An Engineer’s Handbook of Encapsulation and Underfill Technology

Citation

Kingsley, D. (2000), "An Engineer’s Handbook of Encapsulation and Underfill Technology", Microelectronics International, Vol. 17 No. 2, pp. 34-35. https://doi.org/10.1108/mi.2000.17.2.34.2

In today’s environment of increasing markets for consumer electronics the need to reduce manufacturing costs is paramount, whilst maintaining high performance and relaibility.

Since the 1960s, glob‐topping and underfilling with encapsulant materials has replaced many of the early hermetic packages which heralded the birth of micro‐miniaturisation.

In the author’s words, this book aims to assist personnel engaged in the design, development and/or manufacture of encapsulated and underfilled components with the selection and use of encapsulant materials.

“Introduction to glob‐top and underfill encapsulation materials and techniques”, Chapter 1, commences with a historical perspective of resin encapsulants in semi‐conductor packaging, and current trends. An overview of current organic resin encapsulant materials is then presented.

Chapter 2, “The structure and properties of resin encapsulants”, examines in some detail the most widely used materials, i.e. epoxies, silicones and polyimides. Mention is also made of polyurethanes, acrylates and polyxylenes.

Table 2.1 summarises the generic properties of base resins used in encapsulant material whilst Table 2.2 gives the selected physical properties of typical commercially available glob‐top materials. The chapter concludes with notes on the effect of filler additions to encapsulants, including a table of physical properties.

“Failure mechanisms in encapsulated semiconductor devices” is thoroughly examined in Chapter 3. Distinction is made between the die, package and environment.

Die‐related failure mechanisms are briefly described, i.e. aluminium migration, stress in AI tracks, oxide defects and change of surface state.

Principal failure mechanisms related to the effects of device packaging and operating environments are discussed in detail and include mechanical stress, thermomechanical stresses and metallurgical factors.

Brief notes are included regarding environmental and misuse factors such as α‐particle radiation, electrical overstress, electrostatic discharge and solvent exposure. A discussion of AI metallisation corrosion mechanisms then follows.

The chapter concludes with notes on failure modes in underfilled flip‐chipped componens and includes bump metallisation/die passivation effects, fatigue of bump metallisation and amoeba flow. Finally, the effect on adhesion of surface contamination is discussed in detail.

Chapter 4 is entitled “Material properties requirements for encapsulant materials”. The author states, “The aim of this chapter is to determine and discuss the performance characteristics necessary for a material to function adequately as a glob‐top, gravity‐fill or underfill encapsulant”.

Following a literature survey, a summary of performance requirements for a successful glob‐top material is listed.

A review of general material property requirements then follows and includes thermomechanical properties (glass transition temperature, modulus of elasticity, thermal co‐efficient of expansion and cure shrinkage), corrosion failure relationships (adhesion, mobile ion content, water extract conductivity, mositure permeability), electrical properties, thermal conductivity and optical properties.

Information is given regarding solvent resistance of cured encapsulant, mixing and storage of resins, rheology of resin mix and final curing of encapsulant.

Dual‐layer glob‐top systems are next explained, i.e. the use of a layer of hard encapsulant (e.g. epoxy) over a softer compliant material such as silicone, which can minimise thermomechanical stress on the die.

The chapter concludes with notes on the selection of encapsulant materials.

“Test methods for glob‐top and underfill encapsulants”, Chapter 5, summarises the methods and procedures used to test materials and encapsulations and to investigate failed encapsulated devices.

A useful summary of test methods applied to encapsulant materials is given in Table 5.1. These include adhesion, Young’s modulus, hardness, TCE, glass transition temperature, dispensing and flow characteristics, moisture content and electrical properties. All of these procedures are described in great detail, together with their results.

A summary of test methods for encapsulated components is given in Table 5.2, covering topics such as thermal shock/cycling, stress measurement, relative electrical performance, accelerated ageing and salt cell test. Again, these procedures are given in excellent detail and clarity.

Failure analysis techniques cover subjects such as non‐destructive inspection, decapsulation and inspection methods.

Finally, there is a short guide to the analysis of failed components.

Chapter 6, “Processing of encapsulation and underfill materials”, is intended to be of assistance to process engineers and technicians in achieving maximum production efficiency and product quality.

Under the heading, “General Issues”, topics such as preparation of encapsulant materials and surface cleaning/verification methods are discussed.

The subject of die encapsulation dispensing gives details of manual and automated dispensing methods, including the basic engineering principles of fluid dispensing.

Stencil printing of glob‐top encapsulants is briefly discussed.

A review of underfilling methods then follows, and the chapter concludes with descriptions of encapsulant curing methods, i.e. thermal and radiation.

The latest trends are described in Chapter 7, “New developments in encapsulant and underfill technology”.

Encapsulant material development includes polyesters, ormocers, reworkable thermoset epoxies, dimethacrylate systems and filler particle morphology.

A brief review of industry standards and specifications for encapsulant and underfill material is given.

New developments in processing covers curing methods, pre‐deposited self fluxing underfills whilst performance criteria are examined under the following headings: Effect of thermomechanical properties on solder bump fatigue life; Importance of underfill flow properties; Effect of adhesion on component reliability; Effects of moisture absorption in underfill encapsulants; and, finally, A summary of underfill material properties required for optimum solder joint reliability.

This extremely well researched book will be welcomed in the electronics industry by all packaging engineers who wish to know more about a relatively new technology.

Apart from an easy to read style of writing, the author has included many excellent figures and references including a glossary of terms.