This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
2. Tool configuration - XRF configuration, calibration, optimized measuring methodology and the importance of having known standards with the same dimensions of the bumps being measured
3. XRF recipe setting - Measuring accuracy and correlation with ICP and DSC
4. Data interpretation - Ag distribution study in the die and wafer level
One area of interest for Pb-free solder manufacturing is the ability to control and measure the %Ag composition and its variation from wafer to wafer, chip to chip, and C4 to C4.
Invasive Non-Invasive
Atomic Absorption (AA) X-Ray Fluorescence (XRF)
Differential Scanning Calorimetry (DSC)
Inductively Coupled Plasma (ICP)
Electron Probe Micro-Analyzer (EPMA)
Methods to Measure Solder CompositionWafer to Wafer Chip to Chip C4 to C4
Ag% Composition ControlPb-free SnAg solder has become the industry standard for fabricating flip chip interconnects utilizing C4 (controlled collapse chip connection) technology.
Mass Transport Effect The need for the correct geometry comes from the learning that the %Ag is mass transport
controlled. The concentration of Ag in a larger test structure on the edge of a wafer may have a different and most likely higher concentration of Ag than the Ag in an actual C4.
C
Site A has a largest height controlled by diffusion plating compared to B or C
Tool issues such as stage movement accuracy and laser alignment were investigated and then eliminated as possible causes.
The X-ray spot size is 40µm -> a C4 geometry issue was hypothesized.
Reliability dramatically improved after the flattening, proving that C4 geometry impacts the intensity of the signal. Flattening is recommended if the bump height is less than 70um
SEM photos comparing flattened and un-flattened C4’s
The spot size of the X-ray opening is 40 µm.
The detector is collecting photoelectrons from any region that is excited by the X-rays. Flattening reduces the chance of exciting larger regions which would cause artificially higher readings and thus a larger standard deviation.
XRF readings on MG wafers are comparable with ICP data (within 0.2%Ag), but the XRF readings are not reliable on EG wafers due to high background noise
To alleviate this, the XRF collection time was shortened to mitigate primary X-ray penetration.
XRF is a popular, non-invasive inspection method for bump composition.
Several challenges were encountered when establishing this technique.
Balancing X-ray power voltage and current settings to obtain suitable X-ray penetrating ability.
Mitigating background noise, especially from EG chips
Finding an appropriate collection time
Characterizing a tool ability limitation for very low Ag% detection (less than 0.2% Ag).
Through XRF, ICP, DSC comparison, XRF recipes can be developed as an excellent monitor for non-invasive bump composition evaluations. As in all methods, XRF has a unique set of challenges and limitations which can be overcome through proper calibration and verification.
As the packaging trend moves toward finer pitched products, studies of small diameter C4s will become pervasive. If XRF is preferred to measure the bump composition of smaller C4s, geometry will become a key issue. C4 flattening helps mitigate geometry issues on smaller C4s, but may be insufficient for micro-bump features. Future work will involve how to define C4 geometry and the possible creation of composition test sites which will need to have extensive correlation established.
J. Sylvestre, et al.,”The Impact of Process Parameters on the Fracture of Brittle Structures During Chip Joining on Organic Laminates,” 2008 ECTC.
E. Perfecto, et. al, “C4NP Technology: Present and Future,” 2008 IMAPS Device Packaging Conference.
B.. Beckhoff B. Kanngießer N. Langhoff R.Wedell H.Wolff (Eds.) in Handbook of Practical X-Ray Fluorescence Analysis 2006.
Special thanks go to Chia-Hsin Shih and KyungMoon Kim from STATSChipPAC as well as Charles Arvin and Eric Perfecto from IBM for their guidance and support. Thanks also go to John Pennacchia, Stephen McLaughlin and the entire C4 team.