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Test access on modern PCB assemblies is a major challenge. Bead probe technology resolves many of the issues test engineers face. This two-part article provides best practice recommendations for bead probe implementation, PCB design for test (DfT), and product selection.
Test access continues to challenge electronic device and assembly builders. This is especially acute for in-circuit test (ICT), which is fundamentally dependent on electrical access. In the 1990s, ICT came under threat. Increased board performance and limited access due to component and routing density meant that the coverage of in-circuit test decreased while the cost increased. The resulting change in test strategy towards automated inspection, including solder paste, optical and X-ray inspection, lead to new test techniques using built-in approaches such as boundary scan. Bead probes are another method to overcome limited test access on modern assemblies.
What Is Bead Probe?Bead probes are soldered bumps (Figure 1) that can be placed directly onto copper signal traces. The bumps are added using existing solder paste printing principles and reflowed using standard procedures. They add a probe test point for ICT access.
Historically, in-circuit test relies on a bed-of-nails fixture that uses a matrix of probes to connect with internal points on the PCB. The conventional approach is to contact leads, plated thru-holes, certain vias, or to simply include test points or pads within the PCB layout. The provision of test pads within the PCB layout can impose an overhead in PCB real estate. The area available for test pads is much less than the total area of the board and therefore finding the real estate to add multiple test pads can become a real challenge. It also taxes signal Integrity. Adding a test pad on a high-speed trace can impact circuit performance to the point of harm.
Bead probes invert the probing paradigm, and overcome the test access and signal integrity problems associated with traditional test access targets. Rather than use a large test pad on the PCB and a sharp-edged probe, the approach is to use a small test "probe" on the PCB and a flat head target in the fixture (Figure 2).
Figure 2. Bead probe fixture.
The bead probe fills the role of this small test probe and is the same width as the trace but with a given length (typically 3 to 5× trace width) and height (typically 4 mils), as shown in Figure 3.
Figure 3. Bead probe geometry.
This approach consumes no extra surface area on the board, resolving real estate issues. It does not degrade the signal path. It also increases the potential test coverage at in-circuit test.
Evolution of Bead Probe Contacting Technology Bead probe contact integrity is reliant on test probe spring force to partially crush a bead probe and have the probe tip face move laterally across the bead during fixture actuation, removing residual flux and surface oxidation to expose virgin solder. Virgin solder provides a reliable electrical contact.
Numerous experiments proved this connection concept across a large number of test probes. However, it has become apparent over time that other variables exist that may detrimentally affect contact reliability and drive test engineers and production personnel to seek higher yield rates, especially in high-volume production. This paper is narrowed to test-probe-related issues, although there are many PCB-related variables. In this system:
- • A fixture final assembly may require as many as 40 or 50 cycles to verify fit and function;• Programming and debug may require 100, 200, or even more cycles;• In production, the goal is to only have to activate the test fixture once to obtain probe contact on all test access points. Most manufacturers may allow up to 5 or even 10 activations per board to work around manufacturing issues that affect contact;• Re-test may include additional tests to recapture false failures.
The effects of multiple cycles have been captured in various studies, including one conducted by Everett Charles Technologies. A progression of bead compression during cycling is shown in Figure 4. Many factors influence a bead probe's yield strength. These include bead size, solder type (elasticity modulus), spring force, probe bias, contamination, and others.
Figure 4. Bead probe strike progression with a flat tip spring probe.
Spring ForceBased on the original concept of, and experiments with, bead probes, it was considered that spring force would be chosen based on bead size and the elasticity moduli of the bead probe solder. As the technology matures, most users are defaulting to spring forces of 46oz probes, regardless of bead size, with good success.
Spring Probe Bias EffectsMost contemporary test probe manufacturers incorporate biasing to maintain optimum contact between the probe plunger and probe barrel. There are various techniques: ball bias, plunger bias, spring bias, and more. Biasing inherently increases the level of friction in the test probe assembly, which will increase the rate of bead probe compression (Figure 5). Although lubrication can mitigate the effects of friction, it is not uncommon to observe variability in the linearity of the probe spring force rate during test probe actuation.
Figure 5. Variability of bead probe compression due to test probe bias.
Fixture-guided ProbesBead probes have been incorporated into various fixture types. Guided probe fixturing design use a reduced top plate hole size to effectively guide a headless test probe to a PCB test target. If guided probe technology is used with bead probe test sites, use headless bead target probes to prevent probe or fixture operation problems.
Top-side Probing Accuracy Top-side probing accuracy may not be sufficient with standard target probes, due to fixture tolerance stack up. While bottom-side target sizes are typically recommended to be 0.030" with a cut-off of 0.020" top-side probe target sizes must normally be increased to overcome the fixture tolerance stack up. A common topside target recommendation for standard probing is for 40-mil pads with a cut-off of 0.032" pads. Targets smaller than less than the recommended sizes are commonly accessed with guided probe fixtures.
Bead probe technology addresses the test access concerns attributed to fixture tolerance stack up. Bead target probes, which are significantly larger than bead probes, effectively mitigate fixture tolerance issues. Most contemporary probe manufacturers offer oversize test probe tips, if PCB or test fixture tolerance issues are encountered at bead probe locations.
Figure 6A. Poor wetting and missing solder on bead probe sites and B. Poor wetting and missing solder on unloaded R-pack sites.
ContaminationFor years, process engineers have studied the impact of solder paste types on printability and flux residue, working with their bare board suppliers to select optimal pastes and keep their solder mask tolerances in check. These are now considerations for test engineers to investigate when implementing bead probing as well. Ideally, bead probe print deposition undergo solder paste inspection or AOI to ensure print robustness and repeatability. The examples in Figures 6a and 6b are from the same board, showing obvious solder problems. In addition to bead probe formation issues, flux residue on the bead probes may impact contact with the bead target probes. Minimizing residue for bead probe implementation is necessary and attainable with test-friendly solder pastes.