POGO® Spring Probe Considerations for PCB In-Circuit and Functional Test

How do I select the optimum POGO® spring probe for my PCB test application?
PCB test fixture designers have many factors to consider when selecting the proper spring probe for their application. In the world of spring probes, choices and trade-offs abound when it comes to selecting tip geometry, spring force, probe base material and plating.  Choosing the right combination of options greatly impacts test yields, probe life and cleaning frequency.

Tip Selection
Most tip styles can be used for a variety of different applications. However, some test targets are better suited for unique tip styles. Some applications require a non-aggressive tip to be used on pads, such as radius, crown, or flat.  These tips leave little to no witness marks on test pads. More aggressive tip geometries, such as sharp angle blade or pyramid tips, should be considered when coatings or contaminates such as organic solderability preservative (OSP) or solder flux are present on the contact surface.  For through-hole vias one should consider large angle (90+ degrees) blade, star, or pyramid tip styles. These are well suited for contact through the outer ring of the via surface. Over time solder builds up oxide layers, therefore medium to very aggressive tips like the serrated, crown, blade, pyramid and point are used. Posts, pins or screws are more unpredictable and therefore more challenging. For these applications, tip styles like the cup, serrated and crown are best suited. Other applications may require more unique tip styles depending on test target material, size, shape, access and cleanliness.   Learn more on shop.ect-cpg.com/information/tip-selection/

Spring Force
Spring force provides the required compliant force at the plunger tip and the contact force between the barrel and the plunger. Spring force selection must be carefully considered when choosing a spring probe.  Higher spring forces provide more effective penetration through contamination likely to be present on contact points, but will also leave a heavier witness mark on the test point.   The cumulative effect from using high spring force pins may cause flexing of the board under test.  This is especially true for in-circuit test applications where there could be thousands of probes used in the fixture.  The result of this usually means poor contact on areas most impacted by flexing and perhaps damage to the board.    Lower spring forces should be used where no witness marks are welcome or to prevent board flexing on higher pin count applications.

Base Materials
Typical plunger base materials include beryllium copper (BeCu), steel, brass, and tellurium copper. Beryllium copper is a popular choice for plungers because it has both high conductivity and strength compared to other high conductivity alloys. Steel is used when high plunger tip strength is required such as sharp plunger tips and high spring force probes.  Brass is similar to BeCu for conductivity, but is a softer material and is mostly used in applications that do not require high strength. Tellurium copper is a much higher conductivity alloy that is used in high current applications.

Typical probe, barrel, and receptacle materials include beryllium copper, phosphor bronze, nickel silver and brass. Barrels and receptacles are most commonly formed by a deep draw process.  Beryllium copper, as mentioned before, is both high strength and easily formable. It’s typically used for longer and skinnier forms.  Phosphor Bronze when formed has roughly the same qualities as BeCu, however it is more economical. Nickel silver is the most easily formable of the materials, but the trade-off is its lack of strength.  Brass is typically used when the barrel is machined instead of deep drawn.

Typical spring materials include music wire, stainless steel, and BeCu. Music wire is the strongest of the 3, but can only withstand an operating temperature of 105C or less. Stainless steel springs are used in higher temperature applications. BeCu springs are used for high conductivity and non-magnetic applications.

Plating Choices
Choosing the right plating depends on the application and customer preference.  Gold is most frequently used because it provides both excellent electrical performance and good corrosion resistance.  Some suppliers offer other plating options to address the challenges of lead free solder. These platings aim to slow the migration of solder from the test point to the tip of the probe and provide longer cleaning cycles.  If test points are known to be sensitive to contact resistance, an alternative to gold plating should be considered.  Alternative platings also tend to be harder than gold and the tips are much more durable and wear resistive. Learn more on shop.ect-cpg.com/information/base-materials-and-plating/

Tools such as the probe selection filter on shop.ect-cpg.com/shop/  help to select the most appropriate probe solution.