Thermal Fatigue of a Surface Mount Resistor
Introduction
A surface mount resistor is subjected to thermal cycling. The difference in the thermal
expansion between the materials introduces thermal stress in the structure. The solder, connecting the resistor to the printed circuit board, is en as the weakest link in the asmbly. Becau the operating temperature is high when compared to the melting point of the solder, creep deformation occurs. In order to assure the structural integrity of the component a fatigue analysis is performed where the life prediction from two different fatigue models is compared.
Model Definition
A resistor is fastened on a printed circuit board (PCB) with SnAgCu solder. The solder
is connected to the printed circuit board through two copper pads and to the resistor through a NiCr conductor. In reality there are additional thin films around the resistor but they are disregarded in current analysis. A sketch of the surface mount asmbly is shown in Figure 1.
Figure 1: Schematic description of the surface mount resistor.刺绣种类
The resistor is made out of alumina and has dimensions 3.2 mm x 0.55 mm. It is covered on both edges with a 0.025 mm layer of NiCr conductor. The thin layer continues 0.325 mm along the lower and the upper side of the resistor. The printed circuit board is large in comparison with the resistor and is here modeled 0.8 mm thick. It has two copper pads on the top side that are 0.025 mm thick and 1.05 mm wide. The thickness of the solder fillet between the copper pads and the NiCr
conductor is 0.05 mm. The remaining shape of the solder joint varies greatly between each examined solder joint and is here modeled with two reprentative roundings.Becau the out-of-pla
ne dimensions is 1.55 mm, which is significant in comparison with the size of the resistor, the model is simulated in 2D with plane strain conditions. The elastic properties of the materials are summarized in Table 1.MATERIAL Y OUNG’S MODULUS (GPA)POISSON’S RATIO COEFFICIENT OF THERMAL
莜面的营养价值及功效
EXPANSION (PPM/°C)引言怎么写
PCB laminate
22 0.421Copper
1410.3517SnAgCu
500.421NiCr
辣眼睛1700.3113Alumina 3000.228
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The SnAgCu solder material exhibits creep behavior, which can be modeled by a Garofalo model where creep rate is described with
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(1)where εij c is the creep strain tensor, T is the temperature, σe is the effective stress, R is the universal gas constant, and s ij is the deviatoric stress tensor.
The thermal load during an operating cycle is prescribed as a temperature which varies between 20 °C and 70 °C. Each temperature change takes 2 min and is followed by a
叠词的好处TABLE 1: ELASTIC AND THERMAL MATERIAL PROPERTIES
T
d d εij c 2.62105
⋅σe 39.1106⋅-------------------------⎝⎭ ⎪⎛⎫sinh 6.19e 5.32104⋅RT ------------------------⎝⎭⎛⎫–32--s ij σe -----⋅=
3 min dwell. This means that one fatigue cycle requires 10 min, e Figure 2.
Figure 2: Temperature load.
Since the solder material is nonlinear, e Equation 1, veral cycles may need to simulated before a stable cycle is obtained.
Two fatigue models are evaluated. The first is a strain-bad Coffin-Manson type model with the effective creep strain as the damage controlling mechanism and an energy-bad Morrow type mod
el with the dissipated creep energy as the damage controlling mechanism. The material constants for the Coffin-Manson model are
εf’=0.281 and c=-0.51. The material constants for the Morrow type model are
W f’=55.0 J/m3 and m=-0.69.
Results and Discussion
The difference in the elastic and thermal properties introduces thermal stress in the device. Although they are not very high the solder experiences significant inelastic
strains. In Figure 3 accumulated effective creep strain after six cycles is shown.
Figure 3: Creep strains in the solder joint.
The highest strains occur in the thin solder layer just below the resistor. It is mainly the shear strain component which contributes to the effective creep strain in that layer. The slightly higher values around the edge are also affected by modeling of a sharp corner. With a rounding instead the strains will be somewhat lower. Nevertheless the location of highest strain agrees well with the crack path in real applications. In order to evaluate fatigue it is important to obtain a stable load cycle. In applications involving solder joints, frequently either inelastic strain or dissipated energy is ud to predict fatigue. The change of creep strain during the first six cycles is therefore evaluated in a point just below the resistor slightly shifted to the right form the sharp corner. The position of this point can be debated. It is however located in the area where the largest strains occurs and is therefore en as the critical point. In Figure 4 the effective creep strain and the shear creep strain component are shown.
Figure 4: Creep strain development in a critical point below the resistor.
The dissipated energy reprents a combined contribution of changes in stress and strains during a cycle and is in Figure 5 shown with a shear hysteresis. The shear component has been chon since it gives the dominating contribution to the effective creep strain in the critical point.
