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ICT Discussion Probes Future Technology
December 31, 1969 |Estimated reading time: 4 minutes
By Meredith Courtemanche, assistant editor
BOSTON The SMTA Boston chapter presentation on in-circuit testing (ICT), from John Bonsante of Everett Charles Technologies, May 8, covered lead-free choices and testing variations, PCB trends, and spring probe requirements through house- and beta-test results and analysis with ECT's Zeroflex fixture support plates, and other technologies. ECT's findings on higher preloads, working forces, and the range of spring choices revolved around lower test costs, longer spring life, and pointing accuracy.
The electronics assembly industry is always changing, and test suppliers have to modify contact products to accommodate changes such as lead-free, no-clean, high density, and others. That was the message conveyed by John Bonsante, regional sales manager; and Joseph Burns, operations manager, Everett Charles Technologies (ECT). Certain changes denser, more complex boards can be addressed with a design for test (DfT) strategy and precise fixturing. This is the mathematic, rather than "gut" approach, said Burns. Other issues are physical thinner boards and lead-free solder joints increase the chances of damaging an assembly during test and require new materials and technologies to answer industry needs. ECT has developed a proprietary plating material for its pogo pin contacts and has increased preload force to compensate for lead-free properties.
Lead-free solder, no-clean residues, and surface finishes such as OSP are all popular for assembly, and problematic for ICT. "It is the probe manufacturers' duty to provide better tools," said Burns, offering the solution of harder plating, higher preload, and life-cycle predictions that are more in-synch with real-world electronics manufacturing. ECT introduced a proprietary plating on its lead-free pogo contacts (LFPC) to improve upon the industry-standard, gold-based plating material. This product boasts 550 Knoop hardness, which is important for breaking through thicker oxidation layers caused by lead-free and layers of residue left in a no-clean process. The chemistry also allows for a "slicker" contact, a term Burns used to describe how contaminants, residues, and transferred solder fall off the contact, reducing cleaning time and replacement. This finish was actually developed for testing semiconductor packages, such as LGAs and BGAs. Using his palm as a test sample, Burns demonstrated why pogo pins need higher initial force, or preload, to make sufficient contact with lead-free pads. Pogo pins travel the same distance into lead-free and tin/lead joints, but the brittle, hard nature of lead-free makes it more difficult to penetrate, hence higher preload force.
Overall miniaturization has diminished test pads and reduced the area available to place fingers and stops during test, Bonsante explained. To reduce the possibility of damaging a board during test, and to ensure that designers do not produce a board that can't be tested, ECT uses a two-prong DfT approach. By merging computer-aided design (CAD) outlines and bill of materials (BOM) data for a project, ECT generates an accurate 3-D product, allowing them to place constraints without encroaching on components, and to validate the design from a testability standpoint. The company then uses Zeroflex, a product developed to keep the board flush with the top plate, and Guided Probe technology to mill-out test areas on the fixture. This approach "prevents flexure and gives the user more precise targets for ICT," according to Bonsante.
Beyond mechanical changes mandated by lead-free assembly and miniaturization, Bonsante and Burns discussed the technological advancements that allow modeling and analysis for test to accommodate lead-free, dense boards, etc. In a DfT strategy, statistical modeling helps predict the probability of hitting a test pad on an actual assembly. With finite element analysis, ECT can use the CAD/BOM data for milling the test fixture and apply algorithms to test for excessive flexure in the unit under test (UUT). This approach notifies them of any necessary modifications to the fixture to make boards testable without damage. Because the data does not take into account the mechanical properties of the components on a finished assembly (except in rare cases) it is a conservative estimate; components add stiffness, reducing flexure. A trend Bonsante recently has observed is increased demand for strain gauge analysis destructive testing done on a sample board assembled at an EMS provider and fitted with "stain gauge rosettes" to measure "microstrains" in the board during ICT. This approach often is used on assemblies for aerospace applications, to determine acceptable shock and vibration. During test, acceptable flexure is lower on a lead-free board than on a comparable tin/lead assembly, due to the brittle nature of lead-free solder. The strain gauge analysis is performed with the PCB fully loaded with components, to give an accurate, real-world model of test capacities.
Bonsante and Burns see this two-pronged approach to test using technology to better predict test constraints and chemistry to optimize plating materials as a way to promote and support lead-free assembly and denser, thinner assemblies. SMTA Boston chapter will next host "Sucessful Lead-free/RoHS Strategies Conference," June 2021, in Boxborough, Mass. For details, see the SMTA Website, www.smta.org.