May 31, 2009
Or, why our cell phones and MP3 players hardly break anymore.
I dropped my cell phone again, on the driveway as always, then picked it up and expected it to fully perform. It did. I also dropped my electronic Prius car key (replacement cost $185.97 plus $50 programming). Luckily it still works, too. Was I reasonable to hope so?
The answer is probably yes, but this hasn’t always been the case. It depends a great deal on whether the manufacturer verified those designs for durability. Even 10 years ago, many efforts in this area relied solely on physical drop-testing or vibration on a shaker table. Nowadays, engineers have a wide selection of durability-simulation software available to test preliminary designs in more ways than anyone has the time or money to do in a lab. DE spoke to a number of finite element analysis (FEA) vendors to see how their software helps engineers ensure their handheld electronics products meet today’s consumer expectations.
Impact Analysis Requirements
Figuring out what will happen to a handheld device when it hits the ground involves more than just applying a force to a point and seeing whether the unit still turns on. For one thing, you need to decide on the end goal. Do you want to know how well the device will function when subjected to a constant or variable vibration? Is the device going to need special packaging to survive a range of transport conditions? Or are you looking to see what parts will incur the worst damage after a three-foot drop?
Dennis Sieminski at NEi Software explains that for all these goals, you generally need to define four sets of input parameters. The first describes the geometry and material for both the device and the target, such as a concrete floor. The second specifies initial velocity and acceleration to calculate the impact velocity. Third is a range of test values for initial orientation. And the last defines the contacts to correctly mate the meshes when the device hits the target.
Algor simulation software, part of the Autodesk solution for Digital Prototyping, was used to determine the impact stresses resulting from dropping a cell phone housing assembly onto a wooden floor from a height of 26 inches. Prior to striking the floor, a corner of the phone impacts a table top, causing the phone to spin as it continues to fall to the floor below.
The NEi Nastran Automated Impact Analysis simulation tool guides you through this setup, then calculates the distance traveled, creates contacts between meshed surfaces, assesses vibration characteristics, and calculates the impact durations.
These steps are pretty much followed in all FEA software that offers impact testing as part of its explicit (short time duration) analysis features. However, the nuances of doing so, and the way that the data is pre- and postprocessed for interpretation, varies with the package as well as with the processing hardware on which it is run.
ALGOR analysis software, both the Professional MES and Professional Designer versions, can examine nonlinear dynamic behavior under multiple scenarios. Starting with ALGOR’s FEMPRO editor to mesh the model, you can apply materials, set up impact plane and load curve, run the analysis, and view both frequency and energy plots.
“These days you don’t need to make all the modeling simplifications that were needed 10 years ago,” says ALGOR Product Manager Bob Williams. “With the resources of banks of multiprocessors, you can work with the parameters that define the true expected operating environment. People are always going to be dropping cell phones. You can now do a full mechanical event simulation and determine will it shatter, or will the battery cover just come off?”
The visualization capability of Teamcenter from Siemens PLM means analysts can easily share thermal results of this digital camera body with the entire engineering team to help with both the mechanical and electrical aspects of design. Image courtesy Siemens PLM Software
Siemens PLM Software’s family of analysis software offers a number of ways to perform durability analyses. For example, George Laird, a consultant with Predictive Engineering Solutions, used Siemens’ Femap linked to LS-DYNA analysis software from Livermore Software Technology Corp. (LSTC) to investigate why the color LCD touch-screen display on a handheld barcode scanner cracked during physical drop tests. The designers needed a detailed stress analysis to guide redesigning its mounting.
Femap’s application programming interface provided the desired flexible environment for writing a program that would manage element sizing, a critical aspect for controlling the fine level of time steps. The software’s open interface also offered a direct way to write run-ready data decks for input to the LS-DYNA dynamic analysis.
Some years ago Steve Pilz, ANSYS senior product manager, discovered the hard way that his PDA device had not been subjected to thorough drop testing, either physical or simulated. His one-week-old device fell only 2 ft. to a carpeted floor, but the LCD screen cracked. He observes, “Ten years ago it was not realistic to test for the million ways something could hit the ground. It was too slow and too expensive.” Now the resources make it much easier to do simulations, find the worst-case scenario, and design for something that’s reasonable.
ANSYS’ line of software includes three stand-alone products that address durability problems: the base-level ANSYS (for pre- and postprocessing) combined with the LS-DYNA solver, the advanced ANSYS AutoDyn for very high-speed transients, and ANSYS 12.0 Explicit STR. The last package, based on AutoDyn, will eventually be built into the company’s Workbench product. An additional parametric manager tool lets you run a number of different load scenarios and identify the worst case. Pilz adds that with this level of detail, you can even figure out the strength of a solder joint for a chip on a printed circuit board (PCB). Such an analysis could direct the electronics designer to attach a chip capacitor more strongly if it tends to detach after a drop.
MSC.Software also sees the benefits of having several analysis products geared toward different goals. If users simply need a simulated drop-test, MSC Dytran with the Patran pre- and postprocessor form an economical approach. For drop tests combined with thermal and structural simulation, the company steers customers toward a full multidiscipline (MD) solution that includes analysis using nonlinear MSC Marc.
|Getting EEs to Talk with MEs: Cross-Discipline Data Management|
Whether your design team comprises two people across an office or two companies across the globe, your best products will emerge from shared design and analysis data. Addressing the need to couple results across disciplines is a growing focus of product lifecycle management (PLM) tools, including those from Parametric Technology Corp (PTC), Siemens PLM Software, MSC.Software, and Dassault Systèmes SIMULIA.
PTC’s Product Development System helps users synchronize electrical, mechanical, and embedded software designs to improve design reuse and product quality, and cut cost and time to market. The company’s InterComm Expert allows electronic design intent to be more quickly verified by other team members.
