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Innovative Socket Technology Enables Multi-GHz Package Testing
The SM GHz Socket supports high-speed, high-density testing.

Today's electronic packages must perform at the levels of the circuits they protect, handling clock rates through multi-GHz speeds, pin densities that can be finer than a 0.4mm pitch, and pin counts that can exceed 1000. Testing such packages can be challenging, but socket technology offers the means of testing the functionality of high-speed IC packages without damaging them. Effective test sockets require short signal paths for less resistance, good electrical insulation to minimize signal losses, and good thermal management (especially when testing at high power levels). Other parameters are also important, including durability, power consumption, assembly methods, and operating environment. Fortunately, the Silver ball Matrix (SM) GHz test socket from Ironwood Electronics represents a solution that is fast, durable, and supports testing of high-density packaging.
This cross-sectional photograph shows the design of the SM GHz Socket contacts — columns of tiny, precise silver balls.


A test socket is an electromechanical device that provides a removable interface between an IC package and a system circuit board. When properly designed, this interface exhibits minimal loss or reflections, with minimal degradation of signal integrity (SI). Having a removable interface can simplify and save the costs of assembling, reworking, and upgrading a product. For example, when testing an IC with a system printed-circuit board (PCB), the socket is typically semi-permanently attached (without solder) to the system board while the IC can be easily inserted into or removed from the socket for ease of evaluation without disturbing connections to the PCB. Having a socket on a system board can also simplify in-field testing and maintenance.
These plots show contact resistance (CRES) data for the test socket at different cycle intervals.


The SM GHz Socket from Ironwood Electronics, designed to test IC packages with fine 0.3mm pitch, is a sequel to standard GHz sockets. Available in a wide range of models, these SM GHz Sockets are available for applications ranging from compact production units to robust prototype and test units. The sockets are designed so that insertion force is evenly distributed across the top of an IC under test, pushing package solder balls into high-speed, z-axis silver-ball columns. The socket incorporates the unique SM Contact, which uses precise silver balls held together by a proprietary conductive formulation. These conductive columns (with diameter optimized for 50 ohm impedance) are suspended in a nonconductive flexible elastomer substrate with a patented solid core for enhanced durability and reliable performance over time, temperature, and cycling. This flexible substrate is very compliant and resilient and enables the conductive columns to revert back to original shape when the insertion force is removed. The elastomer substrate is the only medium between an IC package and the circuit board. Precision guides for the IC body and the solder balls help position a device under test for an optimum mechanical and electrical connection. A heat sink screw and the socket body provide heat dissipation for the IC in the socket.

Mechanical Characterization
Removable interface requirements are generally stated in terms of the insertion/extraction force and number of insertion/extraction cycles a socket can support without degradation. Insertion and extraction forces grow in importance with the increasing number of pins for a device, and for handling ICs housed directly on silicon as opposed to devices enclosed in packages. A number of tests can be used to evaluate the mechanical relationships of a socket and a device's contacts. The first test examines the relationships of force, contact deflection, and contact resistance. For this test, a displacement force (DF) test station was used to measure contact deflection for a given force. The force increases linearly as the displacement increases. Similarly, the contact resistance decreases as the force increases. Stable contact resistance must be maintained based on the minimum force required. Based on the compliance requirement of each device/application, this first test can help identify a desired amount of displacement. This information is very important for test engineers to set up failure criteria when performing device test using this contact technology.
When tested with applied current, the sockets showed a heat rise of only 14°C for 4 A current.


The second test examines the relationship of the contact resistance over the contact life cycle count. An actual handler was used for this experiment, in which a contact set with 44 leads and 16 ground leads (a QFN package configuration) was mounted on the test board which was then connected to a tester. A gold-plated shorted device simulator was mounted on the plunger head. Since the chosen amount of travel for the SM contact was set at 0.3mm, the test set was adjusted for the head to move down by this amount. The initial contact resistance was measured with the turning on of the automatic-test-equipment (ATE) system, which moves the plunger back and forth which in turn cycles the SM contact. A digital counter measured the cycle count. The test setup operated at ambient temperature and a cycle speed of about 2500 actuations/hour with a dwell time of about 0.7 sec.

From these measurements, the average contact resistance was found to be less than 25 mohms across 1 million cycles. The experiment was repeated for different product lots manufactured at different times. The results were fairly consistent, except that the average contact resistance shifted down by 5 mohms, indicating that process variations can cause at least this amount of variation in contact resistance. Similarly, the contact was tested at various temperatures and an acceptable range was defined as -50 to +150°C.

Electrical Characterization
For these socket interfaces, electrical requirements are generally stated in terms of bandwidth and current carrying capacity. Current carrying capacity is determined by injecting drive current in steps of 10 mA to a maximum value of 5 A. The temperature rise as a function of resistance is measured with a dwell time of 3 minutes per data point, to allow temperatures to stabilize. A temperature rise of 14°C was found for 4 A continuous current. For a test engineer, this indicates that the socket will exhibit no Joule heating effects due to the contact's base structure.
Insertion loss for the contacts was measured as only 1dB at 40GHz.


A final test of bandwidth was made to determine if a contact technology was suitable for a particular test application, using a commercial vector network analyzer (VNA) for the measurements. A test signal was sent from port 1 (the top of the contact) and received at port 2 (the bottom of the contact) to measure bandwidth. The measured insertion loss of 1dB at 40GHz is attributed to 90 percent of the signal passing through the interconnection medium and only 10 percent of the signal being lost at the interconnection transition at 40GHz.

For anyone using high-density wafer-level packages (WLPs), a primary concern is that a socket must provide high electrical performance, with low, stable resistance, and still meet the mechanical mating requirements, such as low mating force and a high number of mating cycles without performance degradation. As the measurements show, the SM GHz Sockets satisfy the requirements for reliable, high-performance test sockets and are suitable for high-speed, high-density, high pin count application needs. The simple design of the socket makes it cost efficient and allows assembly to the target board using existing hardware. A unique feature of this socket is its separable components that can be easily replaced and reused.


Contact: Ironwood Electronics, 1335 Eagandale Court, Eagan, MN 55121 800-404-0204 or 952-229-8200 fax: 952-229-8201 E-mail: info@ironwoodelectronics.com Web:
http://www.ironwoodelectronics.com

 
 
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