Laminated Chip
Carrier Testing

By: Raymond P. Cronin

System provides a flexible solution to a challenging problem

The substrates for advanced chip-scale packages have become, in essence, miniature circuit boards with multiple layers of metallization. While substrate testing is now crucial, delivering an electrical stimulus to minuscule test nodes, ramping up for volume runs, and qualifying a diversity of product types has posed a major challenge for the contract manufacturer. Fabricators and industry watchers acknowledge that current production test approaches cannot provide 100% functional evaluations of organic substrates without sacrificing throughput and accuracy.

A recently introduced system by Everett Charles Technologies changes the status quo by incorporating a new technology for substrate test with the versatility to switch back and forth from prototype runs to high-volume production, and the capacity to handle diverse chip-scale types with a pre-fabricated interface. The new ECT system, along with emerging standards for high-density substrate production, will enable laminated chip carriers to be tested as rapidly and easily as traditional printed circuit boards.

Substrate Test Metrics
The goal of functional substrate testing (both continuity and isolation) is to duplicate the performance of each carrier package as a populated unit. A precise functional test must apply electrical signals to allow measurement of low node-to-node resistance and near-absolute electrical isolation of networks. Testing apparatus should deliver an actual flow of current between all nodes to measure continuity, and a forced voltage between all networks to verify isolation.

The challenge in implementing this goal has to do with the pitch and size of test points on the surface(s) of the substrate. Densities include conductor lines and spaces as narrow as 1-2 mils and vias (vertical connections) which are typically 6 mils or less in diameter. The pads are also easily damaged and their integrity must be maintained for wire bonding or device attachment in the next phase of assembly.

Conventional continuity test. Non-contact capacitance

Conventional continuity test. Cunductive rubber

Issues like these have resulted in a compromise between pad marking, test quality and throughput in current evaluations of organic chip carrier substrates. Various approaches ranging from isolation test only to flying probe to non-contact capacitive sensors tests have been pursued in the past with unsatisfactory outcomes. The increasing complexity of these substrates, the requirement for production throughput and the accessibility to topside and flip side nodes or nets has impacted all methodologies.

Isolation testing on universal grid testers was originally attempted because these systems were widely available at substrate production sites. While the technique offered significant throughput, it could not provide a test of continuity between grid array pads and the fine pitch device pads. Other methods involving capacitance testing generally resulted in less definitive "implied continuity" ratings. While flying probe testers have been successful in prototyping and limited-run assignments, currently available systems lack the throughput for the high-volume production and cost targets of independent substrate suppliers.

Robotic Augmentation
The new ScanMan™ production test system developed at Everett Charles Technologies reinvents the universal grid test by blending this methodology with a precision miniature robot and a flexible scanning contact. Time-consuming custom fixtures are bypassed through the use of prefabricated modular interfaces for many standard chip-carrier sizes. The unit connects to the BGA sites of the organic substrate while the high-speed robotic scanning contact scans the topography of each fine-pitch array. Isolation testing is performed by applying a voltage across networks before the robotic scan. The movement of the robot (for continuity test) is synchronized with the switching matrix of the underlying universal grid. The system's robotic augmentation and proprietary scanning application have influenced new throughput levels for this class of test; typically a complete isolation and continuity test is performed within 4.5 seconds. And when operating in full-automation mode, the ECT ScanMan will test accurately at speeds that are up to ten times that of flying probe systems.

Low-to-High-Volume Runs
A major barrier to automation in high-density substrate test has been the non-standard nature of the product itself. Substrate suppliers and internal production groups grapple with variables ranging from packaging types, physical dimensions, and performance specifications regarding test node counts and placement, to wide swings in job lot counts. Therefore substrate part numbers are processed in sequential, discrete batches, which cancels out the traditional "big iron" approach to test automation for most fabricators. Hard-wired, high-volume test systems are not appropriate for the compressed and fast changing world of contract manufacturing; even less so when the industry is dealing with shrinking product lifecycles as well as newly-introduced processes and materials.

New solutions for test must incorporate modular interfacing and options for rapid scaling to full-volume runs, with fast conversions for other part numbers or prototype assessments. These requirements have been addressed on the ECT system with the following innovations:

• Reusable interface modules for all-standard-sized BGA footprints
• A scanning technique eliminating alignment and realignment time lags
• Four-second docking/undocking automation capacity
• Cycle interrupt controls, Windows NT viewing screens and operating software with an option for remote monitoring or reports on the user's PC network

Part Flow Considerations
The desired mode of operation for the substrate supplier is one that leverages fast response times and capital investments across the customers' diverse needs. The new ECT system permits the use of conventional fixtures for testing dedicated designs but also introduces a set of reusable modular interfaces that meet various industry standards for BGA footprints. This allows the fabricator to avoid fixturing for each customer part. The user installs the prefabricated module for that customer's substrate, loads the appropriate software test program, slides the automation unit in place, and initiates a run.

Adhering to industry acceptance for the JEDEC tray, ScanMan™ is configured to accept standard JEDEC tray loading and unloading while maintaining test speeds. The user has the choice of off-loading failures to a built-in repository or selecting a system-initiated "mark" for failed parts while still returning them to the JEDEC tray with good product. Failed parts may be noted in a software code for another assessment later in the process run.

If the user must detour from a production run to check a prototype substrate, it is generally possible to stay with the same modular interface. The operator backs away the automation assembly, retrieves the robot scanning trace stored in the test system's software, performs the manual test, and re-docks the automation unit, returning to full-speed operation.

Conclusion
Organic chip carrier substrates have become a central part of the semiconductor and printed circuit industries. The test of such substrates has required significant tradeoffs between quality and throughput. The application of a new contact scanning test system available with JEDEC standard automation and universal fixturing has eliminated the tradeoff.

Raymond P. Cronin - -- is corporate vice president of the Semiconductor Test Group, Everett Charles Technologies (a subsidiary of Dover Corp.), Pomona, CA. He can be reached at
(508) 230-9815 or by e-mail: [email protected]