Board Stress Analysis (BSA) has been a useful tool for designers and fixture manufacturers for more than a decade. This tool has allowed designers to minimize induced stresses on the printed circuit board (PCB) under test, and all but eliminate the potential for damage. Until recently, it was only practical to perform a two-dimensional (2D) analysis, which did not include the geometry of components and connectors(1).

New software tools and high-powered desktop computers now make three-dimensional analysis (3D) the best choice. All components on the populated board can be included in the analysis, along with probe forces, and push finger/support placement which yields a highly accurate outcome. With the continued trend of squeezing more and more components on PCBs, this improved software tool may need to become part of standard fixture development in order to prevent damage during test.

Nature of the problem
Standard vacuum and pneumatic fixtures can subject the board under test (BUT) to damaging stress levels if the fixture is not constructed properly. As an example, a test fixture with three thousand 8-ounce spring contacts will push against the board with as much as 1500 pounds of force; Push fingers or vacuum pressure can place additional forces on the board.

This may sound like a huge amount of force, but it will actually not harm the BUT in any way, as long as the forces are applied and counterbalanced properly. For that reason, board stress analysis is an invaluable tool.

The BSA allows a simulation to be performed before the BUT is subjected to potentially damaging forces. Further, once the simulation is run, push fingers, board supports and even probes can be repositioned to reduce flexing and lower stresses placed on the BUT. Typically, the BSA is rerun in an iterative process that successively betters the fixture design until the desired result is achieved.

The forces applied to the BUT by pneumatic and vacuum fixtures are quite different. For example, a vacuum fixture applies uniform pressure (the weight of the atmosphere) to one side of the BUT, which is counteracted by the force of the spring probes and board supports. The pneumatic fixture applies force to the BUT through push fingers that are strategically placed to counteract the force of the test probes. In either setup, an improper application can lead to damage.

Nature and Magnitude of the Stresses As the test fixture is actuated, the forces identified above begin to stress the board and the components mounted on the PCB. The board will flex upward or downward depending on which applied force has the greater magnitude.

The components on the PCB are also subject to these stresses. Leaded devices have significant compliance through their leads and are typically unaffected by the board flexing. But surface mounted components, since they are adhered to the surface of the PCB, are subjected to disproportionate amount of stress. They, in effect, become stiffeners, resisting the flexing, and the stresses are then further multiplied by their rigid mounting and device geometry.

Figure 1. Forces on the Components

Figure 1 depicts some of the reactive forces to which surface mounted devices are subjected. In this example, components mounted on the top surface would be compressed by the board flexing, while components on the reverse side would be stretched. The degree of flexing and the abruptness of the flexing need to be controlled to protect surface mounted devices.

Typical failures of these devices include broken or degraded solder joints, broken components, and false test results. Unfortunately, a marginal solder joint may pass electrical test when it is forced into contact with the pad (2).

Comparison of 2D and 3D Stress Analyses
The stiffening effect of the components and connectors dramatically affects the magnitude and location of the stresses and deflections on the board. To demonstrate, two-dimensional and three-dimensional analyses of the same board were performed.

Figure 2. Calculated Displacements from 2-D Analysis

Figures 2 and 3 depict the calculated deflections and stresses, respectively, resulting from the 2D analysis. Peak stress occurs at a board support adjacent to the region of maximum board deflection, which is near the center of the board. In this case, the maximum deflection was calculated to be .017" and the maximum stress was calculated to be 9370 psi. Thus, the 2D analysis indicates that additional board supports should be added to provide better support in the regions of high deflection.

In contrast, the 3D analysis shows that the maximum deflection of the board is .0065" located near the left edge of the board, and the peak stress is 5056 psi, which occurs at an adjacent board support. The displacement and stress fringe plots resulting from the 3D analysis are shown in Figure 4.

Figure 4. Calculated Displacements from 3-D Analysis

The 3D analysis shows that the components serve to stiffen the board, reducing the maximum stresses and redistributing these stresses to the locations near the component attachment joints. Figure 5 provides a top view of the board illustrating how the stresses have been redistributed. Because of the 3D analysis, the fixture designer's course of action for mitigating board stresses has changed. Whereas the 2D analysis would have indicated a need for substantial addition of board supports across a large portion of the fixture, the 3D analysis allows the designer to focus attention on the left and right-center portions of the board where high stresses threaten component attachment joints.

