Solder Fatigue Analysis

Solder fatigue is available to analyze and predict fatigue damage of the solder joint between a component and the base Printed Circuit Board (PCB).

Solder joint failure between components and the PCB due to thermal loading is a major cause of breakdown in electronic products. It is critical to estimate such fatigue failure due to cyclic loading of solder joints. You can predict the life of solder joints under thermal cyclic loads with solder fatigue. In order to perform solder joint fatigue, TYPE in FATPARM should be set to SOLDER. OptiStruct supports two different approaches for solder joint fatigue analysis.

Damage due to mismatch of local thermal expansion coefficients.

Solder fatigue calculation due to mismatch of local thermal expansion coefficients (CTE) is available using the following solder joint types:

Leadless solder joint
Ball Grid Array (BGA) solder joint

Fatigue damage due to local thermal expansion coefficient mismatch is assessed. The thermal expansion coefficient field (A) on the MAT1 Bulk Data Entry is mandatory.

The Strain range of the solder joint is calculated as:

$Δ γ = C \frac{L_{D}}{h_{s}} Δ α Δ T$

Where,

$L_{D}$: Component length (PKGLEN field on the FATSDR entry).
$α$: Coefficient of thermal expansion (CTE) (A field on the MAT1 entry).
$h_{s}$: Solder joint height. (PKGTHK field on the FATSDR entry).
$Δ α = C T E_{p} - C T E_{c}$: Difference between thermal expansion coefficients of the PCB and the component soldered onto the board.
$T$: Input temperature.
$Δ T = T_{p} - T_{c}$: Difference between temperatures of the PCB and the component soldered onto the board.
$C$: Empirical model constant for solder fatigue (specified using the C4GAMMA field on the PFATSDR entry). It is equal to $\frac{1}{\sqrt{2}}$ for leadless joint type. For BGA type, you can define the empirical model constant.

The strain energy dissipation per thermal cycle is calculated as:

$Δ W = 0.5 \cdot Δ γ \cdot \frac{F}{A_{s}}$

Where,

$F$: Shear force.
$A_{s}$: Cross-section area.

The fatigue life is calculated as:

$N_{f} = (\frac{1}{W_{p} \cdot Δ W})$

Where,

$W_{p}$: Creep energy density for failure defined on the $W_{p}$ field of the MATFAT entry.

The default value is 0.0019. If a user defines a value for $W_{p}'$ , the value must be based on the stress unit in MATFAT. The default value for $W_{p}'$ is based on MPa unit. If user-defined stress unit is not MPa in MATFAT, OptiStruct applies a conversion factor to the default value to get a correct value.

Correct default $W_{p}'$ = default $W_{p}'$ x factor that converts MATFAT stress unit to MPa.