Automated Resistance Mapping of LGA Chip Module Contact Grid

Author(s):
Steven Hoenig - Bloomy Controls
Gareth Hougham, PhD - IBM Thomas J. Watson Research Center

Industry:
ATE/Instrumentation, Manufacturing, Electronics

Products:
PXI-2575, LabVIEW, PXI-4072, PXI-2503

The Challenge:
Verifying uniform force distribution and quality of electrical connectivity on a pressurized land grid array (LGA) interposer and automating a time-consuming and physically prohibitive process involving substantial measurement, data recording, and data presentation.

The Solution:
Using a measurement based on NI LabVIEW software and PXI hardware that automatically cycles through a set of resistive test points and takes two- and four-wire resistance measurements, and then mapping the results onto an equivalent data structure that recreates the LGA layout and presents a visually informative 2D and 3D graphical display.

With the ever-growing input/output terminal count and density of integrated circuit chips, the process of connecting chips to printed circuit boards (PCBs) has become very challenging. Replacing a solder-connected chip involves sending the entire board back to the supplier where it must be removed, replaced, and reconnected. Using an LGA as a chip module-to-board interconnect enables the creation of reversible connections that do not require soldering. Rather, LGAs make it possible for technicians to conduct field replacement, thus saving considerable time and money for the user. Common applications include high-density chip modules found in high-performance computers and servers.

To maintain reliable electrical connections and performance, uniform force distribution across the entire LGA is critical. Variations in contact pressure may exist due to contact defects, chip substrate topography, solder mask misalignment, PCB topography, pad contamination, and similar factors. To verify an acceptable force distribution, technicians must measure each contact’s resistance. The effectiveness of an LGA interconnect system can be validated by the statistical spread of the measured resistance data and, in worst cases, by the identification of open circuits under certain operating conditions.

Validating an LGA interconnect involves measuring all contact resistances and visually and numerically analyzing the uniformity. In addition to the physical challenge of taking resistance measurements for numerous contact points residing within an extremely small physical area, users must gather the data in a meaningful format that facilitates thorough and rapid analysis.

Furthermore, IBM researchers wish to evaluate resistance distributions at varying external pressures used to hold an LGA in place and at various temperatures that mimic a working computer environment around an LGA. They also need to evaluate the performance of an LGA over time to verify its sustainability. Therefore, validation consists of the already difficult process of measuring all the resistances under these different and sometimes dynamically changing conditions including temperature cycling, loss of force to stress relaxation, and contact fatigue. Validation also consists of creating a data presentation framework that tracks the performance between changing conditions.

Clearly, IBM needed an automated means of verification and analysis. IBM contracted us, Select National Instruments Alliance Partner Bloomy Controls, to develop an automated system based on PXI modular instruments and LabVIEW. We connected our PXI system via a MXI-4 interface to a PC running an application developed in LabVIEW.

IBM uses the PXI system as a measurement and switch tool for measuring the resistance of all available contact test points on an LGA. The 679 sets of test points spread judiciously throughout the thousands of actual contacts serve as an accurate representation of the true resistance map while avoiding certain physical constraints involved in measuring every contact.

For this solution, we modified a chip module to act as a shorting block by gold coating around all sides and pulling two signal wires from this envelope to represent the “high” side of every contact. Seven NI PXI-2575 multiplexer modules switch between the pairs of contact test point signals drawn from the other side of the LGA, representing the “low” side of each contact.

We feed the resulting pairs of “high” and “low” signals to an NI PXI-2503 multiplexer, which passes either all four signals for a four-wire resistance measurement or only two signals for a two-wire measurement. The system passes these signals to an NI PXI-4072 digital multimeter (DMM), where it performs the resistance measurement. The four-wire measurement provides a more accurate measurement in the milliohm range, while the two-wire measurement more clearly identifies a higher-resistance value as an actual open electrical contact.

LabVIEW software automates the entire measurement process of setting the two- or four-wire resistance mode via the PXI-2503, cycling through each channel via the PXI-2575, and taking resistance measurements via the PXI-4072. The system generates separate resistance maps for two- and four-wire resistances, as well as for a combined resistance set, in which each contact resistance corresponds to the value of the proper measurement type as defined by a user-specified cutoff level.

We save resistance maps to file in multiple formats for later use by this application and offline analysis tools. This solution also generates a graphical presentation of the data as a 2D resistance map and 3D topography. IBM can use several graphical user interface (GUI) tools to set graphical image parameters such as color, limits, grayscale, zooming, smoothing, and viewing angle. Users can export either image type at any time to an image file containing the graphical display as well as key operation parameters. IBM can use the graphical toolset to analyze new test data as well as previously tested and stored data.

The system expands to multiple LGA sizes and includes tools for calibration, data recall, data analysis, and manipulation. IBM also can set multiple test runs at predetermined time intervals to allow autonomous testing over time.

With this solution, IBM can perform LGA resistance mapping of more than 600 contacts in only five minutes. The previous method required 20 minutes for 400 resistance measurements, limited to a single measurement type, plus an additional 30 minutes to prepare the resistance maps offline. New improvements include integrated analysis and presentation; combined resistance measurement types; and evaluation over changes in time, pressure, and temperature. PXI provides the ability to make a wide range of flexible measurements while LabVIEW delivers the platform for implementing an intuitive, scalable, and vital tool for thorough and rapid testing of LGAs, ranging from experimental designs and prototypes to production units.

Author Information:
For more information on this Case Study, contact:
Steven Hoenig
Bloomy Controls
839 Marshall Phelps Road
Windsor, CT 06095
United States
Tel: (860) 298-9925
Fax: (860) 298-9535
steven.hoenig@bloomy.com

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