Inherently poor adhesion strength between Cu-based leadframe(LF) and epoxy molding compound(EMC) causes popcorn crack phenomena. So high adhesion strength between LF and EMC is required to prevent popcorn crack phenomena.
In this thesis, in order to improve the adhesion strength between LF and EMC, chemical oxidation of copper(COC) is carried out and the adhesion strength is measured by using pull-out specimens, SDCB(Sandwiched Double Cantilever Beam) specimens, and SBN(Sandwiched Brazil-Nut) specimens. Each specimen has its own feature. Pull-out specimen was designed to measure the adhesion strength under shear stress application, and SDCB specimen was designed to measure the critical energy release rate$(G_c)$ of crack lying along LF/EMC interface under quasi mode I loading condition, and SBN specimen was designed to measure the $G_c$ of interface crack under mode I and mode Ⅱ loading condition.
COC was conducted by immersing LF in brown oxide or black oxide forming solution, and the oxidation behaviors have been studied by using XRD, SEM, TEM and AFM.
Brown oxide is composed of fine needle-like shaped CuO crystal only, and thickened up to 150 nm within 2 minutes oxidation time. Such a fine needle-like shape did not change in spite of further oxidation, and affected the
average roughness Ra and wettability of deionized water.
The fine CuO needles belonged to brown oxide grew nearly randomly, and such a microstructure presumably enhanced the adhesion strength between LF and EMC by mechanical interlocking of fine CuO needles to EMC.
Black oxide is composed of $Cu_2O$ and CuO crystal layers. At the early stage of oxidation, the pebble-like shaped $Cu_2O$ crystals were preferentially formed on leadframe suface, and after the thickening of $Cu_2O$ layer up to 200 nm, needle-like shaped CuO crystals were formed atop of $Cu_2O$ layer and grew. The thickness of CuO layer grew parabolically with oxidation time, and showed 1300 nm at 20 minutes oxidation time. The densification of CuO layer have occurred since the CuO layer became continuous layer, and such a microstructural change with oxidation time affected the average roughness Ra and wettability of deionized water.
The CuO needles belonged to black oxide grew with concentrated point, and such a microstructure probably improved the adhesion strength between LF and EMC by mechanical interlocking of CuO needles to EMC.
Pull strength(PS), measured by pull-out test, showed ~8 MPa in case of bare leadframe, but in case of brown oxide treated leadframe, PS showed ~ 23 MPa at 1 minute oxidation time, and such value is maintained with further oxidation. The correlation between PS(MPa) and $δ_{CuO}$(nm) for brown oxide is PS = $4×(δ_CuO)^0.4$. Such a result might be ascribed to the mechanical interlocking of fine needle-like CuO crystal to EMC. On the other hand, in case of black oxide treated leadframe, PS showed ~14 MPa when $Cu_2O$ crystals were dominant on the leadframe suface, and showed ~ 23 MPa when the leadframe suface is covered with continuous CuO layer. Such a result also might be attributed to the mechanical interlocking of needle-like CuO crystal to EMC.
$G_c$, measured by SDCB test, showed ~O J/㎡ in case of bare leadframe, but in case of brown oxide treated leadframe, $G_c$ showed ~80 J/㎡ at 2 minutes oxidation time, and such value is maintained with further oxidation. The correlation between $G_c$(J/㎡) and $δ_{CuO}$(nm) for brown oxide is $G_c$=0.53×$δ_{CuO}$. Such a result might be ascribed to the mechanical interlocking of fine needle-like CuO crystal to EMC. On the other hand, in case of black oxide treated leadframe, $G_c$ showed ~O J/㎡ when $Cu_2O$ crystals were dominant on the leadframe suface, and showed ~80 J/㎡ when the leadframe surface is covered with continuous CuO layer. Such a result might be attributed to the mechanical interlocking of needle-like CuO crystal to EMC.
$G_c$ values through the 20 minutes oxidation are ~ 80 J/㎡ for brown oxide, and ~100 J/㎡ for black oxide. Such a discrepancy is probably ascribed to the size of the CuO crystal, that is to say, the larger CuO needle the higher $G_c$.
According to the comparative study of PS and $G_c$, the correlation between PS and $G_c$ is in existence with the exception of the occasions when PS is low or $G_c$ is high.
The $G_c$ values measured by SBN test are well coincide with the those measured by SDCB at low phase angle, and that is attributable to the accuracy of SBN test. During the SBN test, the interface crack kinked into EMC when ec $G_c$ is ~ 150 J/㎡, but none of interface crack kinked into LF.
XRD, Fractography and EDS studies on fractured surfaces of leadframe parts and EMC parts of pull-out and SDCB specimens indicated that the failure path is quite different with oxide type, oxide thickness, and the loading condition.
