Die Attach and Package Reliability

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  • Topic: DA postcode area, Finite element method, Materials science
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  • Published : October 21, 2008
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THE EFFECTS OF DIE ATTACH PROPERTIES ON DELAMINATION IN EXPOSED-PAD LQFP

Tay, SweeTeck
Assembly Packaging Technologies, Singapore
Advanced Applied Adhesives, San Diego, CA, USA
sttay@aspatek.com

Die attach material has direct impact on package reliability. Its mechanical properties affect the interfacial stresses in the package. Its moisture stability and hot adhesion determine if it has sufficient adhesion to pass moisture soak and 260°C reflow without causing delamination and package crack. Its water extractable cations can form electrolytic solutions to trigger bond pad corrosion process and its anions cause dendrite growth. While the ion induced problems are becoming rare with modern resin purification technologies, and the occurrence of package crack in 260°C reflow reducing with the advance in mold compound technology, the interfacial delamination remains the main cause of package failure today. This paper demonstrates how the die attach mechanical properties affect the interfacial stress in a large exposed-pad LQFP package with the aid of finite element analysis and outlines the approaches taken to help eliminate delamination.

INTRODUCTION

Exposed-pad LQFP is made with a deep downset leadframe such that the bottom side of the die pad is exposed. This en-ables the pad to be soldered directly to the board for better heat dissipating efficiency. Smaller exposed pad LQFPs have been in volume production for several years while the large ones continue to have delamination problem because of the higher CTE mismatch stress from the larger asymmetrically molded mold cap. DA1

Die
65μm
Die back delamination at die corner
Copper pad
Take the 28 x 28 mm exposed-pad LQFP for example, even achieving delamination free time-zero thru scan is a challenge. This paper uses finite element analysis to help analyze the stresses associated with the initial failures of the exposed-pad package and attempts to solve the problem by reducing stress and enhancing adhesion to the problem areas. Figure 2: Time zero die back delamination of DA1

INITIAL TEST
DA1 with properties listed in table 1 was used for the initial evaluation of a 28 x 28 mm ex-posed-pad LQFP. After post mold cure, all the parts showed die corner delamination in thru scan as in figure 1. Figure 2 shows the cross section photo of a failed part. The die corner delamination is at the die back / die attach interface. To study the delamination problem, Algor, a commercial finite element analysis software was used to calculate the stresses on the die back. Table 1 shows the mechanical properties of the materials. Size

(mm)
Modulus
(GPa)
Poisson’s Ratio
CTE
(ppm/°C)
Si Die
8 x 8 x 0.280
130
0.28
2.8
Cu
10 x 10 x 0.120
117
0.3
17
DA1
8 x 8 x 0.050
4.2
0.3
174
EMC1
28 x 28 x 1.4
27.5
0.3
14.7
Table 1: FEA modeling with DA1
Static stress with linear material model was used for the analy-sis. Default nodal temperature was set at 25°C and the stress free reference temperature at 175°C, cure temperature of the mold compound. All the materials were considered to be iso-tropic and homogeneous. Figure 3 shows the isometric view of the three-dimensional model with DA1 in 128-color spectrum. Page 2 of 6 Figure 5: Isometric view of die top with DA1

307 MPa
171 MPa
Figure 6: Isometric view of DA1 top surface
Figure 4: Isometric view of die bottom with DA1
257 MPa
Figure 3: 3D model of the LQFP package with DA1. Displace-ment scale factor set at 5% of model size Figure 4 shows the stress distribution on the back of the die at 25°C in standard 7 color palette. The die has a negative curva-ture and the die back corners that correspond to the die back delamination failure are the stress concentration zones. The von Mises stress is 257 MPa. The maximum stress on the die is on the top surface. Figure 5 shows the stress distribution. The red corners have the maxi-mum stress of 307 MPa which is higher than the die back cor-ners. It did not result in...
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