The primary current path in a probe is through the contact junction of the plunger with the barrel and the barrel with the receptacle. Secondary paths include the contact junction between the spring and plunger and the spring and barrel.
Electrical Probe Resistance
Resistance is dependent on several factors: conductivity ob base metals and plating material, resistance at points of contact between components (which is affected by surface condition), area of contact, force applied at contact junctions, and probe design. For applications requiring very low, very consistent resistance, such as loaded-board test. ECT′s PogoPlus probes employ an enhanced bias ball design that maintains electrical contact between the plunger and the sidewall at all times. ECT probes are self-biasing, resulting in maximum metal-to-metal contact force between components at critical contact junctions. Resistance can also be caused by such factors as: receptacle wire terminations, fixture wiring, test interface, PCB surface contamination, incorrect probe selection like wrong tip and inadequate spring force or high-resistance contacts in the test system. Electrical resistance is included among probe specifications on each data page.
Most probes are rated with a working travel and a full travel position. Full travel is the maximum travel the probe is able to make, before either the plunger recessed into the barrel or the spring bottoms out at full deflection. We call it the spring becomes solid. Therefore we specify a working travel position which is typical 2/3 of the full travel position. This will prevents the probe from bottoming out and extend the life of the spring.
Force values are provided throughout this catalog in both “oz. and (grams)”. Conversion from ounce-force to gram-force: 1.00 oz = 28.35 grams. Conversion from ounce-force to newtons: 1.00 oz = 0.28 newtons.
Plungers are generally manufactured from BeCu (Beryllium Copper), Heat-treated and plated with gold or other plating materials. Some tip styles requiring extended tip life, are made from stainless steel, heat traded and plated.
The spring provides the required compliant force at the plunger tip, and the contact force between the barrel and the plunger. Several spring materials are offered, depending on probe size, spring force and application requirement. Spring material may also be plated with precious metals to improve electrical performance and prevent corrosion.
Higher spring forces will provide you with a more effective penetration through contamination contact points, but also leaving heavier witness marks on the test point. Lower spring forces might be used where no witness marks are welcome or to prevent board flexing on higher pin count applications.
Typical spring force tolerance is ± 20%. A tighter tolerance range can be achieved if required.
BeCu is the weakest of the spring materials, however due to its electrical performance is used on low-resistance applications.
Music Wire is a high carbon steel wire chosen for its consistency and strength.
Stainless Steel is very strong and typically used on high temperature applications or corrosive environment.