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Influence of Salt Residues on BGA Head in Pillow



Influence of Salt Residues on BGA Head in Pillow
In this paper, a series of experiments was carried over to know the influence of specifically NaCl on BGA wetting given Head in Pillow (HiP) as result.
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Authored By:


J. Servin, C. Gomez, M. Dominguez, A. Aragon
CONTINENTAL CUAUTLA
Morelos, Mexico

Summary


The oxide layers are known as wetting inhibitors in component and PCB metallizations. The oxide acts as barrier that prevent the tin diffusion from happening. Besides, in corrosion studies, the role of salt residues -with Cl ion- on some metals is known as being promoters of oxidation or corrosion. On the other hand, most of corrosion studies with tin metallization are focused mainly on the corrosion resistance of tin alloys, but little has been done respecting to the influence of salts on tin metallization wetting.

In this paper, a series of experiments was carried over to know the influence of specifically NaCl on BGA wetting given Head in Pillow (HiP) as result. The aging of components was done following the procedure of J-STD 002C in which the components are exposed to a low amount of steam water for several hours. As a procedure modification, NaCl was added to the water that forms steam in several concentrations. The results show the more time of steam exposition, the more HiPs are obtained.

Additionally, the higher concentration of NaCl in the water, the more HiPs are formed. As a conclusion, NaCl presence could inhibit tin wetting when the proper conditions of energy and environment exist. One interesting fact is during the analysis of BGAs with SEM/EDX; the presence of NaCl was not detected on the solder/BGA ball interface but was found on its edges. This finding would be explained because solder flux tries to remove the contaminants of the interface putting them around the Ball/solder interface. Besides, the oxygen concentration on BGA balls analyzed by SEM increased little or none at all showing the oxide increase cannot be detected by this type of analysis. Because of that, TOF SIMS analysis was carried over showing an increase of oxide on tin surface.

Conclusions


The presence of NaCl under some determined conditions can make wetting decrease in BGA solder balls. In this paper, 3 DOEs were presented together with their results. For the first DOE, the presence of foreign elements such as Na, Cl, Mg and Ca decrease substantially the wetting of solder balls. Several HiPs were created. In the second DOE, we reduced the amount of foreign material and were selective using only NaCl to contaminate the solder balls. The results showed HiPs are still created.

An interesting fact is analyzing the solder balls with SEM/EDX, the concentrations of NaCl on the solder ball surfaces is not uniform but distributed randomly. Besides, the solder balls with low concentration of NaCl still created BGA HiPs. In the final DOE, we completed it with more and intermediate NaCl concentrations. The results completed a range of created HiPs from components with only one HiPs to components with most balls having HiPs. The main conclusion is that there is a direct relationship between NaCl amount on the metallization component and non-wetting issues. Of course, only NaCl presence is not enough but also energy and time are required to create the issues.

Initially Published in the IPC Proceedings

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