The Influence of Contaminant Residues on PCB Prototype Design Spot Weld

a. Electrochemical Migration

Electrochemical migration, short for ECM, refers to ion migration by means of some medium such as flux leftover under the influence of electromagnetic field. For PCB prototype design products, with the change of the environmental humidity, some ionic contaminants in the flux residue such as the active agent and salt will be changed into electrolytes, leading to the feature change of spot welds. When these PCBs are working, under the condition of stress voltage, short cuts will possibly take place between spot welds, causing intermittent fault that decreases the reliability of PCBs. This process is comprised by three steps: path formation, initialization and arborized crystal generation. The path formation starts with the dissolution of metal ion in the electrolyte that is a kind of weak acid formed by the combination of chlorine and bromine residue in the flux and the water in the air. When metal is dissolved in the weak acid, metal filament will be generated. Therefore, the elements including ionic leftover, voltage deviation and humidity must be required to implement the mechanism of the electrochemical effectiveness. Besides, the electrochemical effectiveness is also influenced by temperature, humidity, offer, conductor material, conductor spacing, contaminant type and quantity.

b. Creep Corrosion

Creep corrosion refers to the phenomenon that the sulfide crystal of copper or silver is generated on the surface of PCBs. Different from electrochemical migration, just the existence of contamination source and moisture in the environment is capable of leading to the creep corrosion, without the need of voltage difference. When the sulfur in the air combines with the copper or silver on PCBs, copper sulfide or silver sulfide will be generated. These chemical compounds such as copper sulfide and silver sulfide will grow towards any direction, making fine leads open or short cuts between spacing leads, which will finally result in the bad quality of PCBs. With the PCB size becoming smaller and components miniaturized, the risk of this type of corrosion will definitely improve. Creep corrosion mostly occurs in the fields of industrial control electronics and aerospace because of the more existence of contamination gas in their ambient air. Another reason lies in the HASL implementation on the surface of previous PCBs whose outside copper foil is protected by tin lead. However, with the development of lead-free craft, materials with copper or silver are used in PCB fabrication, welding and plating. Once the wetting doesn't reach the grade in the process of soldering, some copper or silver will be exposed to the air and when the environment becomes bad with the influence of moisture, the risk of creep corrosion will increase greatly.

c. Tin Whisker

Tin whisker is the main concern of professionals. Through a large amount of research based on the chemical and physical parameters generated by tin whiskers, experts indicate that the alloy with tin proliferates with other metals under the effect of high temperature and high humidity, which will help the formation of intermetallic compound (IMC). Under this condition, with the fast increasing of voltage stress in the tin layer, tin ions get proliferated along crystal boundaries, forming the tin whisker that will increase the risk of shortcuts. So in the process of reflow, when tin alloy becomes solid, some halide and bromide in the flux flowing from the tin paste play the role as the ionic contaminant, leading to the mass generation of tin whisker. Besides, tin whisker tends to be influenced by ionic contamination level, which can be concluded that, the higher the ionic contamination level becomes, the higher the tin whisker density will be.

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