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Adhesives Technology for Electronic Applications: Materials, Processing, Reliability

William Andrew

Chapter One

Chapter Outline 1.1. Adhesive types and classifications 2 1.1.1. Classification by form 2 1.1.2. Classification by polymer or chemical type 3 1.1.3. Classification by formulation 5 1.1.4. Classification by curing method 5 1.1.4.1. Heat-cured adhesives 5 1.1.4.2. Ultraviolet (UV)/visible-light-cured adhesives 5 1.1.4.3. Microwave curing 6 1.1.4.4. Moisture curing 6 1.1.5. Classification by function 6 1.1.5.1. Mechanical attachment 6 1.1.5.2. Electrical connection 6 1.1.5.3. Thermal dissipation 8 1.1.5.4. Stress dissipation 8 1.1.6. Classification by intended use 8 1.1.6.1. Die attachment 8 1.1.6.2. Substrate attachment 9 1.1.6.3. Lid attachment 9 1.1.6.4. Surface mounting 10 1.1.6.5. Underfill for flip-chip-attached devices 10 1.1.6.6. Underfill for ball-grid-array (BGA) packages 11 1.1.6.7. Solder replacements 11 1.1.6.8. Particle getters 11 1.2. Summary of packaging technologies 12 1.2.1. Single-chip packaging 12 1.2.1.1. Dual-in-line package (DIP) 13 1.2.1.2. Quad-flat packs (QFPs) 13 1.2.1.3. Area-array devices and packages 14 1.2.1.4. Chip-scale packages (CSPs) 15 1.2.1.5. Lead-on-chip (LOC) 18 1.2.1.6. Chip stacks 18 1.2.2. Surface-mount technology 19 1.2.3. Multichip packaging 19 1.2.3.1. Hybrid microcircuits 20 1.2.3.2. Multichip modules 21 1.2.3.3. High-density interconnect (HDI) overlay process 22 1.2.3.4. Chip-on-board 23 1.2.3.5. Flexible circuits 23 1.2.3.6. Chip-on-flex (COF) 24

1.3. History of adhesives in electronic applications 24 1.4. Comparison of polymer adhesives with metallurgical and vitreous attachment materials 29 1.5. Specifications 29 1.6. Market and market trends 30 References 32

Adhesives are used extensively and are vital in the assembly and packaging of electronic devices, especially in the current proliferation and mass production of electronic hardware. Adhesives are used in assembling semiconductor die, both in single-chip packages and in multichip assemblies. Both bare-chip devices and prepackaged components are attached and electrically connected with adhesives to produce electronic circuits such as printed-wiring assemblies (PWAs), thin- and thick-film hybrid microcircuits, and multichip modules. Adhesives, as pastes or as solid films, are also used in fabricating high-density multilayer interconnect substrates, flexible circuits, flat-panel displays, and a host of other emerging applications including optoelectronics; high-speed, high-frequency circuits; sensors; and smart cards. Due to their low cost, ease of rework, and low-processing temperatures, polymer adhesives have replaced many traditional interconnect materials such as solder, eutectic alloys, and wire, especially for most commercial and consumer electronics. However, because of the large number of adhesives available and the variety of polymer types, forms, and formulations, a basic understanding of adhesives and their properties is necessary in their selection and application to assure performance and to avoid subsequent reliability problems. The prime objective of this book is to provide this basic understanding and define guidelines for selecting and qualifying adhesives based on the product and the conditions the adhesive is expected to meet.

1.1 Adhesive types and classifications

Adhesives used in assembling electronic circuits may be classified based on their physical form, polymer type, molecular structure, formulation, curing method, function, or intended application.

1.1.1 Classification by form

In terms of their physical forms, adhesives may be liquids, solids, or pastes. Liquids may be used where excessive flow or spreading is not a problem. In most electronics applications, however, adhesive flow needs to be contained and controlled during cure especially when used in the assembly of microcircuits. Pastes are generally highly filled with mineral fillers and thixotropic compounds such as fine silica powder to produce semi-solid properties having non-Newtonian flow behavior. They are easily dispensed by forcing them through a needle or by pressing them (squeegeeing) through a screen or stencil. Paste or liquid adhesives, in turn, may be one-part or two-part adhesives.

A one-part adhesive may contain a polymer resin system that cures directly on exposure to moisture, oxygen, UV light, or elevated temperature. Two-part adhesives consist of a separate resin portion, referred to as Part A and a hardener or catalyst portion, Part B, either of which may contain fillers and other formulation ingredients. The two parts are packaged and stored in separate containers. When ready for use, the parts are weighed, mixed, deaerated, dispensed, and cured. Some two-part systems can be formulated as one part by admixing the resin portion with a hardener or catalyst that is unreactive under normal conditions (latent) but decomposes at some elevated temperature to release a reactive curing agent as, for example, the dicy (dicyandiamide) curing of epoxy resins.

Two-part adhesives may also be converted to one-part systems by quickly mixing weighed amounts of Parts A and B, deaerating, freezing, packaging and then storing at temperatures of -40°C or lower. These one-part systems, known in the trade as frozen adhesives, may be purchased from several suppliers in tubes or syringes of various sizes and amounts. Frozen adhesives, generally epoxies, are highly desirable from a manufacturing standpoint since they obviate the risk of human errors by the electronics assemblers of weighing, mixing, and processing small batches of adhesives.

