Effects of Organic Package Warpage on Microprocessor Thermal Performance

Abstract

Efficient heat dissipation is a major challenge for the packaging of high power microprocessors. This paper discusses a novel lid assembly process and characterization techniques that were successfully developed for a high power microprocessor in high volume production. For a high power microprocessor flip chip organic package with underfill, die warpage as a result of CTE mismatch between the silicon and organic substrate is a challenge. A laser-based surface profiling technique was used to characterize the die and package warpage. Die warpage is a function of temperature. There is little die warpage at underfill curing temperatures (stress-free stage), but die warpage increases when the package is cooled from underfill curing temperature to room temperature. The bond line thickness of the thermal interface material between the die and the lid (TIMI) is critical for microprocessor thermal performance, and there is significant TIM bond line thickness variation from die edge/corner to die center as a result of the die warpage. TIM is in compression at the die center and in tension at die edges and die corners. Parallelism between the silicon die and lid surface is another critical factor. If there is serious lid tilt with respect to silicon die surface, one side of TIM will be stretched more that the other side after cooling. Therefore, lid tilt is another factor requires good process control. C-mode Scanning Acoustic Micrography (CSAM) was found to be effective in revealing TIM faults when certain polymers TIMs are used. Another challenge in characterizing the TIM bond line thickness (BLT) is the nature of the gel-type TIM. Even though it is cured, the TIM remains very soft and compliant, which is the desirable property for a good thermal performance and reliability.

A hybrid approach to characterization and life assessment of trilayer assemblies of dissimilar materials under thermal cycles

“…In addition, the material behaviours generally deviate from those of their generic counterparts as the result of their physical or metallurgical interconnection to neighbouring materials [23]. As such, relevant laws governing these behaviours are complex to prescribe [24]; related parameters are difficult to determine for a specific application [25], and book values of the properties as published, show wide ranges of variation [6,26].…”

Thermal modeling for warpage effects in organic packages

“…The seal adhesive and TIM are cured in the same reflow step. The end-of-line package is still warped although the magnitude is much less compared with that immediately after underfill reflow [2].…”

“…Too et al [2] pointed out that the conductive particles in the TIM have better chance to make contact with the silicon and/or lid surfaces at the chip center under compression than at the chip corners as the local Z-direction stress is tensile. This would cause different local thermal resistances at different portions of the chip.…”

In-situ cure assembly and thermal performance verification of electronic packages using chip heat

“…The second package in Fig. 1(b) uses a lid-attach seal material to attach the lid to the chip carrier and can use a lowstructural strength TIM [5]. The third package in Fig.…”