Overcoming Bead Probe Contact Challenges
Part II: Enhancing First Pass Yield With Better Spring Probe Tip Geometries


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By Michael Farrell Sr., Agilent Technologies and Brian Crisp and Ken Snyder, Everett Charles Technologies

Part I of this two-part article discussed the use of bead probe technology and factors that impact probing, such as spring force and biasing. This section covers contact integrity, from the importance of probe head design to probe materials.

During bead probe development, experiments and implementations simply used flat-faced tips and the associated drag of the bead target probe face on the bead probe to ensure contact integrity. This process is highly repeatable, and has been the primary recommendation for a bead target probe tip style. Some PCB manufacturers experience contact issues related to flux residues, contamination, or other factors. Testers attempting to use flat-faced probes on a dirty board have reported it may take as many as 12 fixture cycles to break through and acheive good contact. As such, a standard flat-faced bead target probe may not be the best choice for bead targets on boards entering high-volume manufacturing (HVM).

Several alternative tip face solutions have been tried or developed to improve first pass yield.Standard serrated tips. These tips may seem like a natural choice to improve bead probe contact integrity. However, there are as few as 9 serrations (pyramids) per tip face on a typical 0.100" (Figure 7A) test probe tip; all but the largest of bead probes will center into the valleys between these relatively large serrations.

Micro serrated tips. Micro serrated tip styles (Figure 7B) are an improvement over standard serrated tips in that they feature a significant increase in the number of physical points. This correlates to a reduction in the size of each serration and therefore improved contact and capture of a bead probe. However, one must consider the bead vs. tip alignment issues with a micro-serrated 50-mil probe with 21 full points (+4 partial points). Figure 8, a comparison of the engagement with a large bead (0.006 × 0.025") and a small bead (0.003 × 0.015"), demonstrates this problem.

Figure 8. Hemi-ellipsoid bead vs. micro-serrated 0.050" probe tip.

Micro-textured test probe tips. A micro-textured tip design (Figures 7C) was developed, incorporating closely spaced triangular pyramid tips. These tips are spaced closest to each other along the longitudinal cuts on the probe tip. This is an advantage when contacting beads that are long yet have a small width, such as those placed on a PCB trace. For beads as small as 0.004" in diameter, this tip will make contact by straddling the bead with the closely spaced tips in the worst-case tip orientation (Figure 9). Figure 9. Micro-textured probe tip design to contact 0.004" and larger bead probes.

For beads smaller than 0.004" wide, a flat tip is highly recommended to prevent the beads from slipping between the triangular serrations. The micro-textured design of this new probe also provides maximizes the total area of the tip valleys for contamination to accumulate during use, allowing increased mean time between maintenance cycles. Consider the various witness marks left on bead probes by various tip styles shown in Figures 10 through 12.

Figure 10 shows witness marks left on bead probes by a flat probe tip.

Figure 11 shows the witness marks left on bead probes by a standard serrated tip.

Figure 12 shows witness marks made by a micro-textured probe tip on a bead probe.

Test Probe Selection and Board DesignWith bead probe, costly signal path re-routing during layout to accommodate traditional test pads is unnecessary. During PCB design, confirm that the CAD denotes openings in the solder mask for the bead probe locations. Bead probe test access points typically are designed as pad geometry (normally looks like a test pad) or assigned as a surface mount component with a reference designator. If assigning the bead as a component, exclude the bead probe "components" from the bill of materials (BOM) and silkscreen. Keep-out areas around components and component outlines need to be validated and respected to keep bead target probes from interfering with other components or PCB features. Commercially available tools automate this process with no modifications to copper and with the keep-outs and restrictions automatically maintained.

Control costs and enhance ease of use by following these guidelines. Maximize pitch — keep at least 0.075" spacing to maintain the use of 0.100" bead target probes for ease of design, minimal cost, easier PCB and fixture layout, and reduced maintenance issues. Only use 0.075" or 0.050" bead target probes when necessary to reduce cost and prevent contact issues. Standardize — some major bead target probe manufacturers are standardizing on non-oversize tips at a 0.035" diameter, regardless of pitch, to aid PCB and fixture designers in maintaining simple keep-out rules. Additionally, a common spring force can be implemented for bead probes to ease maintenance. Partner closely with vendors — PCB design and fixture vendors need to be partners in bead probe development, which has the potential to change the fixture cost.

Also strive to use the most advanced test probe base materials and plating options available from major probe manufacturers. Harder, slicker plating material are more capable of withstanding residues and lead-free PCB materials than the traditional hard gold alloy plating types. Standard gold alloy plating hardness varies between 90 to 200 Knoop, whereas newer custom materials can be up to 600 Knoop. This harder plating surface is less porous than standard plating, meaning less solder and contamination transfer will occur, improving mean time between maintenance cycles.

ConclusionEducation and experience are critical for success. Multiple factors influence the effectiveness of bead probe ICT, including PCB cleanliness, solder materials, and probe tips. To successfully implement this method of enhanced coverage at in-circuit test without significant increases in the cost of test, develop standards for effective bead probe contact and tooling requirements.

Michael Farrell Sr., Agilent Technologies, may be contacted at www.agilent.com. Brian L. Crisp, regional sales manager, Southwest/Southcentral/Mexico, Everett Charles Technologies, Contact Products Group, may be contacted at (623) 293-1483; brian.crisp@ectinfo.com. Ken Snyder, senior design engineer, Everett Charles Technologies, Contact Products Group, may be contacted at (909) 445-0520; ken.snyder@ectinfo.com.

Return to Part I here.

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