Thermal performance of underfilled BGAs

To achieve adequate solder joint reliability when using small Ball Grid Array (BGA) components, it is often necessary to use underfill encapsulants. Underfills also have an important role in heat transfer from die to board. In this study, the impact of underfill materials on the thermal performance of BGA´s was investigated and quantified. A fixture and a fully automated data acquisition system were developed to measure the junction to case thermal resistance in underfilled BGAs assembled to laminate substrates. In the developed apparatus, the back of the FR-4 printed circuit boards (PCBs) were instrumented with thermocouples, and then maintained at a constant temperature using a water bath. Special thermal test chips with onboard heaters and diode temperature sensors were encapsulated in the BGAs so that the power dissipation could be controlled and the die surface temperature measured. Using the described system, several studies were completed including the rating of the thermal performance of several underfill materials and the examination of various thermal via designs in the PCBs. A 10-30% reduction in the thermal resistance was observed for underfilled components. After initial testing, the samples were subjected to thermal cycling (-40 to 125°C), to evaluate thermal performance degradation (due to cracking of the thermal solder balls, underfill delaminations, and underfill material property changes). Steady state and transient finite element models of the thermal BGA geometries have been correlated to the thermal resistance measurements.