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Advantages of Hot Dip Tin Coatings vs. Electroplate

1. Metallurgical Bond

a. Hot-Dip: Alloy layer forms between base material and coating which prevents flaking or peeling.
b. Electroplate: Chemical bond is established that frequently results in flaking or peeling, especially near the sheared edge.

2. Dense Structure

a. Hot-Dip: Cast structure, not porous. Few grain boundaries which are paths of diffusion of the base material.
b. Electroplate: Porous structure. The tin grains are piled onto each other to obtain thickness. Many boundaries between grains which are paths for diffusion.

3. Whisker Growth

a. Hot-Dip: Cast structure, large grain size, no inherent stresses. No whisker growth detected.
b. Electroplate: Small grain size, inherent stresses from plating baths. Whiskers readily grow in a pure tin plate. Lead must be added to inhibit whisker growth.

4. Formability

a. Hot-Dip: Cast structure can go through severe deformation with no cracking. (see Figure 2.)
b. Electroplate: Chemical bond, stresses and porous structure. Can crack or peel during severe deformation. Cracks develop at sheared edges. (see Figure 3.)


Figure 1. - (Example of intermetallic growth).
SEM Photographs of the Tin Coatings' Cross Sections, Taken in Backscatter Mode: (a.) Matte Tin on Copper; (b.) Hot-Air-Leveled Tin on Phosphorous Bronze. Light Phase (a) is Tin. Dark Phase (b) is Copper. IMC is Gray Phase in Between.
 
Figure 2. Sheared Edge of Hot-Air-Leveled Tin Stamping; Cracks are Absent.
Figure 3. Cracks in Electroplating Near the Sheared Edge.

Figures 1, 2 & 3 are reprinted with the permission of AMP.


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