Floating and Non-Floating Test Socket Bases – An Introduction

The pogo pin has grown to be one of the most popular interconnect methods in electrical test for its versatility, serviceability, and relatively low cost of adoption in small volume. Their widespread adoption in PCB and semiconductor test, not to mention consumer electronics, can be partially attributed to advancements in the design and manufacturing process over the decades.

Pins are now available in many shapes and sizes with different tip designs, plunger and barrel lengths, spring composition, and electrical coatings to satisfy most modern physical and electrical test requirements. The ability to source from a wide range of pin suppliers allows test socket manufacturers like Robson Technologies Inc. to create test socket designs that extend both test socket and the “Device Under Test” (DUT) longevity for a wider range of devices. RTI’s floating base and non-floating base test sockets are just one example of this.

Non-floating Socket Base – Traditional Use Cases

Non-floating base test sockets are ideal for testing small PCB modules, QFN, LGA, and other IC devices without solder balls or leads.  Devices with larger-sized test points such as pads on printed circuit boards and the bottom side of some packaged ICs can be targeted with a point or crown tipped pogo pin and rely on the device’s body for alignment.  Any possible divergence between the tip of the pin from the center of the pad on the DUT will be accounted for during design while considering DUT and test socket manufacturing tolerances so the pin is guaranteed to make good electrical contact.

Sometimes, non-floating base test sockets are required by virtue of the DUT.  Small PCB devices that use non-plated through-holes or tooling holes for alignment will require a non-floating base design.

In non-floating base test sockets, the device under test rests directly on the pogo pin tips when loaded.  The need for truly vertical compression is greater when using non-floating base test sockets.  Any lateral movement of the DUT across the pin tips as compression is made can damage the leads on the DUT and/or the tips of the pogo pins.

Non-floating bases are typically used for…

• Hand test and with optically aligned auto handlers in IC pick and place test
• Test small PCB modules in lower volumes before scaling up to a test fixture
• When incredibly short electrical lengths are required for high speed and low inductance

Floating Base – What is it good for?

Floating base test sockets expand on the non-floating two-piece architecture by including a third drilled plastic plate between the socket top and the DUT. This additional plate (also known as the floating base) sits on springs, moves independently from the pogo pins and socket body, and acts primarily as an alignment feature for the DUT during loading.

Floating base test sockets are typically used to test BGA and bumped WLCSP devices, though other test requirements may necessitate a floating base design. This is because aligning very fine pitch and large array devices to hundreds or thousands of pins requires more precision than the package outline alone can provide. Relying on the center spacing and positioning of solder balls for fine alignment to the pins is preferred since the balling process is generally considered more precise when compared to outside package tolerances.

Holes drilled in the floating base to match the DUT footprint are slightly larger in diameter than the solder balls on the DUT. This allows the DUT to self-align to the pogo pins without making direct contact with the pin tips when loaded. This also ensures each crowned pin tip rests directly below the center of each ball on the DUT. Devices that have a large package tolerance benefit from the alignment to the balls since it guarantees contact regardless of package tolerance.

Pin positioning relative to the ball is important for preventing damage to the balls, but also in maintaining even electrical length across all pins. The spring force of off-centered pins may drive the pin tip up against the side of the ball and lead to deformation and poor contact. The walls on the floating base surround the ball and pin tip keeping both pin and ball in position during engagement.

A floating base can help ensure near-vertical travel during use, even without the assistance of vertical guides. The DUT and floating base move together during use and the additional width provided by the base prevents excessive rocking of the DUT and rotation of the solder balls when the lid is closed.

The floating base offers an even and hard stop when fully engaged so the pogo pins don’t risk over-compression during use. This also allows for slight variation in travel and force applied between pins – test engineers can quickly identify variation in ball centers or thickness based on differences in the depth and position of witness marks.

Floating bases are typically used for…

• More precise alignment for bumped and array-based packages (BGA/WLCSP)
• Typically used in hand test where devices come to rest before the lid is closed
• Commonly found in universal sockets that rely on ball instead of package alignment
• Allow for integration of quartz or sapphire window lids for 100% optical access

Fundamental Differences Between the Two Socket Bases

Difference in price – additional drilling, hardware, and assembly in floating base test sockets make the non-floating base test socket more cost-effective for non-balled or array-based packages.

Difference in performance – Shorter pogo pins are often sought for high speed and RF operation or low inductance test requirements. The inclusion of a floating base may not be feasible depending on the pogo pin model used.

Difference in modifications – Non-floating base test sockets can be made closer in size to the footprint of the DUT and offer a slightly wider range of design customization options.

Difference in DUTs – PCB modules and QFN/no-lead packages are better suited for a non-floating base socket while BGA, bumped die and chip-scale packages are better accommodated by a floating base socket.

Difference in application – Non-floating base sockets can be made thinner than floating base sockets. This is sometimes necessary when the working space directly above and around the socket is limited during the test process.

Conclusion

There are many test requirements that socket engineers must consider when developing custom pogo pin test sockets. These requirements can help the engineer determine which test socket design is optimal for your DUT and test configuration but ensuring good electrical contact through precise DUT alignment is essential to any design. Whether or not the test socket includes a floating base may seem trivial to the end operator, but it can play an unseen role in the long-term success rate of your socket and test program. Test socket manufacturers like Robson Technologies Inc. can provide test socket assembly drawings with instructions to end-users for ongoing socket maintenance, pin replacement, and general care for both floating and non-floating base designs.

 

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