How does a pogo pin work?

How does a pogo pin work?

This article will look at:

  1. The basic structure of a spring loaded connector
  2. Which kind of inner structure fits which application.
  3. The electrical performance of pogo pins.
  4. The durability of pogo pins

The basic structure of a pogo pin.


A generic pogo pin consists of 3 parts: Barrel, Plunger and Spring. Pogo pins are often inserted into a housing for better stability, but in many cases those pins are just soldered directly onto a PCB.

Pogo Pin Structure Explained

Usually the plunger of the pogo pin contacts a gold pad, called “contact pad” or “mating pad”, that closes the circuit. The Barrel is soldered on a PCB or has a wire attached to it. Pogo pins are often considered the more durable connector type compared to flat spring connectors, as the pin does not scratch the surface of the contact pad.

Every pogo pin has a so-called working height. 

Unloaded Height (Free State) The unloaded height or full height of the pogo pin is the height where the pogo pin is at its maximum expansion. That means the plunger is pressed onto the crimping edge of the barrel. The Crimping Edge is the small inwards bend on the top of the barrel to hold the plunger in the barrel. Pogo Pin at Full Compression
Working Height The working height is the ideal compression of the pogo pin. It is usually 25-75% of the unloaded and full compression height. Using a pogo pin at its working height will increase the durability and performance of the connector significantly. This is especially true for magnetic connectors where magnetic force and spring force need to be carefully balanced.
Full Compression Height     Full compression height is usually reached, when the contact pad touches the edge of the barrel. It can be also earlier, if the spring is fully compressed or the plunger has a longer design.

 

What kind of Pogo is good for which application?



Pogo pins come in a variety of forms and shapes suitable for different applications. While pogo pins seem to be very simple connectors, their inner structure can have a great impact on the performance and lifetime. 

The back drill design is usually used in environments with space constraints, such as wireless earbuds, smart phones, or small IoT devices. They are usually not suitable for high frequency data transmission or fast charging applications.


The bias tail design is most common and cheapest to produce with a good balance between performance, price and size. They come in all kinds of variations and are often used for docking stations or low bandwidth data transmission. Special designs can also be used for antenna signal transmission.


The ball design is the best option for very challenging environments such as heavy duty machines, automobile and medical applications where stability and high current are important.


Electrical performance of a Pogo Pin?


We have learned what types of pogo pins are most common and what kind of design is suitable for which current. But why does a back drill design carries less current than a ball design? The answer is quite easy: Contact resistance and number of contact points inside the barrel.

 

The back drill design has 2 lanes of current flow. A small portion of the current also goes through the spring, but since it is made of SUS or MusicWire the contact resistance is quite high. The current will therefore pass through the plunger and the walls of the barrel. Since the plunger needs some tolerance to slide down, there is a small gap between barrel and plunger. This gap can, due to vibration, result in micro disconnections, which increases the contact resistance. It can even lead to spring burn, if all the current is flowing through the spring.     
The Bias Tail Design has a bias on the bottom of the plunger. This forces the plunger into a small tilt which guarantees, that the plunger stays in firm contact with the barrel. It allows a higher current to pass through, even if the connector is used for example in a car, that has constant vibration. The tilt is slightly exaggerated in this picture.
The ball design maximizes the number of contact points to 4. The ball, usually made of SUS, guarantees a very smooth slide of the plunger since the spring will always have an even pressure on the ball and the plunger.   

  

How durable is a pogo pin? The difference between mechanical and electrical lifetime.


There is no common standard how companies define the life cycles of their products. Many of our competitors just write the mechanical lifetime.

 
What does mechanical lifetime mean?


It indicates how long the spring force is guaranteed to stay within a defined range. For example, if the initial spring force is 100g, the company guarantees that after 1 million compressions (at working height) the spring force won’t be more than +-20% of its base value (80g - 120g). Especially connector manufacturers that sell on platforms like digikey and Mouser use this lifetime. This number however does not describe, how long the pogo pin will function well. More important is the electrical lifetime


What does electrical lifetime mean?


Electrical lifetime indicates how long the contact resistance stays within a certain range. CCP usually indicates this in its drawings. The main point of failure is usually not the spring but the plating of the connector. Over time, the thing layer of (gold) plating gets scratched off the inner surface of the barrel and the plunger, when the connector gets compressed. The exposed base material (Copper or Brass) is less conductive than gold and the contact resistance increases. A high quality pogo pin connector should therefore be coated well on the inside of the connector. Low quality manufacturers often don’t use advanced turning and drilling processes to improve the quality of the inner surface area. This in turn results in an uneven coating, bad contact resistance, inferior signal transmission and low efficiency. While the connector may look great from the outside, there might be no plating on the inside. More than any other connector type, spring loaded connectors rely on their inner values.


In terms of mechanical lifetime, a pogo pin can sustain 500,000-1,000,000 compressions.

The electrical lifetime is very dependent on the plating and the materials used in the pin. If the Contact pad is made of a harder material than the pin, the abbreviation on the pin is higher and the lifetime is reduced. Corrosion is also a big factor that determines the lifetime. Especially corrosion inside the connector can increase resistance and the functionality of the pin. If you use the pin on a smart watch with contact to the body, special platings are necessary to resist the galvanic corrosion.


In general, the electrical lifetime varies from just 5,000 to over 1,000,000 compressions. The hardness of the plating is the key to extend the lifetime. Check our pogo pin introduction page to find out more about the different plating options.