PCB Technical - Surface treatment of lead-free soldering of circuit boards: immersion tin plating and other soldering treatments

Surface treatment of lead-free soldering of circuit boards: immersion tin plating and other soldering treatments

The advent of lead-free soldering of circuit boards has become an inevitable trend. In addition to the replacement of solder, the surface of solder pads, through holes or parts feet can be soldered and must be changed accordingly. From all perspectives, a new situation that will inevitably appear in lead-free soldering has developed many other circuit board soldering methods such as immersion tin plating, bismuth plating, etc. in addition to the OSP and immersion silver plating described above.
1. Immersion Tin
(1) Principles and problems of reaction
Circuit board immersion tin plating has been used in the PCBA industry for many years, but the high temperature bath with a simple formula can only last for one week (because too much invalid tetravalent tin is produced). The immersion tin layer is not only very thin and the color is gray, but also the solderability is not durable. Usually in a general acidic solution, the electrokinetic order of Cu is +0.342V, while the divalent Sn is -0.138V. Of course, it is impossible to "dissolve copper and precipitate tin" as a substitution reaction contrary to nature. But after adding thiourea (Thiourea) bath liquid, the situation of nobleness and inferiority is immediately reversed, so there will be a tin layer on the copper surface being plated out.
The traditional gray tin is a direct replacement reaction of copper dissolution and tin deposition. The new white tin is a bare copper surface that first grows an organic copper complex film, and then indirectly performs replacement tin deposition.

Although the re-emergence of tin immersion still uses thiourea as the main agent, because only in this way can copper be more active than tin, and the displacement reaction of copper dissolution and tin deposition can occur. However, the new generation of immersion tin layer, which has made great progress, not only has a whiter surface, but also has better solderability, and the plating solution is also very stable. Therefore, it is called Immersion White Tin, which is different from the earlier gray tin layer which is of poor quality and easily oxidized. The new immersion-plated white tin can not only be used as a solderable treatment layer, but it is also very likely to become the number one surface treatment of PCB for lead-free soldering. It can also be used as a resist for alkaline ammonia-containing etching, and its manufacturing process is also very simple and convenient. .
(2) Improvement and mass production of white tin
This kind of self-limiting (Self-Iimiting) exchange plating reaction (stannous sulfate or stannous chloride), the resulting tin layer thickness is between 0.1-1.5μm (at least 1.5"m for multiple soldering, The thickness of the residual tin after the replacement of IMC must be more than 0.3 m), this thickness is directly related to the stannous ion concentration in the bath, the temperature, and the degree of porosity of the plating layer. And when the amount of copper in the bath is dissolved Too much time, it may co-deposit and precipitate with tin, which of course will adversely affect the solderability.
Comparison of the scattered tin area of the solder paste after five treatments:
In order to understand the deterioration of the solderability of general immersion tin and FST white tin after aging, the spray tin plate is used as the standard test reference board, and the white tin and the three brands of immersion gray tin are deliberately placed under the same conditions. Aging; then remove the solder paste and weld it, and compare the size of its spreading area with SEM to learn the change history of solderability.

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