Hybrid Microcircuit Technology Handbook

Materials, Processes, Design, Testing and Production

James J. Licari

• Publisher: William Andrew
• Year: 1998
• ISBN:
  9780815517986
• Edition: 2
• Topic: Tecnologia e ingegneria
• Subtopic: Microelettronica

The Hybrid Microcircuit Technology Handbook integrates the many diverse technologies used in the design, fabrication, assembly, and testing of hybrid segments crucial to the success of producing reliable circuits in high yields. Among these are: resistor trimming, wire bonding, die attachment, cleaning, hermetic sealing, and moisture analysis. In addition to thin films, thick films, and assembly processes, important chapters on substrate selections, handling (including electrostatic discharge), failure analysis, and documentation are included. A comprehensive chapter of design guidelines will be of value to materials and process engineers, chemists, and electrical engineers who design and test hybrid circuits.

1

Introduction

1.0 CLASSIFICATION OF MATERIALS FOR MICROELECTRONICS

Before proceeding to a discussion of hybrid circuit and multichip module processes, it is important to have a general understanding of the materials most commonly used in today’s electronic equipment. Indeed, it is difficult to dissociate processes from materials. All electronic devices and circuits are based on combinations of three general types of materials. These are conductors, insulators, and semiconductors, and are defined in terms of their abilities to either conduct or impede the flow of electrons.

1.1 Conductors

Conductivity is a function of the number of free electrons and their mobility. Conductors conduct electricity readily because they have many free and mobile electrons available to carry the current. The best electrical conductors are metals, generally elements residing in Groups I, II, and III of the Periodic Table. Elements in Group IB, typically metals having a valence of 1, are most conductive. Because these elements contain only one electron in their outermost shells, the electron is easily released from the forces of attraction of the nucleus. Thus, elements of Group IB (copper, silver, and gold) are among the best conductors known and the most widely used in electronics. Metals with valences of 2 or 3 are also good conductors, but not as good as those of Group IB. Among these are aluminum, palladium, platinum, nickel, and tungsten. A comparison of the resistivities (inverse of conductivities) of various metals is given in Table 1. The metals and alloys most widely used in the fabrication of electronic circuits and systems and their general applications are given in Table 2.

Table 1

Electrical Resistivities of Metal Conductors.

Metal	Symbol	Resistivity (ohm-cm × 10^−6)
Silver	Ag	1.62
Copper	Cu	1.69
Gold	Au	2.38
Aluminum	Al	2.62
Tungsten	W	5.52
Nickel	Ni	6.9
Indium	In	9.0
Platinum	Pt	10.52
Palladium	Pd	10.75
Tin	Sn	11.5
Tin-lead	Sn-Pb (60–40)	14.99
Lead	Pb	20.65

Table 2

Commonly Used Conductors and Their Uses in Electronics.

Conductor	Symbol	Use
Gold	Au	Wire, plating, thick films, thin films, filler in epoxy conductive adhesives for device attachment
Gold-platinum	Au-Pt	Thick-film solderable conductors
Gold-palladium	Au-Pd	Thick-film solderable conductors
Gold-silver	Au-Ag	Low-cost thick-film solderable conductors
Gold-silicon	Au-Si	Eutectic attachment of devices for ohmic contact
Gold-germanium	Au-Ge	Eutectic attachment of devices for ohmic contact
Silver	Ag	Low-cost conductor, thick-film plating, thin film, EMI, filler for epoxy conductive adhesives
Silver-palladium	Ag-Pd	Thick film (low silver migration)
Silver-platinum	Ag-Pt	Thick film (low silver migration)
Copper	Cu	Thick film (low cost), thin film, foil and plating for printed circuit boards, wire, filler for epoxy conductive adhesives
Aluminum	Al	Wire, thin-film metallization for devices
Tungsten	W	Thick-film conductors for co-fired tape substrates and packages
Molybdenum-manganese	Mo-Mn	Thick-film conductors for co-fired tape substrates and packages
Nickel	Ni	Barrier metal, low conductivity metal, plating
Tin-lead alloys	Sn-Pb	Solder for attachment of components, plating for circuit boards
Indium and alloys	In	Low-temperature solder for attachment of devices and components
Tin-silver alloys	Sn-Ag	Low-temperature solders

Metals have a positive temperature-coefficient-of-resistivity (TCR); that is, as the temperature of a conductor increases, its resistivity increases or, conversely, its conductivity decreases. However, for the highly conductive metals, (Al, Au, Ag), the effect is not significant in the temperature range over which circuits are tested or used (room temperature to about 150°C), Fig. 1.

Figure 1 Electrical resistivity of metals versus temperature. (note positive TCRs)

1.2 Insulators

At the other end of the spectrum from conductors are the insulators, which are very poor conductors of electricity. Their valence electrons are tightly bound and not free to move within the structure. In an insulator, the bonds that hold the atoms together, as a rule, are covalent (shared electrons) and possess high bond strengths (the energy in kilocalories per mole to break apart a bond pair). An important class of insulators are compounds based on the carbon atom—hydrocarbons, polymers, and plastics fall within this group. The bond energies for C-C, C-H, and C-O bonds are 83, 99, and 110.2 kcal/mol, respectively—thus extremely difficult to break apart to create free-moving electrons. Sufficient energy, of course, could be applied in the form of extreme heat or radiation to eventually rupture the bonds, but this is done only at the expense of causing irreversible damage to the material. Free radicals and smaller molecular fractions would then be formed which are quite different in physical properties from the original material.

Insulators generally reside in Groups V, VI, and VII, of the Periodic Table. Examples of excellent insulators include: plastics, rubber, ceramics, glass, sapphire (a form of aluminum oxide), other metal oxides (aluminum oxide, beryllium oxide, and silicon dioxide), and metal nitrides (silicon nitride, aluminum nitride). Insulators may be solids, liquids, or gases, but only solids are extensively used in electronics. Applications include substrates on which resistors, capacitors, and metal conductors are formed, substrates within which semiconductive, resistive, and conductive areas are generated (ICs), and passivation layers protecting active circuits from moisture and other harsh environments. Solid insulators have resistivities ranging from several megohm-cm to greater than 10¹⁴ ohm-cm, but caution should be used in their selection because in some cases the insulating properties can be rapidly degraded by ionic impurities, moisture absorption, or thermal and radiation exposures. In contradistinction to metals, insulators have negative TCRs (resistivity decreases with increase in temperature). For the ceramics that are most widely used as circuit substrates or as dielectric insulating layers, this decrease is signifi...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• MATERIALS SCIENCE AND PROCESS TECHNOLOGY SERIES
• Preface to the Second Edition
• Preface to the First Edition
• NOTICE
• Chapter 1: Introduction
• Chapter 2: Substrates
• Chapter 3: Thin Film Processes
• Chapter 4: Thick Film Processes
• Chapter 5: Resistor Trimming
• Chapter 6: Parts Selection
• Chapter 7: Assembly Processes
• Chapter 8: Testing
• Chapter 9: Handling and Clean Rooms
• Chapter 10: Design Guidelines
• Chapter 11: Documentation and Specifications
• Chapter 12: Failure Analysis
• Chapter 13: Multichip Modules: A New Breed of Hybrid Microcircuits
• Index