Authored By:
J. Servin
Continental Cuaulta, Mexico
Summary
Lead Free Technology has brought new materials and different quality concerns to the Electronics Industry. For that, the creation of new methods to determine the quality of materials is needed and preferred. Intermetallic compounds for example can grow faster in lead-free metallization and decrease the possibility to form good joints. For that, a new method to measure IMC layer thickness is presented.
This new method uses a combination of X-Ray Fluorescence Method (XRFM) and Coulometric Stripping Method (CSM). XRFM is capable to measure percentage of elements and correlate the values to their layer thickness. This procedure makes XRFM not so suitable to measure intermetallic thickness when it is growing because the elements only combine each other and are not removed; therefore XRFM gives similar values in thin metallization layers of tin-copper with thick or thin IMC layer, for example. On the other hand, CSM removes pure element layer using an electrochemical depletion.
The combination of both methods allows evaluating the IMC layer thickness in a more precise form. For validation of the method, SEM/EDX and Auger microanalyses were made to compare values. Besides, several experiments were carried over in several temperatures and reflow profiles to measure IMC growth in Chemical Tin PCB's. The results were used to develop a more precise equation to predict the IMC growing. The new equation uses the Activation Energy depending on the IMC thickness.
Conclusions
The application of the XRF/CS Method has been very helpful to determine more acutely IMC layer growth or diffusion in Immersion Tin PCB's. This allows getting an important finding: there is also a linear relationship between the activation energy and the IMC thickness. This was observed especially in temperature dynamic-state experiment with the reflow profiles; although it is also applied to temperature steady-state experiments.
Another important finding is to see clearly the acceleration of the diffusion rate after a determined temperature in Immersion Tin PCB's. It was found the intermetallic formation was accelerated approximately at 217°C, very similar temperature to the eutectic point of the copper-tin-silver. Because of that, the corroboration of Ag presence had to be done with SEM/EDX analysis in the case of Immersion Tin pads. This phenomenon would also justify the fact of using different Activation Energy Equation over 217°C. Applying a relationship between the Activation Energy and IMC Thickness resulted in a linear equation with an approximate constant of 67680 J/mol for temperatures below 217°C and 32000 J/mol for temperatures over 217°C. This indicates the energy is to start the diffusion over 217°C is almost half that of below 217°C.
The linear dependency between Q and IMC is positive; this means we will need more energy as long as the IMC growth. This would be explained with the fact more energy would be needed to diffuse more atoms through a stable IMC layer, this hypothesis needs to be proved. Although with the application of the new method to measure IMC layer and the proposed set of equations is possible to simulate any thermal degradation of Immersion Tin, such as, passing PCB's through baking or several reflows, so quality issues can be prevented.
Initially Published in the IPC Proceedings
|
