BY DR. PHIL GARROU, Contributing Editor
At the 2016 ECTC Conference, TSMC discussed their UFI (UBM-Free Integration) Fan-In WLCSP technology which they claim enables large die fine pitch packages.
Development of low-cost WLCSP for large die with high I/O count is desired for broadening its applications. Reliability issues including solder cracking and high chip warpage are known to be the main challenges for extending the die size of conventional WLCSP to more than 5×5 mm2 with ball pitch smaller than 350 μm.
TSMC has discovered that by controlling the maximum strain location and optimizing materials, chip warpage and the stress between silicon and the PCB can be reduced which improves both component and board-level reliabilities of WLCSP packages. Packages as large as 10.3×10.3 mm2 with both 400 and 350 μm ball pitches have been developed.
UBM is used as an interfacial layer between the metal pad of the integrated circuit and the solder ball. The formation of UBM/ solder intermetallic compounds (IMC) limits the board level reliability of the package due to the poor mechanical robustness of IMCs. When the die size is increased, stress increases which promotes cracking at the UBM/solder ball interface.
TSMC claims their UFI WLCSP fabrication cost is lower than conventional WLCSPs due to the elimination of the UBM. Removal of the UBM also reduces the thickness of the package by 30%. Figure 1 compares the structures of a standard WLCSP vs the TSMC UFI WLCSP. In the UFI WLCSP, the solder balls are directly mounted to the Cu RDL followed by the polymeric PL (protection layer which secure the balls.
Very similar removal of UBM and subsequent thickening of the copper pad has been reported before by Amkor in 2010 [1].
TSMC simulation results showed the solder joint fatigue life decreases with increasing die sizes for both UFI and the conventional WLCSP. Predicted solder ball fatigue life was found to increases with decreasing die thickness. The authors suggest that decreasing the die thickness not only reduces the thermal expansion difference between the die and the PCB, but also causes the die to bend more under thermal loading. In addition, simulation results imply that solder joint creep strain for solder mask defined (SMD) structures is 72% higher than for non-solder mask defined (NSMD) structures because of its reduced flexible solder joint height and the constraint of the solder mask. Thus they concluded that it is better to use NSMD type of PCB for UFI WLCSP. The use of NSMD structures to increase reliability has been known since the work of Bell Labs Ejim [2].
The UFI WLCSP passes all component-level tests and exhibited board-level thermal cycle life that is 1.4 and 2.3 times longer than that of the conventional WLCSP in terms of the first failure and the Weibull distribution, respectively. 10mm UFI WLCSP have passed component-level reliability tests such as TCB1000, uHAST96 and HTS1000, and board- level reliability tests of TCG500 and drop tests.
To demonstrate the possibility of higher interconnect density, they fabricated UFI- WLCSP with multiple RDL layers. The package with two RDL layers had die size of 10.3 x 10.3 mm2 and ball pitch of 350 μm (Figure 2). Again such structures passed all component level reliability testing.
References
1. http://imapsource.org/doi/abs/10.4071/2010DPC- tha32?journalCode=apap
2. TI Ejim et. al., “Reliability performance and failure mode of high I/O thermally enhanced ball grid array packages” Electronics Manufacturing
Technology Symposium, 1998, p.323 – 332.