Test results show these versatile components are durable and stable for such harsh environments as medical and military technology.
BY BRUCE VALENTINE
When selecting a connector for a given application, design engineers are faced with the challenge of balancing many needs, both technically and commercially. While many connector systems can be purchased off-the-shelf or through traditional sales channels, many standard products trade performance for wider market acceptance.
Spring contact technology delivers a diverse range of building blocks, including:
- Simplifying product design through elimination of mating counterparts;
- Enabling higher-density connections where needed (0.4-mm between centers is possible from stocked components);
- Shortening board-to-board heights to less than 3 mm;
- Extending product lifetime with greater than 50,000 reliable mating cycles;
- Exceptional blind mating characteristics, allowing for simple product assembly or relaxation of critical dimensions.
In addition, correctly specified spring contacts also offer a stable, highly consistent interface, even under the harshest conditions. These characteristics are why many top defense and medical technology companies choose spring contacts and connectors for interconnections.
How the signal ‘sees’
Differentiating what appears to be a simple electromechanical component is critical in understanding how to select the correct contact for a given application. It is useful to understand how the internal design of these contacts aids their overall performance and durability.
When defining “stable” and “consistent” in relation to a spring contact, it’s important to consider how the signal “sees” the connection medium and how this varies between mating cycles or during changes in the external environment, such as temperature or shock. This “path of least resistance” is useful in understanding how the signal will behave. The signal will transfer along the contact at different but distinct points, which for a single-sided contact can be classified and defined as R1, R2, and R3. Total resistance can be defined as R1+R2+R3, and so forth.
•R1 is influenced by several factors, including the material and condition of the target and plunger, the surface area and shape of the plunger tip, and the amount of force applied. In general, the highest bearable force should be selected to minimize contact resistance; however, spring contact force can be modified easily by changing the internal spring design and/or material, or simply changing the compressed mechanical height at which the contact finishes. This lets you balance the total force of interconnect against board thickness or reinforcement to minimize flexing. For arrays of connectors, this can be a critical factor, as in eliminating additional weight for airborne or portable applications, which is normally required with incumbent connector technology.
One important benefit of spring contacts is that force is generated linearly and requires significantly less printed circuit board (PCB) real estate than conventional bent-metal or plug-and-socket systems, providing more room for functional components in the finished article.
Customized tips can also be accommodated to pierce or wipe away contaminants or surface oxides, although for the majority of applications, a spherical tip provides significant surface area to satisfy the most demanding of requirement.
•R2 is the point of signal transfer between plunger and barrel. To ensure the signal avoids the long, thin, variable path that is the spring, mechanical systems referred to as bias are used. These systems reliably hold the plunger in contact against the internal barrel wall, providing a constant, sliding, low-resistance connection. The bias system ensures the resistance remains stable and consistent, irrespective of height, shock, or vibration. Several systems of bias exist.
In addition to bias, the manufacturing technique and plating of the barrel itself influences R2 over the lifetime of the contact. Deep drawn parts offer the ability to pre-plate the internal working faces with gold, while also work-polishing and hardening the wall to optimize product lifetime. Conversely, machined parts allow for flange-like mounting features, but are more challenging to internally plate.
 FIGURE 2. These reliable connector systems all use spring contacts. |
The bias-ball system is the most aggressive, providing a highly consistent connection between the plunger and barrel by compressing a ball against an angled plunger. With this system, contact lifetime is approximately 50,000 cycles, after which internal wear creates a less-stable connection.
Bias-plunger and bias-spring systems are less aggressive, and subsequently deliver a less consistent, yet still acceptable, connection between plunger and barrel. Both systems use the spring or plunger instability against its counterpart to create the bias effect. The lifetime of such a system typically is 100,000 cycles maximum.
 FIGURE 3. The eccentric bias system, which uses a contact with a backdrilled plunger, exhibits this typical resistance stability over multiple mating cycles. |
Patented technology called eccentric or E-bias uses a contact with a backdrilled plunger, maximizing possible stroke length and increasing the contact area between barrel and plunger to minimize signal loss. The backdrilled hole can be offset or angular to the spring’s natural center line and exploits the spring’s natural tendency to straighten, generating a sound side loading between plunger and barrel at all times. The benefit of such a system is stable low loss, coupled with a minimum 100,000-cycle lifetime.
•R3 resistance is the point of transfer between the barrel and PCB. This figure is often negligible for many designs, but is influenced by board-termination technique. Standard surface-mount technology or through-hole techniques provide adequate performance.
Shock treatment
Performance under shock and vibration is often critical in performance applications. To better understand how spring contacts operate during such conditions, we have carried out extensive product testing on key bias types. The objective of the test process was to examine how the signal-path loss varied after exposure to different levels of vibration, and if discontinuities could be forced through long-term fretting wear or by unbiasing the spring contact with excessive shock.
 FIGURE 4. This cross-section of a typical spring contact shows its basic elements, all of which have an effect on the contact’s performance. |
Testing was conducted in accordance with EIA 364, with the product resistance measured initially and at each subsequent change in vibration, as follows:
- Mechanical shock: 58 Gs, 11 milliseconds, 1⁄2 sine wave, 3 blows/direction/axis, 3 axis;
- Initial vibration: 3.1 Gs RMS to EIA 364 Procedure 28
- Power spectral density 0.02;
- 50 to 500 Hz;
- 1 hour/axis, 3 axis;
- Secondary vibration: 5.35 Gs RMS to EIA 364 Procedure 28
- Power spectral density 0.02;
- 50 to 2000 Hz;
- 1 hour/axis, 3 axis;
- Tertiary vibration stage: 7.56 Gs RMS to EIA 364 Procedure 28
- Power spectral density 0.04;
- 50 to 2000 Hz;
- 1 hour/axis, 3 axis;
- Final vibration stage: 9.26 Gs RMS to EIA 364 Procedure 28
- Power spectral density 0.06;
- 50 to 2000 Hz;
- 1 hour/axis, 3 axis.
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The results of this testing process demonstrate how any bias system improves contact performance, but an eccentric bias enables a reduction in piece-part count while retaining stability.
 FIGURE 5. This sample random vibration profile at 9.26G root mean square (RMS) on the Y axis shows stable performance. |
Biased battery contacts offer exceptional stability during the most severe of shock conditions.
The force needed to create a disconnect can be calculated using the following formula:
Force = mass χ acceleration.Therefore:
Acceleration = force ÷ masswhere force is the spring force at rated stroke, and mass is the mass of the plunger.
For a typical bias ball spring contact:
Force = 65.2g, mass = 0.01587g, and acceleration = 65.2 ÷ 0.01587, or 4108.
 FIGURE 7. The eccentric bias system, a patented system shown on the far left, is fundamentally different from other bias systems, including ball, plunger, and spring. |
To achieve disconnection, the contact must be exposed to well in excess of 4000 G shock.
Spring into action
Spring contacts in all form factors can be quickly engineered into highly reliable connector systems, including cabled, coaxial, surface-mount, and double-sided varieties.
 FIGURE 8. This is the classical shock profile derived from the EIA 364 Procedure 28, which is a vibration test. |
Spring contacts can be a useful tool in your design toolbox, exhibiting durability, stability, and the adaptability to meet many interconnect challenges.
BRUCE VALENTINE is European business development manager for Interconnect Devices Inc. (www.idinet.com), of Kansas City, KS. Valentine is based near Oxford, England.