Solid one-part adhesives may also consist of preforms (B-staged or partially cured polymers) that, with additional temperature and pressure, will bond two surfaces by flowing, wetting, and fully curing as with copper-epoxy prepreg laminates. Other solid adhesive forms include film and tape adhesives. Film and preform adhesives often contain acarrier or "scrim" consisting of glass or plastic mesh or cloth for easier handling and bondline thickness control during cure. Film and preforms may be precut to the exact size and configuration needed to attach components to printed-circuit substrates, lids to packages, and insulation to circuit substrates for 3-D module assemblies.

Solid adhesives may also be "hot melt" thermoplastic films or pellets that liquify on heating and resolidify on cooling and are useful in the fabrication of high-density multilayer circuits.

1.1.2 Classification by polymer or chemical type

Adhesives are often referred to by their polymer type. The major polymer types used in electronics packaging are epoxies, silicones, acrylics, polyurethanes, polyimides, and cyanate esters. Some generic properties apply to each type but, in generalizing, one must be careful since there are hundreds of formulations of each type on the market, each with minor or major differences in properties. Even for a specific formulation, an adhesive's processing conditions, such as its cure schedule, can affect the final properties. Chapter 3 provides a discussion of the major polymer types and their properties.

Polymer adhesives may be further classified as either thermoplastic or thermosetting, depending on whether their molecular structures, after curing, are linear or crosslinked. The linear polymers may be straight chains or branched chains (Fig. 1.1). Thermoplastic adhesives melt and flow at a specific temperature or within a narrow temperature range and then quickly resolidify on cooling. Thermoplastic adhesives are sometimes called hot-melt adhesives and are particularly advantageous in their ease of processing and reworking. Most polyurethanes and polyamides fall in this category. Although thermoplastic polymers are linear, not all linear polymers behave as thermoplastics. Many linear polymers have a high aromatic or heterocyclic content in their structures, thus increasing their thermal stabilities to a temperature that, instead of melting, char and decompose like thermosetting polymers. Among these are polyimide pastes. Other thermoplastic film adhesives that are fully polymerized, high-molecular-weight resins behave in the same way; examples are polyetheretherketone (PEEK®, a trademark of Victrex), polyetherimide (ULTEM®, a trademark of General Electric), fluorinated ethylene propylene (FEP), polysiloxane imide, and some epoxies.

Thermosetting adhesives, on the other hand, soften as the temperature is increased to the glass-transition temperature, but do not melt and resolidify. Instead, because of their highly cross-linked macrostructures, they decompose and char on reaching their decomposition temperatures. Epoxies, cyanate esters, and phenolics are generally thermosetting types.

A unique adhesive type is silver-glass; a paste consisting of a glass matrix blended with silver particles, a polymer binder, and a solvent. Silver-glass adhesives are not considered polymer types since the polymer binder is burned off during final processing. The polymer resin is added to the formulation to give the paste, the thixotropic properties needed during dispensing or screen-printing. Silver-glass adhesives are processed at much higher temperatures (400–450 °C) than either polymer adhesives (75–175 °C) or solders (180–300 °C). These high temperatures are required to burn-off the polymer binder, melt the glass, and fuse the glass and oxides.

1.1.3 Classification by formulation

In many adhesives formulations, the resin portion is the same or similar and what determines its properties is the hardener or catalyst that is used to cure the resin. Thus, depending on the hardener, epoxy adhesives may be referred to as amine-cured, anhydride-cured, polyamide-cured, or novolac-cured. Polyurethanes may be polyolcured or hydroxypolyester-cured.

Formulations may also be either solvent-based or 100% solids. One-hundred percent solids adhesives contain no solvents and are rapidly replacing the solventbased types because of environmental concerns over volatile organic compounds (VOC), government regulations restricting their use, and reliability concerns.

1.1.4 Classification by curing method

The curing process consists of transforming low-to-moderate molecular-weight resins (monomers or oligomers), generally liquids, into high molecular-weight solid polymers. Most curing mechanisms involve polymerization requiring a catalyst or hardener and are initiated by some form of energy.

1.1.4.1 Heat-cured adhesives

Heat curing by convection in box ovens is the most widely used and simplest method for curing adhesives. Heat is applied until a temperature is reached at which polymerization occurs and continues until the polymerization is complete. Time–temperature curing schedules are developed for each adhesive for which optimum properties are achieved. For each of their adhesives, suppliers provide several cure schedules that are considered equivalent relative to final properties. Users may also develop curing schedules to achieve properties specific for their application and maintain production flow. This can be done experimentally by measuring a desired parameter such as electrical resistivity, dielectric constant, hardness, or bond strength as a function of cure temperature and time. Optimum cures may also be determined from the degree of polymerization as measured by infrared spectroscopy, thermomechanical analysis (TMA), or differential-scanning calorimetry (DSC). The cure times and temperatures required are largely dictated by the hardener or catalyst used. With the rise in mass production of consumer electronics and the need to reduce energy costs, epoxy adhesives known as snap-cured adhesives have been developed that cure in less than 1 minute at temperatures of 160–170 °C. Snap-cured adhesives are generally cyanate ester or modified cyanate esters that form high-temperatures-table triazine structures on curing.

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Excerpted from Adhesives Technology for Electronic Applications: Materials, Processing, Reliability by James J. Licari Dale W. Swanson Copyright © 2011 by James J. Licari and Dale W. Swanson. Excerpted by permission of William Andrew. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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