In the bath the following chemical equilibrium:
HBF4 +3 H2O <==> H3BO3 +4 HF
HF <==> H + + F-
2F-+ Pb2 <==> PbF2 ↓
When the boric acid content of the bath is too low, the three balance all move to the right, eventually leading to the formation of harmful PbF2 precipitation. Therefore, a certain amount of free boric acid from the stable free fluoroborate.
PbF2 the solubility product for the KSP = 4.0 × 10-8moι / ι,. According to the solubility product principle, when the [F-] 2 [Pb2 +] ≥ KSP = 4.0 × 10-8moι / ι, the bath may generate PbF2 precipitation. Based bath [Pb2 +] 0.68moι / ι, (140g / ι,), [Pb2 +] KSP of values ??into the above equation have:
[F-] 2 × 0.68 ≥ 4.0 × 10-8moι / ι.
Economic calculated:
[F-] ≥ 10-3.62moι / ι = 0.0002399moι / ι
= 0.0045556moι / ι
= 4.556mg / ι
4.556ppm
By the above calculation, when the bath free [F-] is greater than 4.556ppm PbF2 precipitate makes bath turbid. In this bath, plating, coating, pitting, pits, rough and blistering and other defects.
Thus, a sufficient amount of free boric acid in the bath solution, by inhibiting the free fluoride boric acid from the free [F-] control the degree lower than the PbF2 precipitation, which in a stable bath.
When the high content of free boric acid in the bath, in the case of lower temperatures will precipitate a lot of crystalline plating coating is very rough, and affect the binding force between the substrate and the substrate.
4.2.4 cathode current density (DK)
With the rise of the DK, the increase in tin content in the coating, and deposition speed and production efficiency. DK lower, plating crystal refinement.
, In order to accelerate progress in the actual production will rise DK; But what DK rose to the number of suitable by the main salt ions in the bath, especially the lead ions (Pb2 +) content constraints.
According to [7] reported that when the bath [Pb2 +] up to 333g / ι, DK can reach 4A/dm2 and coating reflection surface {111} of the structure factor is still as high as 1.42, indicating crystallization of very thin. When the bath [Pb2 +] 222g / ι make the coating reflection surface {111} of the structure coefficient of 1.47, DK can only rise to 1A/dm2; DK rose 2A/dm2, reflective surface of {111 } structure coefficient dropped to 1.14, the coating crystalline in greater detail. Reduced to 111g / ι, the bath of Pb2 +], if DK rose 2A/dm2 the reflecting surface of {111} structure coefficient of only 0.63 at this time of the coating in a micro perspective is not careful, to achieve more detailed coating, DK only l 1.5A/dm2 below. [7] claimed that the rise of the DK lead to the rise of the cathode potential. When the cathode potential of more than 560mV (relative to the saturated calomel electrode), regardless of how the concentration and other process parameters change, the advantage of the crystal orientation from {111} changes to {100}, the crystallization of the coating by thin thicken. This change, pyramid-shaped crystal surface formation is the rate-determining step of nucleation and crystal growth controlled by surface diffusion or nucleation changes the results of the liquid phase mass transfer control.
4.2.5 stabilizer content
Oxygen in the air (O2) has a certain solubility in the bath, it will bath part of the divalent tin ion (Sn2 +) oxidation of tetravalent tin ions (of Sn4 +). Sn4 + in the solution of the acid in high concentrations, also a very small solubility of Sn (OH) 4, followed by lost part of water to generate stannate (H2SnO3) colloidal suspended solids affect the quality of the coating.
The way to solve the above problems is to join in the bath the 2 ~~ 12g / ι, the hydroquinone (a hydroquinone), between a hydroquinone or phenol antioxidant. The basic principle is that they react with oxygen to generate the oxidation state. Power, this oxidation state can be restored at the cathode into the original material. And so forth, which greatly reduces the oxygen content of the bath, delaying the Sn2 + oxidation, the stability of the bath.
4.2.6 of the additive content
0.1 ~ 5g / ι gelatin, peptone or peach gum and other cathodic polarization can improve and refine the crystallization to improve the dispersion ability, and also help to improve the content of the coating of tin in the plating bath. Additives too little causes the coating loose, rough, black; too much can make coating crisp.
Bearing anti-friction coating, plating by liquid phase mass transfer, pre-conversion, charge transfer, surface diffusion or nucleation and the formation of crystalline five steps composition. In the formation of the coating crystal is divided into two processes simultaneously, namely, the crystallization process of the formation and growth of the core (nuclei). The speed of these two processes determine the extent of the thickness of the coating crystallization. Nuclei generation rate is faster, but slower growth rate after nucleation, a larger number into the grain, the grain smaller. On the contrary the grain on the coarse. In other words, when the nuclei generated faster than the growth rate of nuclei in the electroplating process, you can get a detailed, tightly packed crystalline coating. Nuclei generated faster than the growth speed of nuclei, the greater the degree of coating crystalline more detailed, close.
Crystallization of the thinner coatings, protection, functionality and appearance quality is better. The practice shows that the coating electrocrystallization cathodic polarization effect can increase the generation rate of nuclei, to facilitate access to the detailed crystallization of the coating. But when the cathodic polarization of more than a certain range, it will lead to hydrogen is a lot of precipitation, so that the coating becomes porous, rough, loose, burnt, or even powder form, the worse the quality. When the additive content and the free fluoride boric acid content is too high or the salt concentration is too low, the coating will these serious defects.
4.2.7 The temperature of the impact
With increasing temperature, the dissolution of each ingredient in the bath increased the concentration of desirable upper range. This is conducive to DK increased deposition speed. But the temperature is too high anodic dissolution too fast, increase in anode slime, to enhance the solution turbid, coating nodulation. Free fluoride boric acid, volatile organic stabilizer accelerated increase in odor, and environmental degradation.
Low temperature the free boric acid is easy to precipitate, to enhance the solution turbid, rough coating; DK l up, slow deposition rate, low production efficiency.



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