Vibration simulation software has been optimized specifically to handle printed circuit boards.
Craig Armenti, Mentor Graphics, a Siemens business
Board Systems Div.
To get a feel for how vibration problems can affect product design, consider that the U.S. Air Force has estimated shock and vibration causes 20% of all mechanical failures in avionics. That level of failure is a problem for any kind of product, but it is particularly challenging for expensive equipment slotted to have a long lifetime. Military avionics, for example, is expected to last between 10 and 30 years.
Reliability has become important for most products regardless of the length of their useful life. Though cell phones only have a projected lifetime of 18 to 36 months, they still must work reliably over that time. Fortunately, engineers know the causes of shock and vibration failures in electronics. They arise primarily from broken component lead wires, cracked solder joints, cracking of component bodies, plated hole cracking, broken circuit traces, board delamination, and electrical shorting.
Mathematical relationships have been developed to relate fatigue life of various components to the dynamic displacement of the PCB on which they sit. Some of these relationships are simple enough to have been developed into rules of thumb. For example, designers have long followed what’s called the octave rule which says the natural resonant frequency of the chassis should be at least one octave from the natural frequency of the PCB.
Nevertheless, mathematical rules of thumb can only go so far. Most design teams rely on physical testing to reveal such issues. Physical vibration and acceleration testing, known as Highly Accelerated Lifecycle Testing or HALT, provides a way to ensure the reliability of a product and identify potential failures caused by environmental factors. It takes place just before volume manufacturing and requires expert technicians. HALT testing involves applying a much higher fatigue than the actual product will undergo, thereby forcing failures and identifying weak spots as the product is rigorously tested. The process is, however, costly and destructive, potentially requiring up to four months while averaging $30,000 (USD) or more for each design.
For military equipment, HALT takes place before qualification testing. Military products (as well as transportation and machinery) are built for high-reliability, and they must perform in the harshest of environments. So their HALT work tends to be severe, taking place in a range of environmental conditions.
A point to note is that there is no one set of HALT specifications for specific kinds of products. HALT procedures are defined depending on the stresses the specific product will see, how many samples are available, what circuits are active during the expected stresses, and so forth. HALT plans usually incorporate incremental temperature stress levels, fast thermal transitions, incremental vibrational stresses, and combinations of these conditions.
However, there are complicating factors: Results can vary depending on the test chamber, possibly concealing problems that could lead to failures in the field. Additionally, only a few samples typically go through HALT because of the expense.
Improving validation with simulation
No wonder, then, there has been a move to develop simulators able to predict how PCBs and electronics will behave in vibration and acceleration. This software does not eliminate the need for HALT. But it can highlight potential problems on a layout, so design teams can reduce HALT expenses and let reliability specialists have more time to focus on tougher issues. Bridging the electrical and mechanical design disciplines, vibration and acceleration simulation can shorten design cycles and help boost product reliability.
During vibration and acceleration simulation, the software typically applies an acceleration load in all directions. For efficiency, the analysis takes place concurrently with component placement. The software streamlines simulation setup by automatically reading the physical stackup and board materials along with the component weight and boundary conditions. The user need only define the sweep frequency and constraints or boundary conditions to use during simulation. Finally, the simulation lets the user directly view the harmonic frequencies and applied stress on all components that could potentially cause a failure.
As a best case, two options for quick design simulation are available within the layout domain: First, a vibration mode calculates relative stress and deformation values to pinpoint weak links in contact areas of component leads and pins. The results can later be translated into a probability of potential component failure. Second, a constant acceleration mode provides a linear static analysis which applies a constant acceleration to the design. This analysis can find von-Mises stress, deformation, and safety factors, all of which can later be translated into pass/fail values for components.
Once failed parts have been identified, a post-processor interface that animates the model of the board can help determine the exact variables and factors causing the part failure. A simplified post-processor view can highlight problematic parts, so designers can note and correct potential component failures. This facility allows engineers or designers with no product reliability expertise to easily decipher fatigue and vibration issues.
An advanced view mode can let the product development team identify the direct cause of a failure. Besides letting the designer customize the visualization of the virtual prototype, the post-processing utility provides a way to configure the deformation intensity of the critical areas so designers can directly view how the board will deform. The deformation of the board can be animated so designers can see how the board will bend and move during vibration analysis.
A few examples show the kind of physical testing that simulation can eliminate or reduce. In one case, a company tests its products for transportation stress by shipping samples to multiple destinations around the world and then back. Products returned without problems are considered acceptable. If problems arise, the design is revised and the process of physical testing repeats.
To test for issues during use, another company secures its product to a warehouse floor and performs physical vibration analysis. This process is, of course, expensive, time-consuming, and potentially destructive.
A third company that designs boards containing large connectors would run physical tests without the connector(s) in place to make room for the required test jigs. The inaccurate test environment gave rise to false positive results.
Virtual vibration and acceleration simulation can provide more reliable results than the physical tests these examples illustrate. Modern simulators incorporate an algorithm for failure prediction as well as a rapid and accurate fully automated finite-element analysis.
In essence, the simulator runs a virtual HALT sequence during the PCB layout stage. The simulation helps visualize and identify problematic parts, detect components on the threshold of failure that would be missed during physical testing, and analyze pin-level von-Mises stress and deformation to determine failure probability and safety factors. Additionally, designers can simulate all designs as opposed to just those that are high-risk.
All in all, the ability to run virtual simulation on every board while in the layout domain lets engineers and designers detect issues early in the product development process before the board goes to the manufacturer.
Though vibration simulation has long been available, it is only recently that versions of these simulators have been specifically optimized for use on PCB designs. In that regard, Mentor, a Siemens business, offers Xpedition Design for Reliability (DFR), the first PCB design-specific vibration and acceleration simulation package. It is specifically optimized for the PCB layout designer. Its component modeling library includes over 4,000 3D solid models used to create highly defined parts for simulation. The 3D library allows users to easily match geometries to their 2D cell database. Designers can assemble the part models on the board and automatically mesh them for performance analysis including any required stiffeners and mechanical parts.
Patented post-processor technology lets designers quickly see high-failure-probability components and analyze boundary conditions, material properties, and environment profiles. To simulate stresses, Xpedition applies a force vector to each component. That information is used to determine the stress applied to that particular component and to understand if there is a potential failure. Currently, this is an early analysis looking for component failures. Designers can see the stress intensity and deformation of the entire board and any resulting failures due to placement.
When problems arise, the program lets designers easily move components around. And though this may be the age of artificial intelligence, it is still the human operator who must see the results and identify a resolution. Xpedition (DFR) provides layered results to make the process easier and animations for visualization.
Finally, it is worth noting that Xpedition (DFR) employs a mesh size a factor of four to eight times higher than that of most ordinary mechanical analysis software. This is in the interest of accurate modeling and typically works best on systems containing 32 GB of RAM.