Evaluation of thermal enhancements to flip-chip-plastic ball grid array packages

Enhancements to thermal performance of flip chip plastic ball grid array (FC-PBGA) packages due to underfill thermal conductivity, controlled collapse chip connection (C4) pitch, package to printed wiring board (PWB) interconnection through thermal balls, a heat spreader on the backside of the die, and an overmolded die with and without a heat spreader have been studied by solving a conjugate heat transfer problem. These enhancements have been investigated under natural and forced convection conditions for freestream velocities up to 2 m/s. The following ranges of parameters have been covered in this study: substrate size: 25-35 mm, die size: 6.19 x 7.81 mm (48 [mm.sup.2] area) and 9.13 x 12.95 mm (118 [mm.sup.2] area), underfill thermal conductivity: 0.6-3.0 W/(m K), C4 pitch: 250 [micro]m and below, no thermal balls to 9x9 array of thermal balls on 1.27 mm square pitch, and with copper heat spreader on the back of a bare and an overmolded die. Based on our previous work, predictions in this study are expected to be within [+ or -] 10% of measured data. The conclusions of the study are: (i) Thermal conductivity of the underfill in the range 0.6 to 10 W/(m K) has negligible effect on thermal performance of FC-PBGA packages investigated here. (ii) Thermal resistances decrease 12-15% as C4 pitch decreases below 250 [micro]m. This enhancement is smaller with increase in die area. (iii) Thermal balls connected to the PTHs in the PWB decrease thermal resistance of the package by 10-15% with 9x9 array of thermal balls and PTHs compared to no thermal balls. The effect of die size on this enhancement is more noticeable on junction to board thermal resistance, [[PSI].sub.jb], than the other two package thermal metrics. (iv) Heat spreader on the back of the die decreases junction-to-ambient thermal resistance, [[THETA].sub.ja], by 6% in natural convection and by 25% in forced convection. (v) An overmolded die with a heat spreader provides better a thermal enhancement than a heat spreader on a bare die for freestream velocities up to about 1 m/s. Beyond 1 m/s, a heat spreader on bare die has better thermal performance.