EVALUATING THE ASSEMBLY OF MIRRORED LARGE HIGH-DENSITY BGA PACKAGES

摘要

The mirrored BGAs application has been used in memory cards or other applications because of real estate and improved electrical performance constraint. Normally low pin count BGAs were selected for such applications and the mirrored BGAs are considered as one component from the design point of view. The application of mirrored large high-density BGAs has not been popularly adopted in the industry due to the testability / manufacturability / reworkability / reliability concerns and the possibility of increasing PCB layers which means the increase of material cost and assembly process difficulty. Besides, good heat transfer designs may be required for mirrored high-density BGAs with high heat dissipation level. Heat-pipes can be installed especially for the bottom BGAs to improve heat transfer efficiency. 3D Laminograpy X-ray system is capable of detecting the solder joint integrity for mirrored BGAs. Beside the testability concerns, the manufacturability and reworkability of the application of mirrored high-density BGAs were also validated in this evaluation. The eutectic Sn-Pb solder joints of mirrored FCBGAs (test vehicles) were subjected to temperature cycling between 0℃ to 100℃ with 40 minutes per cycle for 1000 cycles. There is no failure found after the 1000 cycles. One of the findings in this study is the stand-off heights of bottom mirrored FCBGA solder joints were elongated with average 1.2 mil taller than that of top mirrored FCBGA solder joints due to gravitational stretch during the second reflow process. Since there could be the difference in solder joint (thermal) fatigue life between the top and bottom mirrored FCBGAs and we would like to characterize the thermal cycling life distribution of the mirrored FCBGAs, the temperature cycling test will be resumed until the failure found on both top and bottom mirrored FCBGAs.