研究目的
To investigate innovative WLCSP structures and materials, specifically ball support mechanisms and Bismuth-bearing solder balls, to improve solder joint reliability in temperature cycling without underfill while maintaining drop test performance.
研究成果
The new WLCSP structures with ball support mechanisms and SACQ solder ball significantly improved temperature cycling reliability without compromising drop test performance. This allows for the use of larger WLCSPs in harsher environments and existing mobile designs, providing a viable alternative to underfill.
研究不足
The study focused on specific WLCSP structures and alloys; results may not generalize to other package types or conditions. The multi-modal failure distribution in some legs was not fully explained, and the impact of SACQ alloy on drop test reliability requires further investigation. Testing was limited to 3004 cycles for temperature cycling and 1000 drops for drop tests.
1：Experimental Design and Method Selection:
A Design of Experiment (DOE) test matrix was constructed with five different large die WLCSP test vehicles. Variables included ball support structures (front-side mold and 5-side mold) and solder ball alloys (SAC405 and SACQ). The study involved motherboard assembly and board level reliability testing, including drop testing and temperature cycling with in-situ electrical monitoring for Weibull analysis.
2：Sample Selection and Data Sources:
WLCSP test units with daisy chain connections were designed with a 7.525 x 7.525-mm die size, 18 x 18 full ball array, 0.4 mm ball pitch, and 324 balls. All units were manufactured with 300-mm wafers using a standard 4-mask WLCSP process.
3：525 x 525-mm die size, 18 x 18 full ball array, 4 mm ball pitch, and 324 balls. All units were manufactured with 300-mm wafers using a standard 4-mask WLCSP process.
List of Experimental Equipment and Materials:
3. List of Experimental Equipment and Materials: Equipment included event detectors for in-situ monitoring, drop test apparatus, temperature cycling chambers, and cross-section analysis tools. Materials included SAC405 and SACQ solder balls, polyimide dielectric, copper redistribution layers, under bump metallurgy, and PCB test vehicles with OSP finish.
4：Experimental Procedures and Operational Workflow:
Packages were assembled onto PCBs using SAC387 solder paste and a Pb-free reflow profile. Assembled boards were subjected to drop tests (1500 G peak acceleration, 1.0 ms pulse duration) and temperature cycling (-40 to 125°C, 2 cycles per hour) with continuous electrical monitoring. Failure analysis was performed via cross-sectioning and optical microscopy.
5：0 ms pulse duration) and temperature cycling (-40 to 125°C, 2 cycles per hour) with continuous electrical monitoring. Failure analysis was performed via cross-sectioning and optical microscopy.
Data Analysis Methods:
5. Data Analysis Methods: Data were analyzed using 2-parameter Weibull statistical distribution to determine characteristic life (Eta) and shape (Beta) values. Failure modes were identified through cross-section analysis.
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