IWLPC (Wafer-Level Packaging) Conference Proceedings


TAILORING THE COEFFICIENT OF THERMAL EXPANSION OF A PRINTED CIRCUIT BOARD OR INTEGRATED CIRCUIT SUBSTRATE

Authors: Carol Burch, Kris Vasoya
Company: ThermalWorks
Date Published: 11/3/2005   Conference: IWLPC (Wafer-Level Packaging)


Abstract: As the roadmap for silicon die mounted directly onto a printed circuit board advances, many technologies such as Die Attach Film, ACF/NCF, ACP/NCP, underfill and adhesion materials have moved quickly into mainstream use, gaining acceptance in products such as cellular phones, PDA’s and disk drives. The die used in these applications is usually smaller in size, and solder interconnect failures are not as common with these lower shock and vibration applications.

As the world moves to integrate large scale die onto the board with flip chip and direct die attach methods, dangerous Coefficient of Thermal Expansion (CTE) mismatches between the printed circuit board or substrate and the die must be addressed. Carbon has emerged as a new way to constrain the core of a printed circuit board or substrate, therefore allowing a silicon die that has an expansion rate of 2.5ppm/C to sit on organic material that has a similar 2-5ppm/C (parts per million per 1 degree Centigrade) expansion rate.

Decreasing or even eliminating thermally induced strains at the solder joint allows the electronic world to attach silicon without the need for underfill and adhesion material. Until control of the stress that occurs at the flip chip die surface becomes possible, the risk of tomb stoning and cracks at bump level is eminent. Carbon composite laminate is available today which allows the designer to tailor the surface CTE of an organic substrate or printed circuit board down to the silicon die expansion rates and attach even large scale ASIC and Chip technologies directly to the PCB or substrate.

The CTE of an organic PCB can also be tailored to the 6-8ppm/C expansion rate which enables to mount ceramic packages like CBGA reliably. In the following paper we will discuss the properties of a carbon core printed circuit board laminate known as STABLCOR® which will be referred to as composite laminate throughout the study for practical purposes. The benefits of carbon go beyond surface CTE expansion control and include high tensile modulus (rigidity), low weight, and high thermal transfer rates. We will discuss these additional benefits in practical applications as well.

Key Words: Carbon, Constrained CTE, Thermal Benefits, Rigid, Lightweight Printed Circuit Board or Substrate



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