September, 2013
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Page 53
Innovative Socket Technology -- Multi-GHz Package Testing Continued from page 51
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 mechani- cal and electrical connection. A heat sink screw and the socket body pro- vide heat dissipation for the IC in the socket.
Mechanical Characterization Removable interface require-
ments are generally stated in terms of the insertion/extraction force and number of insertion/extraction cycles a socket can support without degra- dation. 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
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.
devices enclosed in packages. A number of tests can be used to eval- uate the mechanical relationships of a socket and a device’s contacts. The first test examines the relationships of force, contact deflection, and con- tact resistance. For this test, a dis- placement force (DF) test station was used to measure contact deflec- tion for a given force. The force increases linearly as the displace- ment increases. Similarly, the con- tact resistance decreases as the force increases. Stable contact resistance must be maintained based on the minimum force required. Based on
These plots show contact resistance (CRES) data for the test socket at different cycle intervals.
the SM contact was set at 0.3mm, the test set was adjusted for the head to move down by this amount. The ini- tial 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 con- tact resistance. Similarly, the contact was tested at various temperatures
Continued on page 56
When tested with applied cur- rent, the sockets showed a heat rise of only 14°C for 4 A current.
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 perform- ing device test using this contact technology. 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
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