Tin layer change by storage/ thermal stress

In chemical tin coating surfaces everyone speaks of diffusion layers.

But what are diffusion layers anyway?
Diffusion layers is Intermetallic phases which form is a direct function of time and temperature at the phase boundary of two metals. This growing takes place in the inner layer of the structure. They may have different compositions. From the outside, they are difficult or impossible to be seen (unless they have penetrated to the surface). But the growth of these intermetallic phases also depends on the conditions when applying the output layers (here the layer of tin).

What is the situation at smarttin® layers?
Through an extensive investigation, it was to find out how the growth of diffusion layer at smarttin® layers. Here, it was determined how much pure tin for real-time storage and/ or thermal stress is still present. The investigations were carried out solely by metrological methods.

First, the X-ray fluorescence method (X-ray method) was used. Here with the total thickness of the tin layer, including the tin content in the diffusion layer, is measured non-destructively. These measurements were carried out in the home APL.

Secondly, the coulometric method was applied. In this method, only the pure tin layer, ie without the tin content in the diffusion layer, is measured destructive. These measurements were carried out by the Research Institute for Precious Metals + Metal Chemistry (FEM) in Schwäbisch Gmünd.

The thickness of the diffusion layer can now be calculated as follows:

diffusion layer thickness = total tin thickness (X-Ray-value) - pure tin thickness (couloscope value)

The study was performed on real time stored reference samples of process test boards, as they are produced in APL daily performed. As a starting layer thickness nominal 0.8 micron was chosen with a

cpk of ≥1,333. The storage periods were 0, 1/2 and 1 year. In parallel, the printed circuit board of different thermal loads of 0, 1 or 2 reflow processes have been subjected. (peak 245 °C, cycle length 480 s)

Would like to read more? An elaboration on the subject of diffusion layer is available in the download area.

Author: Mr. Dirk Kaschel

[1]   entsprechend IPC-4554 (Verwendet wurde ein X-Ray-Gerät (Firma Fischer, Typ XULM) mit der Messapplikation chemisch Sn <3 µm Cu/Br.)

[2]   entsprechend DIN EN ISO 2177 (Zum Einsatz kam ein Couloscope (Firma Fischer, Typ CMS-Step) mit einem Messzellen­-Ø 2,2 mm; Elektrolyt 170 ml/l Salzsäure und Ablösestrom 0,2 mA.)