Tools manage PCB design and test

Judicious deployment of design, manufacturing, and test software can cut costs and speed time to market.

Rick Nelson, Senior Technical Editor -- Test & Measurement World, 10/1/2002

Various techniques—ranging from x-ray and optical inspection through functional electrical test—have emerged to shorten time to market and reduce test costs while maintaining thorough fault coverage. Each technique brings its own strengths:

• Optical inspection can verify component part numbers and orientation.
• X-ray inspection can determine the adequacy of solder joints.
• In-circuit test can exercise components individually.
• Boundary-scan test can evaluate components as well as interconnects.
• Functional test can test for faults that may have escaped other stages while providing confidence that a board functions properly.

Throwing all these approaches indiscriminately at a PCB would be a costly mistake, however. Expensive pieces of capital equipment would duplicate each other's efforts, with x-ray systems and boundary-scan-capable testers both looking for the same solder-ball problems. You'd not only be misusing your equipment, but you'd also be needlessly extending test times.

There is a rule of thumb that can guide you toward the most effective use of inspection and electrical-test systems: Deploy your equipment to catch each class of fault as early as possible in the manufacturing process. For example, don't wait until the in-circuit test stage to determine whether a diode is inserted in the proper polarity; an optical-inspection system can check that for you before a PCB undergoes solder reflow. You'll save the cost of soldering a bad board and of subjecting it to subsequent optical and x-ray inspection stages; in addition, rework won't require a skilled technician to remove the incorrectly soldered part.

Figure 1 The Agilent Access Consultant software package helps to effectively deploy x-ray inspection (via the AwareTest xi link) and boundary scan (via the Silicon Nails link).

Unfortunately, on complex boards having thousands of nodes and components, you may not be able to see the trees for the forest. The sensible way of testing a diode's orientation can be obscured by the plethora of other components, each competing for testability analysis with its own common-sense solution. For assistance, you can turn to testability-analysis software packages that can help you deploy common sense on the large scale demanded by today's complex products. One study has found that such tools can save $1 million and cut three weeks from time to market (Ref 1).

Of course, these tools need a description of your PCB-under-test's design that they can understand. That usually comes in the form of an input file from PCB design-automation programs ranging from Allegro to Zuken-CFF. Test companies have accommodated these various formats, but at the cost of translation efforts that you as a PCB-tester customer ultimately shoulder. Efforts are underway to provide standard formats for file conversions.

One such standard manifests itself in the XML schema employed in the 2.0 version of the industry-standard GenCAM (www.gencam.org) format, released in February by the IPC—Association Connecting Electronics Industries (webstds.ipc.org). The goal, according to a statement released by the organization, is "a single file, able to completely describe a printed board, a printed board assembly, an assembly array, multiple assemblies on a sub-panel, a board-fabrication panel, quality-assessment coupons, and assembly/test fixtures." The GenCAM standard (pioneered by GenRad, now a part of Teradyne) will help permit bidirectional data transfer among "the design shop, the bare-board fabricator, the assembly house, and the test-fixture house in order to maintain information synchronization and ensure the quality of the final product" (Ref. 2).

Despite the benefit of standards, standards-based products often lag their proprietary counterparts in performance and functionality. Tools based on standards such as GenCAM have yet to achieve the capabilities of proprietary test-assessment tools including Agilent Technologies' AwareTest and Teradyne's D2B (Design to Build) software families, both of which aim at distributing test functions across various pieces of test and inspection equipment.

Agilent's AwareTest, in its current AwareTest xi implementation, targets distribution of test chores between an Agilent 5DX x-ray inspecton system and Agilent 3070 in-circuit test system (Ref. 3). Under AwareTest xi's direction, the 5DX looks for solder defects on your board; it then transfers 5DX program information to a 3070, which performs the in-circuit test necessary to provide full yet non-redundant test coverage across the board. Two other Agilent programs complement AwareTest xi: OmniNET software analyzes CAD data before board layout to make recommendations for removing test probes, and Agilent 3070 Access Consultant performs a similar function after layout. OmniNET forms the basis for a dialog between design and test engineers, helping them collaborate on developing functional, testable PCBs.

AwareTest xi presupposes that you've settled on combining x-ray inspection with in-circuit test. How do you know whether that's the best solution for your PCB test chores? Agilent proposes a simple calculation:

Complexity index

= [(C + J) / 100] * D *M * S

where

C = the number of components,

J = the number of solder joints,

S = 1 for a double-sided PCB and 0.5 for a single-sided PCB,

M = 1 for a high mix and 0.5 for a low mix of components, and

D = 0.01 times the number of joints per square inch.

For a high complexity index (above 125), Agilent suggests you combine x-ray inspection with in-circuit test; for a medium index (between 50 and 125), combine x-ray or optical inspection with in-circuit test; and for a low index (less than 50), use optical inspection or in-circuit test alone.

In June, Teradyne's Assembly Test Division extended its D2B software to support its Optima automated optical inspection (AOI) platforms, complementing the D2B suite's support for automated x-ray inspection and in-circuit test.

D2B software comprises Windows-based tools that employ an open architecture that lets you integrate test and inspection platforms with minimal data-manipulation overhead. D2B software modules include these components:

• GR Force / A3 lets you acquire data from more than 30 CAD design formats and generate programs for a variety of test and inspection equipment. It supports design-for-test analysis early in the design cycle, before the cost of change becomes prohibitive.
• GR Force / Strategist analyzes the design of each PCB, models the production line configuration, and determines the optimum strategy for the programming of the test and inspection equipment.
• GR Force DesignView allows you to import board schematic information in labeled HPGL format. Its graphical viewer can then simultaneously update both schematic and physical information whenever you select a component or signal, enabling the simple browsing of complex design data.
• D2B Framework allows you to modify design data and manage parts libraries. Its data editor allows the common management of all part numbers, making it easy to implement engineering changes and manage new product introductions.
• D2B GenCAM Viewer allows you to view GenCAM v1.5 design data files in a graphical format. It also reads GenCAD and GR Alchemist III files.

Testability-analysis tools can help you implement cost-effective and fast test strategies that incorporate x-ray and optical inspection plus in-circuit test segments of an overall PCB test strategy. Agilent plans to introduce by year's end a software tool that can contribute to the development of effective functional test strategies. For now, it remains your job, in concert with design engineering, to ensure adequate deployment of boundary-scan test capabilities and to determine what level of functional test you'll need to ensure PCB quality. As test-awareness software tools mature, they'll provide further integration of all PCB test options.