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Lead-Free Solder Ball Attach Improvement on FCPBGA with SOP Pad Finishing
Keywords: Flip Chip Plastic Ball Grid Array, lead-free solder, solder on pad
For Flip Chip Plastic Ball Grid Array (FCPBGA), recent industry trend is changing from ENIG pad finishing to Solder-on-pad (SOP) pad finishing to improve board level reliability due to black pad issue with ENIG. However, SOP has posted a challenge in ball attachment process due to higher oxide level on SOP pads. The use of Sn3.8Ag0.7Cu solder ball on SOP pad finishing has been facing slanted ball and wrinkled ball issues after ball attach reflow. Slanted ball is a defect as it fails the solder ball radius-true-position and coplanarity specifications. Wrinkled ball surface topography may cause solder voids in the solder joint after board mounting and therefore is of critical importance to solder joint reliability. In this paper, Sn3.5Ag solder ball on 33x33mm FCPBGA with SOP was being compared with Sn3.8Ag0.7Cu solder ball. After assembly, samples were subjected to laser scanning for slanted ball inspection. Visual inspection under low power scope was done to check for wrinkled balls. Cold ball pull (CBP) was used to evaluate the solder joint strength after assembly (T0), after 6x reflow and after 168 hours high temperature storage (HTS). Tray and packing drop test were conducted to assess solder joint integrity due to handling and impact force. Solderability test was performed to assess board mounting reliability. The result of this study showed that Sn3.5Ag solder ball resolved the slanted and wrinkled ball issue. The cold ball pull result showed no significant different within Sn3.5Ag and Sn3.8Ag0.7Cu ball at T0, 6x reflow and 168 hours HTS. Also, no ball drop in tray and packing drop test. All the samples also passed solderability test. Finally, Sn3.5Ag is recommended to replace Sn3.8Ag0.8Cu ball for 33X33mm FCPBGA package to improve ball attach yield by resolving the slanted ball issue and to improve the solder joint integrity by resolving the wrinkled ball issue.
Wong Tzu Ling, R&D Engineer
Freescale Semiconductors Malaysia Sdn Bhd
Petaling Jaya, Selangor 47300,
Malaysia


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