Effects of the solder oxide layer on high frequency signal propagation in Pb-free interconnections

In this paper, the effects of the surface oxide layer thickness of a Pb-free solder (Sn_3.4Ag_0.8Cu) and pure Sn on high frequency signal propagation have been investigated. To simulate the thermal effects induced by extended device powering, the Pb-free materials were annealed at 200°C for different times (up to 30 days) to increase the oxide layer thickness. The test modules consisting of lead-free microstrip lines (pure tin and SnAgCu) on a low-loss liquid crystal polymer substrate were fabricated and measured up to 50 GHz. The insertion losses of thick and native oxides were compared using transmission line theory. The results were in good agreement with the predictions, and the comparison of the insertion losses of different oxide thicknesses pointed out that the natural oxide layers induce less resistive loss than the annealed materials at microwave frequencies. It was evaluated that the resistive loss of the highly oxidized pure Sn was 0.6 dB/cm larger than that of its native oxide layer. In the case of the highly oxidized Sn_3.4Ag_0.8Cu the insertion loss was 3.5 dB/cm higher at 50 GHz as compared to that of its natural oxide.