Siemens offers its Teamcenter PLM products as an avenue for tying together MCAD and ECAD results. The software lets designers working with model elements from different applications share data across domains (e.g., if the ME discovers and corrects electrical component stress issues, the information is readily available to the EE who can perhaps relocate a part or change a PCB outline).
MSC.Software’s SimExpert addresses system-level issues, making it easier to link the outputs from one analysis package to another and allowing use of older models.
SIMULIA now leverages PLM capabilities from Enovia, another Dassault Systèmes brand, with its own expertise to offer Simulation Lifecycle Management. This effort recognizes the need to collect and organize not only CAD data, but analysis results to allow re-use and collaboration across projects and disciplines. —PJW
Digging Deeper into the Details
At Cranes Software and its sister company Engineering Technology Associates (ETA), the emphasis is on a line of vertically integrated products that allow you to go deep into the details of what may cause a failure. Vipul Kinariwala, NISA product manager for Cranes, says that six or seven years ago, they realized a need to go beyond just determining whether a product will still work after shock and vibration. He says, “We needed to simulate a drop test to find out not only what parts are breaking, but what accelerations did they see?” This specific information could then be used in designs to isolate the more sensitive components and influence packaging design for shipment.
Manually setting up a simulation to get to the chip and component level consumes much time. Now, users of ETA’s VPG product line can automate the process of defining initial and boundary conditions for multiple test cases with VPG’s Drop Test Simulation module. Output includes hundreds of thousands of elements and data formatted for direct input to LS-DYNA software.
With so many handheld devices operating via touch-screen, ETA has recently tackled the need to evaluate indenting as a type of force, simulating component damage behind the LCD. Crane’s NISA software also plays a role in durability analysis, with its FEAP product targeted specifically to analyzing PCBs for stress, random vibration, fatigue life, and more. Tim Palmer, product manager for ETA’s VPG, explains, “You can take the results from a VPG drop test simulation and pass them along to VEAP to study the durability of solder joints, giving you an end-to-end toolset.”
Dassault Systèmes’ SIMULIA Abaqus/Explicit software brings its own brand of high-end performance to nonlinear simulations. For example, it can model the rubbery gasket around an LCD display and include pre-compression effects from device assembly in the impact simulation. The company’s industry lead for electronics, David Cadge, notes that the sophisticated pre- and postprocessing capabilities, including extensive automation and scripting tools, also help users investigate impact stresses down to the level of solder joints and individual LCD layers.
At ADINA R&D, engineer Nagi Elabbasi believes that defining contacts forms the major difficulty for a detailed impact analysis. When you consider the number of components involved in a typical electrical device, it’s certainly a challenge to simulate how a sudden shock will shift one component with respect to another. Elabbasi says that currently they, like others, don’t directly couple back to electronics analyses, but see this as a growing field in the future, particularly for such applications as hearing aids, small speakers, and many MEMS devices.
Over the Wall or Full Circle?
It’s not surprising that most electronics engineers still pass their designs “over the wall” to the packaging engineer and industrial designer to complete, with no direct feedback. Did a physical shock erase a flash memory? It’s still hard to tell. Connecting the results from drop-test simulations back to the designer of a hard-drive or barcode scanner is usually limited to identifying the location of a visible break. Electronics are certainly designed to operational standards, whether military or commercial, but how well they function when placed in the next level of assembly is the question.
The faster you can evaluate the effects of changing a design factor, the better. Systematically analyzing such changes falls in the realm of design of experiments (DOE), whether applied to design, operation, or testing. SIMULIA’s Abaqus/Explicit supports an add-on product called Isight that provides engineers with a suite of tools for creating simulation process flows. Users can automate design explorations including optimization and DOE. The package uses a graphical drag-and-drop library of components to simplify the planning task.
The NEi Nastran Automated Impact Analysis simulation tool is used here to assess vibration characteristics and calculate impact durations of a laptop computer falling to the floor.
MSC.Software recognizes that manufacturing tolerances play a key role in electromechanical performance. Its Insight module helps analysts examine the effects of variations in such factors as the nominal width of a solder trace or the density of a material, so that information can go back to a designer.
Greg Brown, Parametric Technology Corp. (PTC) product manager, points out that drop-test simulations generate data of a statistical nature that should drive realistic product decisions. For example, if the cell phone you’re designing is a premium product that needs to last, the mechanical design will likely differ from that of a low-cost, short-life version. Simulation data will guide you to the areas to be strengthened for the more durable design.
PTC supports this capability in its Pro/ENGINEER Mechanica software. The company’s CoCreate software can also play a role in this task by helping designers quickly run through first-cut model changes.
A more general approach to improving designs for durability comes from Vanderplaats Research & Development. Its VisualDOC general-purpose optimization software adds design optimization capabilities to almost any analysis program, and can perform both linear and nonlinear operations. VisualDOC includes a design of experiments module and offers discrete variable optimization.
Value in the Big Picture
It used to be that management just asked designers if the unit passed or failed. Now it’s likely they’d say, product A works, but we need to make it 10 percent better to overtake the competition. Stephen Endersby, Dassault Systèmes SolidWorks product manager for simulation, says durability simulation can help by highlighting trends. “During the early stages of design, this is more important than (exact) answers,” he says. “You first consider the extremes of use, then ask all the what-if questions.” SolidWorks Premium and Professional both support these tasks; a white paper on their capabilities can be found at the company website.
Big picture and fine detail—both uses of simulation have their place in designing durable electronics. Remember that next time your phone bounces across the floor.
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Pamela Waterman is an electrical engineer and freelance technical writer based in Arizona. You can contact her about this article via e-mail sent to DE-Editorsmailto:[email protected].
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