This paper mainly introduces the cause of the jaffini effect of PCB chemical silver sinking surface treatment process and the production control method.Firstly, the inevitability of the effect is introduced, and then the main factors are analyzed experimentally. Finally, the method to control the effect is obtained.
Keywords: giovanni effect, lateral erosion
The RoHS regulations limit the use of lead, cadmium, mercury and hexavalent chromium compounds in PCB manufacturing and board assembly, forcing PCB manufacturers to switch from hot-air leveling welding to lead-free alternatives.
In general, lead-free surface finishing processes commonly used in the manufacture of circuit boards include HASL, OSP (organic solderable protective coating material), electroless nickel/gold, electroless nickel/electroless palladium/gold, solder, silver, and electroplated tin.Compared with other kinds of chemical silver impregnation, the process of nickel impregnation has its own advantages.Organic solders are easy to operate and cost effective in PCB fabrication, but are limited in assembly.Chemical immersion silver makes the line very flat, suitable for high-density line, pin pitch SMT (surface mount), BGA (ball foot array package) and chip direct installation.The operation of chemical silver immersion is simple, the cost is not high, the maintenance is less, the use of relatively small equipment can have high yield.Based on these advantages, chemical silver is more applied to surface treatment of PCB.
2. Principle of chemical silver and giovanni effect
As we all know, the reaction mechanism of chemical silvering is very simple, namely the simple metal displacement reaction. The reaction process can be expressed as:
Ag++ e - = Ag e = 0.799
Cu++ + 2 E - = Cu E = 0.340
Ag + + ag Cu = 2 + 2 Cu++ E = 0.459
Under normal circumstances, the reaction is spontaneous, which can be expressed as the following figure:
After a certain thickness of deposit, the chemical silver layer can achieve the protection of copper surface, and successfully complete the task of welding protection, but it often backfired.The process of silvering is not smooth. We all know that the surface of PCB is printed with ink before surface coating, while the copper surface of PCB will present a different scene after ink printing.The following figure can be used to represent:
In the process of solder making, due to the effect of development ink more or less exists side erosion, so the processed ink often presents the following figure:
This form of ink structure provides a good environment for the so-called "giovanni effect" of silver melting.The innovative ink holder usually forms small cracks in which solutions cannot be exchanged and cannot provide enough Ag ions.But the Cu in the electrolyte solution constantly loses electrons and becomes Cu ions, while the Ag ions in the solution continuously gain electrons, which are deposited on the bare copper surface.The reaction will not stop until the Ag ions in the solution have gained electrons and Cu have lost electrons in equilibrium.So the position of the "giovanni" is that the silver is thicker than normal.The entire process can be represented as the following: