TABLE OF CONTENTS Immersion silver is another lead-free surface finish formed by galvanic reaction. Silver is selected due to the following four considerations: (1) electromotive potential of Ag (+0.80 V) relative to copper (+0.344 V) allows use of immersion deposition process; (2) high electrical conductivity of Ag is compatible with touchpad applications, in-circuit probe test processes, and signal transmission requirement; (3) Ag is a noble metal and thus promises good stability; (4) Ag dissolves into solder quickly and thus promises good solderability.1 copper from the base metal for silver in the silver nitrate bath, as illustrated by Fig. 14.27.1 As discussed in the section on the immersion Au process, a particular benefit of immersion processes is the self-terminating feature; that is, the process terminates when the coating completely covers the base material. As a result, the plating thickness is consistent and easily controlled.54 The process can be horizontal or vertical, with 8 min of cycle time for a conveyorized process. Typically, 120 panels/h can be treated in 8-m equipment at 50 C process temperature. Figure 14.28 shows an example of PCB with an immersion Ag surface finish.1
Fabrication Process. The immersion silver coating process consists of four baths, a precleaner, a microetch followed by the conditioner, and finally the plating bath. Table 14.6 shows the detailed process conditions for organic modified silver (Sterling
