Table 3 III. Breakdown of the 676 I/O and 256 I/O BGA assemblies
Related Figures (18)
Table I. Investigated components Table II. Reballing process matrix *No-clean alpha UP78-OSP; °No-clean alpha OM338. (1) Sn-Pb solder profile: 70 s to 80 s above 183°C, peak 215°C. (2) Sn-Pb modified solder profile: 100 s above 183°C, peak 215°C. (3) SAC solder profile: 50 s to 60 s above 217°C, peak 230°C. (4) SAC solder profile: 50 s to 60 s above 217°C, peak 240°C. Fig. 1. Test vehicle with six BGAs on each PCB; board dimensions: 8 inches (203.2 mm) x 4.5 inches (114.3 mm) x 93 mils (2.36 mm). Fig. 2. Weibull plot of 676 I/O reballed BGA assemblies with different ball removal procedures (SW, solder wick; LTWS, low- temperature wave solder). Vlicrostructural Analysis of Reballed Tin-Lead, Lead-Free, and Mixed Ball Grid Array Assembli Under Temperature Cycling Test Fig. 3. Weibull plot of 256 I/O reballed BGA assemblies with differ- ent ball attachment methods (PF, preform; BD, ball drop). Table IV. Cycles-to-failure data of 676 I/O and 256 I/O BGA assemblies Fig. 4. Unreliability of 676 I/O lead-free and mixed BGA assemblies. Fig. 6. Separated intermetallic layer at the component side of a reballed 256 I/O BGA assembly (LTWS + PF) (bright-field optical microscopy). Fig. 5. Unreliability of 256 I/O lead-free and mixed BGA assemblies. Fig. 7. Microstructural comparison of as-received (a—c) and temperature-cycled (d—-f) Sn-Pb solder joint (failed at 2401 cycles) of reballed 256 1/0 BGA assemblies (LTWS + PF). Phase coarsening is visible all over the solder joint, but intense phase growth can be seen in the crack area (ESEM: bright = Pb rich, dark = Sn rich). Microstructural Analysis of Reballed Tin-Lead, Lead-Free, and Mixed Ball Grid Array Assemblie Under Temperature Cycling Test Fig. 8. Crack propagation in temperature-cycled reballed 256 I/O BGA assemblies (LTWS + BD, ESEM). (a) The crack propagates along the phase boundary of a Pb-rich layer at the board side (solder joint failed at 1989 cycles). (b) The crack propagates through the bulk solder close tc the component side (solder joint failed at 2858 cycles). There is also a Pb-rich layer at the interface, which did not cause the failure. Fig. 9. Changes in grain structure in reballed Sn-Pb solder joints (LTWS + PF): (a) as-received solder joint, and (b) temperature-cycled solder joint (failed at 2201 cycles). Detailed pictures of the phase coarsening area close to the crack are shown in (c) and (d). (d) Sn grains in this area show grain refinement and changes in orientation (256 I/O BGA, cross-polarized optical microscopy). Microstructural Analysis of Reballed Tin-Lead, Lead-Free, and Mixed Ball Grid Array Assemblie Jnder Temperature Cycling Test Fig. 10. Microstructural comparison of non-reballed SAC305 as-received solder joint (a—c) and temperature-cycled solder joint (d—f) (failed at 5230 cycles). IMC coarsening is visible all over the solder joint, but an intense phase growth can be seen in the crack area (256 I/O BGA, ESEM). Fig. 11. Changes in grain structure in non-reballed SAC305 solder joints: (a) as-received, and (b) after temperature cycling (failed at 2197 cycles). Detailed pictures of the phase coarsening area close to the crack are shown in (c) and (d). (d) Sn grains in this area show grain refinement and changes in orientation (256 I/O BGA, cross-polarized optical microscopy). Microstructural Analysis of Reballed Tin-Lead, Lead-Free, and Mixed Ball Grid Array Assembli« Inder Temperature Cycling Test Fig. 12. Pb distribution in an as-received mixed solder joint: (a) overview of a solder joint, showing full mixing but heterogeneous distribution of the Pb phase (dark phase) over the solder joint; and (b) detail, showing £-Sn cells surrounded by intermetallics a Pb-rich phases (bright-field optical microscopy). Microstructural Analysis of Reballed Tin-Lead, Lead-Free, and Mixed Ball Grid Array Assemblie Under Temperature Cycling Test Fig. 13. Microstructural comparison of non-reballed mixed (a—c) as-received solder joint and (d—f) temperature-cycled solder joint (failed at 4973 cycles) (SAC305 + Sn-Pb paste, approximately 3 wt% Pb). Coarsening of IMCs and Pb phase is visible. Intense phase growth cannot be seen clearly in the crack area (256 I/O BGA, ESEM). Fig. 14. Changes in grain structure in non-reballed mixed solder joints (SAC305 + Sn-Pb, approximately 3 wt% Pb): (a) as-received solder joint; and (b) after temperature cycling (solder joint failed at 2827 cycles). Detailed pictures of the crack area are shown in (c) and (d). (d) Sn grains in the crack area show grain refinement and changes in orientation (256 I/O BGA, cross-polarized optical microscopy). Fig. 15. Differences in the phase size and shape of the Ag3Sn intermetallics between as-received (a) mixed and (b) lead-free assemblies. These differences may be caused by the change in the solidification sequence and temperatures due to the presence of Pb.