Figure 5. Stress Plot From 3-D Analysis

Generating the 3D Analysis
Evolving CAD software technologies have enabled 3D BSA to become more automated than its 2D predecessors. The 3D BSA uses the board and component geometry directly from the board designer's data, through the use of Electrical CAD (ECAD) to Mechanical CAD (MCAD) translation software. MCAD translation software currently supports thirteen different ECAD packages.

The 3D finite element analysis (FEA) software runs within the MCAD software providing complete associativity between the solid model and the analysis. This means that design changes in the solid model (such as adding or relocating board supports), are automatically reflected in the analysis model, allowing multiple design interactions to be analyzed quickly and accurately. Probe forces are applied in the FEA model via a custom generated program, which uses the application-programming interface of the MCAD software.

Another benefit of the latest generation of FEA software is improved accuracy coupled with decreased processing time. Typical early 2D models employed relatively coarse mesh and still took several hours to process. Today's FEA software supports mesh sizes orders of magnitude smaller than the 2D models while cutting processing time down to typically less than one hour.

Added Benefits
Once the finite element analysis has been completed, fixture fabrication files can be automatically generated through the use of computer aided manufacturing (CAM) software that also runs within the MCAD software. The software allows precise placement of board supports in the fixture top plate or pocket milling for component clearance in those fixtures designed for the board to sit flush on the top plate.

The process involves creating a model of the fixture top plate; features such as tooling holes and board support mounting holes are located using an external reference to the board geometry. An external reference in the MCAD software indicates that a feature in one part is dependent on a feature in another part for its solution. If the referenced feature changes, the dependent feature also changes. This ensures that the tooling holes and board support mounting locations in the top plate will always mirror the board.

For fixtures designed for the printed circuit board to sit flush on the top plate, the process is similar. The MCAD software offers a mold cavity tool that is used to create a negative image of the board in the top plate. Component offset clearances can be globally selected, or tailored to individual components. Once the top plate geometry has been established, the CAM software automatically extracts the machinable features from the top plate and generates a machine-code file for the milling/drilling machine.

The other beneficial outcome of having CAD data in such detail is that it is now possible to build finished fixtures without waiting for an actual PCB. The ability to build a simulated PCB from the CAD data is a long-awaited advance in this industry, and holds the promise of reduced time to market.

The same process for pocket milling the top plate is used, but in this case molds are created of both the top and bottom images of the board, as well as a blank representing the board itself. The molds are then used to make positive images of the board using hard gasket material. These images are adhered to the board blank resulting in a very accurate representation of the populated board.

Summary
3D board stress analysis is a vital fixture development tool that provides a realistic view of the relative forces that will be placed on the PCB. It is the best option available for eliminating potential damage to the PCB and its components during production test.

A broken PCB can cost thousands of dollars, and lost time. Based upon the desktop computer processor power now available, the migration of PCB designs to 3D CAD, and the need for improved quality, the means are there to make 3D BSA a practical part of the fixture design process. It would seem that this will be a natural evolution for test fixtures in the 21st century.

References
1. St.Onge, G., A method to control board flex, Evaluation Engineering, September 1989

2. Green,S., Hidden costs of board stress in test, Electronics Manufacture and Test, May 2000

About the author
Gary St.Onge is Vice President, Test Fixture Group of Everett Charles Technologies. He holds Master's and a Bachelor's degree in Mechanical Engineering from Union College. Mr. St.Onge is also a licensed professional engineer in New York State, and has received 8 U.S. patents related to test fixturing.
Everett Charles Technologies, Test Fixture Division, One Fairchild Square, Clifton Park, NY 12065. Phone: 518-877-7042,
e-mail: [email protected]

Tom Newhall is a Product Design Engineer for Everett Charles Technologies. He holds a bachelor's degree in Mechanical Engineering from Clarkson University and has 15 years of engineering experience with ECT and General Electric.
Everett Charles Technologies, Test Fixture Division, 1 Fairchild Square, Clifton Park, NY 12065. (518) 877-7042,
e-mail: [email protected]