Reinventing the spring-loaded probe pin

Author : Plastronics

03 November 2016

By reinventing and fully automating the manufacturing process, spring probe pins can now deliver high performance at price of a stamped contact for high density, fine pitch applications.

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For decades the spring-loaded probe aka pogo-style pin has delivered excellent mechanical and electrical performance in a highly compliant contact.  However, this often came at a high cost given that each pin is constructed of 3-4 discreet parts manufactured and assembled in a laborious, less-than-fully-automated process.  

The cost can be large, in fact, that when significant volumes of pins are required, many opted to utilise less compliant, lower performance alternative contact technologies to reduce costs. This approach is becoming less viable, however, with the increasing miniaturisation of integrated circuits, electronic components and devices that pack more circuitry into smaller footprints.  

A single test socket for reliability and burn-in testing, for example, can require hundreds and even thousands of spring-loaded probe pins in a fine pitch configuration.  The same applies to board-to-board compression connectors.  When factoring in multiples of test sockets as well as production-level quantities of connectors, pin quantities can literally run into the millions.

Enter, or perhaps re-enter, the spring probe.  With a new approach to pin design and a complete re-invention of the manufacturing process, miniaturised spring probes as small as 0.2mm are now available that provide a high temperature, current and bandwidth performance pin at the price of a stamped contact.

Traditional pogo-style pins

Although designed and manufactured in subtly different ways, the pogo-style pin is typically constructed of a pin, two plungers and a spring encapsulated in a metal shell.

This style of pin is highly compliant, which means it is designed to compress or “comply” during insertion. This is critical when attempting to maintain a good connection despite potentially uneven surfaces, varying heights, errors in parallelism and flatness, or pivoting or rotating elements.

Although compliancy can be achieved by other techniques, such as bends, buckles, or cantilever-style contacts, additional space between pins is required during compression.  Spring-loaded pins operate in a purely vertical fashion, so the maximum space occupied at any time is defined by its diameter. This allows for placement of spring-loaded pins in fine pitch distances as low as 0.2mm.  

The trend toward more compact, high density electronics design – defined as the number of pins in a small area or the distance between pin centers – is already impacting several markets.  

One market that regularly utilises fine pitch spring probes is for electronics reliability testing, including burn-in, HTOL, HAST, THB and other testing protocols.  To conduct this type of testing, sockets are manufactured to provide an intermediate (temporary) connection through an array of pins between the PCB and the components or multi-chip modules being tested.  The PCB is then connected to a computer or other device for data capture and analysis to determine pass or fail.

The other market is compression-style board-to-board connectors used in electronics for telecom, automation, medical, aerospace, and military applications.   These specialty connectors utilise spring probe technology to create a one-sided connector that is mounted against pads on the PCB.   The benefits of spring-loaded connections include the elimination of receptacles to reduce costs, space savings, “one-touch” attachment and removal, and high durability.

Whether creating a temporary connection, such as for testing, or as a permanent board-to-board interconnection, the common denominator is more pins, less real estate.

“Devices used to have 2,000 pins in a two inch square area.  Now they want the same 2,000 pins in a one-half inch square and the only way to do that is to reduce the pitch of the device,” says Ila Pal, Chief Operations Officer of Ironwood Electronics, a manufacturer of high speed sockets and adaptors for characterization, burn-in, and production testing.

Ironwood Electronics was utilising spring probe pins on a 1mm pitch design and more recently at 0.5mm.  But during the last year, requests came in to shrink the pitch to 0.4mm.  Now, the company is moving even further down to 0.35mm. 

Germany-based test socket manufacturer EP Ants GmbH, is experiencing the same market trend. 70-80% of the test sockets the company manufactures today are for high density applications.  

Another market driver is price.  Higher density means a higher volume of pins per test socket.  Multiply that by the number of sockets required for parallel or serial testing at a single facility and customers expect companies to deliver the best possible price without sacrificing performance.

The H-Pin

Plastronics has developed H-Pin for this type of application. H-Pin is a stamped spring probe that delivers the mechanical, electrical and thermal performance of a pogo-style spring probe.  The highly compliant pin has a working range up to 1mm with a flat spring rate and can be utilised up to 15GHz with -1.0dB loss, carry up to 4 amps of current and withstand temperatures up to 200°C.

Although there are a few design tweaks, the real departure is in the manner in which complete pin assemblies are manufactured using a high volume BeCu stamping process and a 100% automated, high speed assembly and inspection process that can produce up to 400 pins per minute.  

H-Pin is available in various lengths and pitch sizes as low as .2mm. Depending on quantities, the spring-loaded probe pins can cost 30-50% less.  With thousands of pins potentially in a single test socket or board-to-board connector, the savings can be significant.  

Meeting high performance demands

For spring-loaded probes pins, high performance characteristics are defined by the ability to withstand high temperatures required for burn-in and other tests, ability to handle increasing amounts of current over increasingly smaller pins, and the ability to handle high frequencies.

The ability of the pins to withstand higher levels of current, despite shrinking in size, is also an increasing concern – particularly with the prevalence of higher power output lithium batteries.

For high performance applications, there can be concerns about the construction of traditional spring-loaded probe pins and how its design can affect the quality of the connection under compression, the potential for unreliable test results requires all finished pins be tested and binned according to performance results. 

The performance of the pin as measured by its contact resistance is also a benefit in high frequency testing.   Whereas standard spring-loaded probe pins can experience wide variations in contact resistance, potentially leading to false failures, the variation in the H-Pin is minimal and well within accepted levels.

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