Fixturing for Surface Mount (SMT) transformers
The increasing drive towards smaller and cheaper electronic devices has resulted in a growth in the number of transformers being manufactured as surface mounted devices.
The advances in Surface Mount PCB technology over the past 20 years have resulted in smaller, more reliable electronics, as well as allowing a greater degree of automation in the assembly process.
Here are two examples of AT Tester fixturing for typical SMT transformers
One is a “classic” SMT device with gull wing pins
The second is a “planar” type device with only flat contact pins on the underside of the device.
Please click images to zoom
1. Surface mount package with gull wing legs.
Above is shown a fixture for a Coiltronics CCFL transformer (CTX21607-R)
The socket used is a ZIF (zero insertion force) style socket.
The lever above the socket opens and closes the contact pins in the horizontal plane.
This means that no vertical force is applied to the gull wing transformer pins, which would result in damage to the legs.
Each pin still has two connections, thus allowing a true Kelvin connection to the item under test.
This in turn allows the AT Tester to compensate out the effects of the fixtures (natural contact resistance, wire resistance and wire capacitance) from your readings. Hence you get a true value each and every time.
The quick release fixture allows the AT tester to be instantly reconfigured for a different bobbin type, and also minimizes the part-to-part change over time.
This is especially important when coil resistances are low (typically <0.5Ohm) where contact resistance becomes more of a factor.
Blade lifetime is in excess of 20,000 insertions.
This fixture was manufactured by our fixturing partner Wilco Control Systems . Typical cost around 400 USD
2. Planar package with flat contacts.
Above is shown a fixture for a WURTH SMPS power transformer (#750341144)
The part has 6 flat SMT contacts on the bottom of the transformer, and as such presents a problem for standard Kelvin pin type connections
The solution here is to use two vertical spring probes for each pin, mounted in a locating recess.
The locating recess provides a quick and easy location guide for the transformer.
The yellow plate is spring loaded. When the clamp is engaged, the plate lowers and the transformer is accurately and evenly pushed onto the test pins..
Each pin still has two connections, still allowing a true Kelvin connection to the item under test.
This in turn allows the AT Tester to compensate out the effects of the fixture wiring.
This is especially important when coil resistances are low (typically <1.0 Ohm) where contact resistance becomes more of a factor.
Operation lifetime is in excess of 20,000 insertions.
This fixture was manufactured by our fixturing partner Wilco Control Systems. Typical cost around 400 USD