The effect of wire coating on the occurrence of fatigue failures during TMCL-testing

The effect of wire coating on the occurrence of fatigue failures during

TMCL-testing

By: Arno Vissers

Jasper Winkes

Objective:

• Study the wire-shape.

• Development of parametric 2-D FEM model.

• Calculate the wire plastic strain with and without a coating.

• Calculate the total strain with and without a coating.

• Recommendations for the coating material

Wire shape

2-D FEM Model

2-D FEM Model

2-D FEM Model

• Variable geometry: - wire thickness;

- coating thickness.

• Mesh creation: af_planar_quad_trimesh *.

• Contact Body with contact type glue between materials.

• Initial conditions: Cooldown from 150 to –65 deg C.

• Material definitions: -Variable E-modulus of coating

• Generalized plain strain elements (Type 19)

Results

Results

Fig. 1 Total Strain (thick Wire)

Fig. 2 Total Strain (thin Wire)

Fig. 3 Plastic strain (Solder bump place)

Fig. 4 Plastic Strain (Lead place)

Effect of coating on equivalent total strain on wire

No Coating:

Wirethickness 15 um ets 0.17

Wirethickness 30 um ets 0.15

Wirethickness 50 um ets 0.17

With coating

Same ets for all wirethickness, coatingthickness and coating elasticity modules

ets 0.02

Effect of coating thickness on total equivalent plastic strain (teps) in the wire

Wirethickness 15 um (coating elasticity 5 MPa)– No coating teps 0.21

– CT 1 um teps 0.18

– CT 5 um teps 0.14

– CT 10 um teps 0.09

– Substantial reduction of plastic strain with increasing coating thickness

Effect of coating elasticity on equivalent total strain on coating

Wirethickness 15 um (coating thickness 5 um)– 1 MPa ets 1

– 5 MPa ets 0.7

– 10 MPa ets 0.5

Thicker wire results in higher ets in the coating

Thinner coating results in higher ets in the coating

Uncertainties

• Coarse mesh while simulating gives uncertain results.

• No solder bumps were modeled.

• Fracture analyses was not taken into account.

Conclusions

• No plastic strain in coating since E-wire >> E-coating

• Use of coating leads to reduction of plastic strain in wire.

• Coating elasticity Total strain in coating • Thickness coating Total strain in coating • Wire thickness Influence of coating

thickness on plastic strain in wire

Future perspectives

• With ever decreasing wire thickness (which leads to reduction of material costs) coating is a good way to prevent the occurrence of wire fractures.

• If the wires are isolated with a coating there is less chance of electronic short-circuit while injection molding the compound.