Characterization of Mechanical Properties of Thin Film and Intermetallic Compound for Lead-Free Flip Chip Packages
Date Issued
2005
Date
2005
Author(s)
Tsai, I-Ting
DOI
en-US
Abstract
Technologies for flip chip bonding boomed to meet the requirement of smaller packages with higher performance that can sustain a substantially high temperature even under normal operating conditions. The coefficient of thermal expansion (CTE) mismatch between the chip and the carrier would make mechanical issues of the contact bumps even more severe for the reliability of flip chip packages during temperature changes. When employing the numerical simulation such as finite element methods to predict and optimize the reliability, only if the inputs of material properties are correct, can the outcome be referable. Under bump metallization (UBM) and intermetallic compounds (IMC), which usually have thickness of the thin-film order, are important parts in a contact bump. However, there are rare studies about the material properties of UBM and IMC, which are usually difficult to obtain because of the thickness of the thin-film order. The goal of this study is to develop a method to obtain the mechanical properties of materials with thickness of few micrometers. Copper films are employed due to the emergence as an important material in IC packages in the last few years, and IMC is set as another target because it might dominate the reliability of the interconnections owing to its characteristic material properties.
In this study, methods for determining elastic moduli and CTE of sputtered copper films and Cu3Sn IMC on silicon substrates are presented. The mechanical properties of silicon substrates were measured first. In addition, reflection moiré was developed to measure the slope change of the laminated composite structures subjected to thermal loading. As for analysis methods, the genetic search algorithm (GA) with FEM using ANSYS was then used to inversely obtain the elastic modulus and CTE of thin films with three different thickness and Cu3Sn, which was formed when tin reacted with copper. A large amount of data points such as 9600 points from whole-field slopes of two orthogonal directions make the algorithm more robust and immune to noise up to S/N ratio 10.
The optimally obtained elastic moduli are 105.8 Gpa, 97.5 GPa, and 92.5 GPa for copper films of 2.2 um, 3.7um, and 4.5um thick, respectively. Elastic moduli are less than that in bulk, but increase monotonically with decreasing thickness. The optimally obtained CTE values are 32.5 ppm/°C, 30.0 ppm/°C, and 29.9 ppm/°C for copper films of 2.2um, 3.7um and 4.5 um thick, respectively. CTEs are larger than that in bulk, and decrease monotonically with decreasing thickness. The results obtained are also discussed with further examination in this study.
This study also presents methods for obtaining elastic moduli and CTE of IMCs formed at the interface of lead-free solders and substrates. Up to tens micrometers of IMCs are formed on metal substrates. Such sufficient thickness of IMCs makes nanoindentor competent to obtain the elastic moduli of IMCs. The results of Cu6Sn5 and Cu3Sn agreed well with those in previous literatures with nanoindentation. The result of Ni3Sn4 is limited, and Cu33.5Zn66.5 from Sn-9Zn is new.
The conclusions of each topic are narrated respectively in the end. When materials are formed of thin-film order, it is not proper to neglect the discrepancy in material properties of thin films and bulks. In addition, as the elastic modulus and CTE for Cu3Sn are significantly different from those in Cu, the need to incorporate these material properties into stress analysis in solder bumps of a flip chip or wafer level packages might be essential.
Subjects
薄膜
介金屬化合物
熱膨脹係數
Thin Film
IMC
CTE
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-94-D89543005-1.pdf
Size
23.53 KB
Format
Adobe PDF
Checksum
(MD5):7ab50736f9a7bb7e6fd0dc63321d9ebe