An Evaluation of Thermal Enhancements to Flip-Chip-Plastic Ball Grid Array (FC-PBGA) Packages

Abstract

Flip-chip plastic ball grid array (FC-PBGA) packages are fast becoming the industry norm, in particular in the performance and cost driven consumer electronics sector. Since high thermal conductivity (k∼15–20 W/(m K)) ceramic substrate is replaced by a low conductivity (k∼0.2–0.5 W/(m K)) organic substrate in the FC-PBGA packages, enhancement of thermal performance of these packages to meet ever increasing demands is crucial for their wide spread use. In this study, enhancements to thermal performance of FC-PBGA packages due to material and design changes and external means such as heat spreaders and overmolding of the packages have been evaluated by solving a conjugate heat transfer models using the methods of computational fluid dynamic. The thermal enhancements evaluated in this study include the effect of thermal conductivity of the chip to package interconnect due to change in underfill material and the C4 bump pitch, effect of package to printed wiring board (PWB) interconnection through the use of thermal balls, effect of a heat spreader on the backside of the die, and overmolding the die without and with a heat spreader. Thermal performance of the FC-PBGA packages have been studied using junction to ambient thermal resistance, Θja, junction-to-board thermal resistance Ψjb, and junction to case thermal resistance ΨjT under natural and forced convection for freestream velocities up to 2 m/s and the for following ranges of parameters: Substrate size: 25 to 35 mm, die size: 6.19×7.81 mm (48 mm2 area) and 9.13×12.95 mm (118 mm2 area), C4 pitch: 250 mm, 150 mm and below, underfill material thermal conductivity: 0.6 to 3.0 W/(m K), no thermal balls between the package and the PWB to 9×9 array of thermal balls on 1.27 mm square pitch, and with copper heat spreader on the back of the bare and overmolded die. Based on previous experience, predictions in this study are expected to be within ±10% of measured data. The following conclusions are drawn from this study: 1. It is concluded that the thermal conductivity of the underfill materials in the range 0.6 to 10 W/(m K) is negligible. 2. It is also concluded that the bump pitch can decrease thermal resistances by 12 to 15 %. The change may be smaller with large die area. 3. Thermal balls (C5) connected to the PTHs in the PWB can decrease thermal resistance by about 10% to 15% as the number of thermal balls & PTHs increase zero to 9×9 on 1.27 mm pitch. The effect die size on this thermal enhancement is more noticeable on Ψjb. 4. Heat spreader on the back of the die decreases Θja by a small amount (6–7%) in natural convection and a large amount, about 25% in forced convection. 5. Overmolded die with heat spreader on the top of the overmold provides better thermal enhancement than heat spreader alone up to about 1 m/s. Beyond 1 m/s, heat spreader (without overmold) performs slightly better.