TABLE OF CONTENTS Electrolytic tin with an electrolytic nickel underlayer is a low-cost alternative for precious-metal finishes, such as Ni-Au or Ni-Pd. Tin has an excellent solderability, probably only next to Sn-Pb finishes. Nickel is often used as a migration barrier under the tin to prevent copper from forming a thick, brittle layer of intermetallics with solder. The cost of plated tin is approximately the same as HASL. The cost of the tin plate over nickel is more than the HASL process, especially if the copper, nickel, and tin-plating processes are not in the same continuous plating system. When comparing it with any of the noble metal finishes, tin over nickel will be the least expensive finish.
The thickness of the Ni/Sn finish is plating chemistry and process dependent. For instance, Hinton, at Hinton PWB Engineering, reported a semimatte to bright tin finish with 8 to 15 m thickness on top of the nickel underlayer (1 to 2 m).78 On the other hand, Ludwig et al., at Lucent, reported a satin-bright tin (3 m) electrolytically plated on a 2.5- m nickel layer, which is electrolytically plated on the base metal copper.47 Melton and Fuerhaupter considered the typical electrolytic tin layer to be 7.5 to 10 m in thickness.11
Fabrication Process. The fabrication process of EN itself can be described as follows, and it is the same as that in Sec. 14.4.135:
Use the cleaner to clean the copper surface.
Etch the copper surface.
Plate the nickel at 52...
