Editor's Note: This article originally appeared in the June 2012 issue of SMT Magazine.

Abstract

The industry trend toward reduced feature size and faster operating speed has generated increased demand for flip-chip devices. Flip-chip technology enables design flexibility and offers many benefits, but, at the same time, the devices are susceptible to thermal and mechanical stresses and require the use of an underfill to achieve the desired reliability. In addition, advances in the device design to “pack in” more power and functions often impose more demands on underfills.

In many cases the cooling capabilities of these advanced packages still limit device performance, especially when there is limited space for conventional heat dissipation techniques, such as thermal greases, heat sink, and/or fan. To address the issue of thermal dissipation in flip-chip applications, LORD Corporation has developed a thermally-conductive underfill which combines the processing properties and reliability improvement from a traditional underfill with a thermal conductivity of 1.2 W/mK. The thermally conductive underfill is ideal for flip-chip applications where there is limited space for traditional heat dissipation techniques and/or in hermetic packages where no air movement is available to cool the device.

This paper describes the key challenges in developing underfill technologies imposed by package geometries and performance requirements. These demands on underfills require innovative formulation design using novel polymeric materials and specially-selected fillers. We will discuss the design of a non-anhydride underfill with low viscosity, small particle size filler, fast flow, and high reliability.

Introduction
 
Continued advances in flip-chip design and technology have made flip-chip devices more versatile and multi-functional. It has allowed penetration into many different applications with various restrictions and performance requirements. All of these FC devices still require an underfill to achieve desired reliability, but each is unique by itself to fit the specific requirements and restrictions. Thus it creates challenges for material suppliers to formulate different underfills for different device types. When the flip chips are designed with increased functionality, and more power is packed into the ever-smaller form factors, heat dissipation and thermal management become challenging, especially in a confined space.

Figure 1: Heat dissipation and thermal management become challenging in a confined space.