High Power Large Size HFCBGA Thermal Characterization

Abstract

Nowadays, the integrated infrastructure for big data and intelligence analytics is more and more important; which drives the requirement of high power large size integrated circuit (IC) package for data center. High performance flip chip ball grid array (HCBGA) is the mainstream of this kind of application, no matter integrates the functions by system on a chip (SoC), multi chips module (MCM), fan-out (FO) or 2.5D. A 65x65 mm2 one-piece hat type HFCBGA with up to 400 W power is chosen to be the thermal test vehicle (TTV), 5 thermal interface materials (TIM) include the middle to high thermal conductivity gel type TIMs $(\mathrm{K}=3.8\sim 11.5 \mathrm{W}/\text{mK})$ and sheet type graphite TIM are chosen for thermal performance evaluation. The result shows paste type TIM has good coverage with low modulus; but the standard deviation of the bond line thickness (BLT) ∼20%, from theoretical point of view, which may result significant deviation of junction-to-case thermal resistance. However, this phenomenon does not appear from the experiment result in this paper. Graphite TIM has much better BLT control capability, lower BLT standard deviation, ∼6%, due to its BLT is well controlled by the initial sheet thickness. However, its elongation is much lower than paste type TIMs and then resulting lower coverage. Thermal result shows BLT deviation has smaller impact than coverage deviation. And the shortage of sheet type TIM is enlarged in this paper. When the compression force on the graphite TIM is not enough, large thermal resistance will be observed. For the paste TIM with good coverage, the difference of the junction-to-case thermal resistance is no more than 16%, even the thermal conductivity range of TIMs are large (3.8∼11.5 W/mK). In the other hand, the performance of thermal modules has larger difference than package with different TIMs. Several cooling solutions have chosen for testing, the result shows the thermal module chosen dominates the maximum allowable thermal dissipation of package.