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Strategies to Successfully Select a Custom Socket

Feb, 01 2016
Strategies to Successfully Select a Custom Socket

Greater steps are being taken every day to standardize the test socket and packaging industry.  At the same time, increasingly more complex test requirements drive the need for new, non-standardized, innovative test sockets.  The demand for new products in the global market combined with the innate evolution in technology continuously pushes the limits in package architecture.  Traditional packaging has expanded to include higher performance requirements in broader applications, requirements that can only be addressed with a custom test socket.  We’ll briefly investigate some emerging requirements and explain how you can successfully identify and convey your requirements to a vendor to be sure you receive a test socket that matches your product.
Many sockets must now provide complete access to the package surface for both test and FA applications while providing ease of insertion and reliable contact continuity.

Medical applications have introduced new requirements for biologically inert contact methodology with no contact abrasion.  In some cases this also necessitates a socket solution providing an environment held at a constant temperature with little variation in tolerance. For high performance, high density  ASIC’s,  designs are now optimizing ball arrays using non-uniform spacing to more effectively allow high speed signals to escape the array in tighter PCB trace routes. Designing these types of sockets often requires a customer supplied CAD file to accurately place test pins.
Typically packages are loaded “live bug” in the socket base allowing topside access through a hole in the lid and backside access through a hole in the socket and PCB.  However, there are times that backside package access is required with the package loaded “dead bug.”  For example, during optical testing this allows optics to face down into light/pattern source while being contacted from the backside of the package.  Signals are routed from the package up through the lid via pogo pins to an interposer PCB, then back down to the DUT board through the socket base via transfer pins.  This design requires a package specific custom socket.
How to successfully identify your requirements

Two words: “plan ahead.”  Custom sockets require from 2 to 8 weeks to design, manufacture and assemble.  Give your vendor time to do it right.  Planning ahead now and defining the test scenario as accurately as possible can prevent oversight and errors in the final product.  Many times one test requirement effects other requirements. Failure to express a critical requirement could needlessly cost time and money.  Here are some points to consider when discussing your requirements with a potential socket vendor.


  • What are the planned uses for the socket? 
Will you be using the socket for failure analysis or on an automated handler, performing burn-in test, or engineering characterization or a combination of the above?Understanding how the socket will be used is paramount to test socket and lid design strategies.
  • Do you need access to parts of the package?
Cutouts in areas of the socket body and lid can provide access to parts of your UUT during test.The type of access required may also determine the best way to load the package in the socket (“dead-bug” with contacts up, or “live-bug” with contacts down).
  • How many test sockets will you require?
Planning ahead and ordering sockets for future use can save money and time, especially if sockets are required at multiple test facilities or for multiple test applications.
  • Does the test socket need to match a predefined footprint on an existing board?
Custom test sockets can be designed to mount directly to an existing footprint, in most cases.  An additional interposer board or alternative interconnect may be necessary in the event that the custom socket cannot be made compatible with the current PCB layout.
  • Are socket dimensions critical, including maximum socket size and total height?
This is particularly important when interfacing to other fixtures used in conjunction with the applications board or DUT. In addition to footprint size, interfaces to auto-handlers have their own special needs. Temperature forcing equipment that must fit over the socket may have other requirements.
  • What are the package tolerances of your device?
One of several factors that impacts the socket architecture and a major reason package drawings are required to provide an accurate quotation.Many of today’s packaging applications involve small modules with SMT components, sometimes under metal lids. There is commonly a wider range of thickness tolerance for these types of packages which may necessitate a special pressure plate to be designed into the lid.
  • At what frequencies do you plan to test?
In the last two years advancements in pogo pin technology have more than doubled the upper frequency limit.The test frequency and current requirements help determine the type of contactor that will be used.Pogo pins, elastomeric, diamond particle, etc.
  • At what temperatures do you plan to test?  
The type of pogo pin and socket material used will need to be rated for the temperatures you test at.Standard ranges 0-80°C, extended ranges -40° to +125°C. Some applications require as much as 200°C!
  • Will there be components on the DUT board in the area immediately around the pads?  
Pockets can be milled in areas on the bottom of the socket base providing clearance of SMT components that need to be near the test pins.
  • How thick is the PCB on which the socket will be mounted?  
Thin PCBs may require the use of a backing plate in conjunction with a compression mount socket.This plate mounts to the opposite side of the PCB and reinforces the structure to prevent flexing when the socket is mounted. Like the socket base, clearances for SMT components may be required.
  • How many insertions will be performed?
Using a lower cost contactor with a shorter life expectancy may be appropriate for sockets that will only undergo a handful of insertions.
  • What type of pad or lead material does your package use?
Tin-solder and Pb free contacts require contactors with different characteristics. Using a pin rated for the wrong lead material may work initially but be problematic later.
  • What lid type do you require? 
Sometimes custom sockets are only as useful as the type of lid used.Optimal lid design depends heavily on the socket’s application and the package type.Low Profile open top lids, often called Failure Analysis lids, are intended for testing devices that have been decapsulated or opened for analysis.These lids allow a full view of the die surface by pressing down on the perimeter of the package or leads.There are increasing numbers of chip Scale Packages (CSP) in use today that offer a challenge to the socket manufacturer, especially when the engineer requires a complete view of the die for optical fault localization methods.Standard F/A lids that rely on contacting the perimeter of a package obscure the view of the die’s periphery.An F/A style lid with a fused quartz window allows for 100% visibility of the Device Under Test (DUT).Since fused quartz is transparent to IR light, a variety of IR imaging and IR laser methods can be employed.
Unfortunately, even the best designed custom test socket may not meet all of the strictest requirements simultaneously.  However, working with a vendor that has experience producing the type of socket and lid you require offers the peace of mind that the final product works correctly, consistently, and meets your test requirements.  Successful testing starts with a vendor that fully understands your application, can meet your performance requirements, is ready to fully support their product, is easily accessible, and communicates well with your technical team.

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