PSI - Issue 23

C. Skotarek et al. / Procedia Structural Integrity 23 (2019) 463–468 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

464

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1. Introduction Press-fit connectors are solder-free electrical connections in high-current mechatronic devices connecting a printed circuit to a control module. The contact unit consists of a circular plated through-hole and a plated Cu-pin which is pressed into the hole. A typical example is shown in Fig.1. A cold-welded zone develops in the contact zone between the pin and the plating of the hole by material diffusion. The width of this contact zone amounts to a micrometer or less and is decorated by pores. (see Fig.2).

200 µm

Fig. 1. Cross section of press-fit connector with bore hole and plated Cu-pin

plated pin

plated pin

cold-welded zone

cold-welded zone

plated bore hole

plated bore hole

a) optical micrograph

b) close-up by SEM

Fig. 2 Metallographic analysis of connector with cold-welded zone

The reliability of the contact depends very much on the integrity of this cold-welded zone. Upon loading, cracks may develop in this zone, and the contact between the pin and the hole is lost. Further on, oxides gather between the crack faces and may cause a serious disturbance of electric connectivity [Hannel et al.]. Hence, the electrical reliability of the connector can be traced back to crack initiation in the cold-welded zone, and its mechanical strength becomes an issue. However, the mechanical behavior the cold-welded zone cannot be assessed directly, as it is a system property in the sense that any cutting procedure opening up the hole and giving direct access to the cold-welded zone will cause a relaxation of the contact stresses, which have developed during the insertion procedure, and will hence damage the contact zone. This is the reason why mechanical properties of the press-fit joint can only be determined by an inverse procedure, in which a digital twin of the connector is generated by taking both the exact geometry and the inserting process into account. The response of this model connector to mechanical loading can then be directly compared to experimental findings. This paper shows the details of this procedure and relates the observed mechanical behavior to the damage process in the cold-welded zone.