solution quickly, he immediately ruled out trial and error laboratory testing and focused on obtaining technology that would accurately simulate the phenomenon on a computer. 

"Accupak/VE was a good match for us in terms of cost and the capability to simulate mechanical events over time with linear and nonlinear material models. In addition, the ease-of-use of the Release 12 interface and the level of the support offered for the product enabled us to get results quickly," said Hall. A novice to FEA, Hall took the introductory, hands-on education seminar, which helps ALGOR customers to take full advantage of software features and capabilities by using example models drawn from real-world situations. "Back in the office, technical support helped me to troubleshoot issues specific to modeling the drawbridging phenomenon," said Hall.

Although there are two masses of solder, Hall decided to focus on simulating forces on only one side of the component to simplify the model. Hall began by building a 2-D model of the fuse and later created 3-D model using Superdraw III, ALGOR's single user interface for FEA and precision finite element model-building tool. 

The final 3-D fuse model Hall developed consisted of solid brick and truss elements. The first group of brick elements represented the fuse, which consists of conductive plastic sandwiched between metal foil and coated with various other materials. For the purposes of the MES, the plastic nylon material from the standard ALGOR material library closely matched the aggregate material properties of the fuse. A second group of brick elements was assigned material properties similar to lead, a stiffer and denser material than the nylon-like material of the fuse. This group was fix with boundary conditions on the bottom surface to simulate that the solder paste attaches firmly to the PC board. 

Truss elements were placed around the edges of the lead-like material group and assigned an initial axial force of 2 dynes to simulate the surface tension exerted by the solder when it cools around the fuse. Hall defined the event to take place over .5 second with 200 time steps per second. The length of the event was based on timing the real-world event during the manufacturing process. 

The FEA-based MES was then processed, realistically simulating motion and flexing in the mechanical event. MES computed and showed results on the computer model at each instant in time with the software's built-in visualization capabilities. Hall captured these analysis results over time in an AVI animation file. 

In the case of the drawbridging simulation, Hall was interested only in the displacement results. In the 2-D simulation, the fuse experienced an unacceptable degree of drawbridging. Consequently, Hall experimented with the 3-D model by changing the dimensions of the fuse where it contacts the solder paste. By minimizing the area that touches the solder in the vertical (Z direction) and also slightly in the across the end of the fuse (Y direction), Hall was able to reduce the drawbridging effect to an acceptable level in the final 3-D model.

		Although there are two masses of solder, Littelfuse Senior Scientist Tom Hall decided to begin with a 2-D model of the fuse and to focus on simulating forces on only one side of the component to simplify the model. In the 2-D ALGOR simulation, the fuse experienced an unacceptable degree of drawbridging.
		Littelfuse's Hall experimented with the 3-D model by changing the dimensions of the fuse where it contacts the solder paste. By minimizing the area that touches the solder in the vertical (Z direction) and also slightly across the end of the fuse (Y direction), Hall was able to eliminate the drawbridging effect in the final 3-D model. Based on the ALGOR MES results, Hall was able to suggest design directions for a fuse geometry that would eliminate the possibility of drawbridging. This type of analysis and development could be used to alleviate the phenomenon for many other types of printed circuit board components.

"The AVI file of the analysis replay were invaluable when I presented my results first to others at Littelfuse and then to the OEM who had requested that we investigate drawbridging," said Hall. "Showing the simulated motion over time helped to explain the mechanical principals behind the drawbridging phenomenon to those that weren't familiar with it. The accuracy of the results and the clarity of results presentation helped us convey to our customer that we understood and modeled the problem accurately. The final model we presented proved that the geometric changes we were making would significantly help to alleviate the problem." Laboratory tests on prototype fuses verified the accuracy of the MES displacement results over time.

"Expanding use of mathematical modeling in product development had been a goal for some time," said Hall. "This first application of MES was a real learning experience and just the beginning. Now we are applying software at a higher level and using it to solve different problems."