-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Webcast Probes Accessibility Possibilities
January 26, 2007 |Estimated reading time: 3 minutes
By Meredith Courtemanche, assistant editor PALO ALTO, Calif. Senior scientist and co-inventor of the Medalist bead probe technology, Kenneth Parker of Agilent Technologies presented a Webcast on in-circuit testing (ICT) technology. Parker explained the limitations of traditional test pads, and examined many facets of bead probes shape, resiliency, implementation possibilities, and the reported benefits to PCB designers dealing with close traces and densely populated formats.
Parker focused on the limited access to test points created by dense designs and fine signals on many PCBs. He also noted that high-frequency circuits are open to interference from traditional test pads. As routing adjacent to round, flat test pads becomes increasingly difficult, designers face loss in either performance or testability, thus loss in reliability of a given design.
The bead probe's primary appeal in such a design environment is its width no wider than the signal trace it lies on. This eliminates the design step of routing traces around test pads, and also allows designers to place several beads on one trace without compromising performance, so long as they observe a minimum bead-to-bead distance. The beads run along the trace and stand a few mils in height with a flat top.
Because the effect on signal integrity is negligible, and routing is unaffected, bead probes can be designed into various locations, top- and bottom-side, improving test accessibility and giving test operators more flexibility. Parker said that this layout-independent test-point theory can increase synergy between test and design departments, which often are spread across a global footprint. To test, the spring-loaded probes, which traditionally incorporate a point to pierce a test pad, must be adjusted to a matching flat-headed pin.
Parker manifested this concept with a small potato and a water glass, showing how, when pressure is applied from the flat bottom of the glass to the relatively flat top of the potato, the outer skin breaks, causing uncontaminated potato to come in contact with the glass. This concept applies to flux, dirt, oxides, and other contaminants that form a "skin" around the bead, especially with no-clean manufacturing. Contact between the probe and bead breaks this surface, generating good contact and a reliable test. Parker explained that this natural wiping action can enhance reliability with subsequent tests, as a new surface is exposed each time.
Parker also chronicled the process of forming bead probes, and explained some development issues that had to be resolved. Bead probes are categorized as solder-mask-defined and metal-defined, depending on whether they are placed on planar copper or on a trace. Both are made by opening a hole in the solder mask at a desired location, and then opening a square or rectangular aperture on the solder stencil. Width and length must be engineered carefully to avoid structural and shape problems post-reflow, explained Parker. The next steps are concurrent with standard SMT processing solder paste (in this case lead-free) is squeegeed over the stencil and the board is reflowed. To create the beads, which reflow into a hemielipsoid shape about 2 4 mils tall, operators must overprint apertures, creating enough volume for the finished beads to stand further off the board than components. Stencil-wiping direction can create slight variations in results, as well as registration inconsistencies. Parker and his team used test vehicle boards to quantify the effect of various manufacturing factors probe force, registration, aging, etc. He also noted that communication is key when implementing bead probe technology; if operators are not aware of changes, the rectangular apertures may be dismissed as errors in the design or as off-mark registration holes. Factory-to-factory differences can also affect probes.
In total, Agilent has 16 patents or patents-pending on the bead probe process and technology. Parker suggested that the investment in communications and ICT changes will benefit OEMs and original design manufacturers (ODMs) with dense, high-frequency boards. Designers interested in maintaining board layout without interference from test pads may also benefit.