NASA Office of Logic Design

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space flight systems,
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The NASA ASIC Guide: Assuring ASICs for SPACE

Section Four: Part Acceptance


Introduction

Section Objective:

To present to ASIC managers, designers, quality assurance people and others, the major aspects of ASIC quality assurance.

How true to the specification is this ASIC likely to perform throughout its lifetime? Whether NASA flies an ASIC depends on the answer to that question. This section presents ways to assure that a flight ASIC meets all requirements, especially those that can be explicitly contracted. The approach described here represents state- of-the-practice methodology and is the best approach known today to assure success in an ASIC's final application. It includes a combination of complete requirement definition and specification, conscientious supervision, informed design, and proper part acceptance.

ASIC part acceptance requires a verification approach different from off-the-shelf devices. Off-the-shelf devices have reliability data bases compiled from previous applications. This data base provides reliability assurance that reduces or eliminates the need for verification tests. In contrast, an ASIC design has little or no previous applications and therefore lacks significant reliability history. Therefore, unlike off-the-shelf parts, ASICs require extensive verification under all anticipated conditions. Since the user is the designer, the user must be deeply involved in this verification process by specifying tests and screens, supplying tests vectors, providing test diagnosis, and verifying engineering (prototype) parts.

This section focuses on those activities that constitute traditional off- the-shelf part acceptance, with additional material for ASIC devices. However, before we address this material we will identify part acceptance work that occurs as subordinate activity in other sections of this guide.

Part Acceptance Throughout the NASA ASIC Guide

Part acceptance runs through many ASIC activities, from initial requirement definition to analysis of test and screening data used in determining the disposition of delivered parts.

PART ACCEPTANCE IN FUNCTIONAL AND PERFORMANCE SPECIFICATIONS

Part acceptance largely consists of comparing specified part behavior with actual part behavior. Section Three: Chapter 1: "Technical Specification" addresses in detail the ways to specify part behavior requirements. Devices must be accepted or rejected on the basis of how well they match these specified requirements to assure the operational quality of the system in which they will function.

PART ACCEPTANCE IN PROCESS AND DESIGN QUALIFICATION

The methodologies adopted for designing and fabricating an ASIC prove critical to part acceptance. Section Two: Chapter 2: "Technical Evaluation" covers the details of validating fabrication technologies, design approaches, and CAD tool sets. These evaluations consider issues central to successful part acceptance, such as testing and process control.

The part acceptance process provides no guarantees. Part acceptance is a set of commonly accepted activities that verify each part produced can pass certain contracted tests. These end-of-the-line tests may reveal anomalous defects but they do not reveal every possible failure mechanism to which the part may be vulnerable throughout its lifetime. Fabrication process quality directly relates to the presence and severity of failure mechanisms. Therefore, we cannot overstate the importance of choosing a vendor who has a qualified process line.

PART ACCEPTANCE IN GOVERNMENT QUALIFICATION PROGRAMS

We have based the guide on two government programs that qualify manufacturers to produce ASICs: QML (MIL-I- 38535) for gate arrays and standard cell devices, and QPL for gate arrays (MIL-M-38510/605/606/607/608).

Government-qualified services greatly reduce the cost of part acceptance by generically qualifying vendors to produce many types of parts. This alleviates most of the qualification expense that would be required per part without government qualification.

The appendix "Government Qualification Programs" has detailed discussions of these programs and how they relate to part acceptance.

PART ACCEPTANCE AS IT RELATES TO QUALITY AND RELIABILITY

Part acceptance aims to obtain parts with high quality and reliability. Therefore, designers and their managers must know how to assess quality and reliability.

Definitions for Quality and Reliability

"The user must be deeply involved in the verification process, specifying tests and screens, supplying test vectors, providing test diagnosis, and verifying engineering parts."

The terms "quality" and "reliability" have vague meanings in government standards and the integrated circuit industry. For clarity, we define these concepts as follows. "Quality": the extent to which the user organization gets what it requires explicitly in its part contract and implicitly in the vendor's in-house specifications. "Reliability": the likelihood of a part to operate according to its specification throughout its specified lifetime.

Part Acceptance through Quality Assurance
The vendor attempts to deliver quality parts by passing them through in-line and end-of-line tests. To make an unbiased assessment about the actual quality of these parts requires an impartial group of engineers. Quality Assurance, an organization independent of vendor production engineers and the ASIC design team, performs this role by administering tests to determine whether the ASIC conforms to its specification.

Part Acceptance as it Relates to Reliability

"Part acceptance ensures parts with high quality and reliability."

Reliability calls for prediction, making its requirements more difficult to assess than quality, which only requires description. But these predictions determine whether a part will be accepted for flight. The vendor and sometimes the user apply many tests that yield information that helps determine how long a part is likely to last. These tests, along with test structure measurements, provide the necessary data to make reliability predictions.

We discuss how tests and test structures provide predictive data in the appendix "Reliability".


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