Adriana Zambova

A study balancing physical properties for an optimized thermal interface material

Thermal dissipation requirements for advanced flip chip devices are becoming significant. High end flip chip devices that used lidded packages are now being redesigned to utilize even more advanced materials. Devices that previously did not require a lidded package are now being forced to utilize such a package. The latter scenario can also can benefit from more advanced materials. Initial thermal performance is a key factor for selecting a candidate thermal interface material (TIM), but how well the package can survive reliability conditions is also important. Cost pressures on the overall structural design can also be a significant factor for some types of applications. Polymeric TIMs can address many of these issues by having a high initial thermal performance and by allowing adequate mechanical stress relief to survive reliability conditions. The TIM properties, such as elongation and modulus can be as important as the thermal conductivity. Properties such as thermal resistance and thermal impedance are even more important. This paper will detail how these properties are inter-related and will demonstrate that an optimum balance between them can be achieved.

Fundamental investigation of lid interactions with TIM1 and adhesive materials for advanced flip chip packaging

As flip chip packaging evolves, there are increasing demands on the overall package design in order to maintain reliability while the devices themselves are increasing in performance. A key aspect for advanced flip chip applications that require a heat sink, is to ensure the integrity of the interface between the silicon chip and the heat sink (often a referred to as a lid or heat spreader). The challenging trends for these types of packages include: increased silicon surface areas, changes in substrate type(s), thinner overall package z-heights, and other thermo-mechanical demands which can all result in new, or re-introducing previously-solved, mechanisms that disrupt the thermal interface which is critically important for sufficient heat dissipation. In this research, the relationship between the lid and the materials at this interface is investigated. There are two types of lid/material interactions studied: the lid-to-TIM1 interface and the interface at the perimeter where, for large devices, an auxiliary band of adhesive/sealant is used to improve package reliability. Among the various techniques identified to assess the interactions at this interface, the adhesion performance is considered a primary method. A modified die shear adhesive method was optimized specifically for this application and determined to be the most suitable among several competing adhesion characterization methods. This investigation also includes characterization of the adhesion performance for numerous types of materials using standard microelectronics industry reliability conditions. It was found that following the failure modes, in particular, (as opposed to only the adhesive force data) at select intervals during this battery of stress testing can improve understanding of the interfacial behavior. Additionally, correlation studies were conducted to compare adhesion results with the other techniques typically used at the flip chip package-level, such as manufacturing line parametric tests (e.g. acoustic microscopy) or in the failure analysis lab (dye penetration, cross-sectioning, etc). Finally, an analytical chemistry assessment of the lid surface functionalities was also performed in order to correlate potential chemical interactions with this material set. Subsequently, the feasibility of improving the compatibility between the lid surfaces and the materials was evaluated. A variety of cleaning methods and other techniques were utilized to identify which methods seem to provide some performance benefits without significantly compromising other aspects of the overall package robustness.