SO Packages

Challenges of and Solutions for Contacting Small Outline Devices

  • Features

    • wide variety of pitches, sizes, and pin counts
    • a wide range of applications (MOSFETS, drivers, switches, MCUs, sensors, etc.)
    • low cost packages
    • typically NiPd or tin plated
    • can have exposed ground pad
    • very high volume
  • Contacting Challenges

    • high currents
    • high voltages
    • coplanarity issues
    • device asymmetry
    • Kelvin test
    • tin migration
    • high contact spring wear on NiPd platings
    • tin oxide on leads
  • Contactor Features for SO Packages

    • self-cleaning wipe
    • wide spring tips
    • high contact force
    • relatively high surface area for heat dissipation
    • relatively high thermal mass
    • durable one piece design
    • low and stable contact resistance
    • high current carrying capability
    • large mechanical operating window for stable contactor performance

SO Packages

SOICs are leadframe based packages that are usually encapsuled in a rectangular plastic body with lead extending from two sides. For better thermal performance, many SOICs have exposed pads, typically on the bottom of the device.
SOICs are industry standard sized and the leads are typically spaced at pitches ranging from 0.4 mm to 1.27 mm.


SOICs can be tested as single devices or in Strip form. In both cases ensuring a stable contact even if there is tin oxide on the device leads. With multiple retests at different temperatures the contact pressure and wipe need to be aggressive enough to penetrate the tin oxide layer without resulting in exposed cooper.
Also, since SOICs are an industry standard packaging that is suited for many different applications, the test requirements can vary a lot, with voltage ranges up to several kV and currents far above 100A.

For singulated devices, due to the trim and form process, the geometry of the leads is not always symmetric resulting in varying standoffs or different lengths of the contactable part of the device lead. Since the contactable portion of the device lead is usually at an angle, it causes a lateral force at the point of contact.

For strip applications in cases where SOICs are tested with extremely high currents, a contacting technology that can handle these high currents, a contacting technology that can handle these high currents without sacrificing the capability of testing at high parallelism is required.

Addressing the Challenges

Since there is a wide variety of tin alloys and NiPd based lead platings, Xcerra offers a wide variety of Kelvin and non-Kelvin cantilever spring with different tip styles, geometries and platings, designed to address the issues of tin oxidation and wear out.
Xcerra cantilever spring have a high compliance window to compensate for coplanarity issues of the SOIC leads.

The durable one piece design of Xcerra’s BlueLine contactors allows for lateral forces at the point of contact, without damaging the contact spring tip.

Each socket is manufactured out of high quality materials and custom, designed to match the customers test requirements.