Stripping
In the production of PCB boards, there are only two steps in which stripping is used. The D/F stripping process is simple, using horizontal equipment, and the chemical solution used is mostly 1~3% NaOH or KOH by weight.
Line etching
1. The mechanism of copper etching
(1) copper ions in the alkaline environment solution is easy to form copper hydroxide precipitation, in order to prevent this precipitation phenomenon, you need to add enough ammonia to make the wrong ion group to produce ammonia copper to inhibit the precipitation occurs. It also allows the copper's original amount and its continued dissolution in the liquid to form a very stable wrong ammonia copper ion. Such divalent copper ammonia ion can be used as an oxidizing agent to dissolve the zero-valent copper metal by oxidation. Still, during the redox reaction, monovalent copper ions will be produced.
In this reaction, the solubility of sub-copper ions is very poor, it needs to be assisted by ammonia, ammonia ions, and a large amount of oxygen in the air to continue to oxidize into soluble divalent copper ions, and then become the oxidizing agent for etching copper again and again until the amount of copper is too much and slowed down. Therefore, the general etching machine pumping air in addition to the exclusion of ammonia odor can supply fresh air to accelerate the etching of copper.
(2) In order to make the above copper etching reaction proceed more rapidly, the etching solution has more additives, such as.
a. Accelerator Accelerator can promote the above oxidation reaction more quickly, and to prevent the precipitation of copper ions.
b. Bank protecting agent (Banking agent) to reduce side corrosion.
c. Suppressor Suppressor inhibits ammonia dispersion at high temperatures, inhibiting the precipitation of copper to accelerate the oxidation of the copper etching reaction.
2. Equipment
(1) In order to increase the etching speed, it is necessary to increase the temperature to more than 48℃, so there will be a lot of ammonia odor diffusion need to do proper extraction, but the extraction volume is too strong will be useful ammonia also a lot of extraction caused by waste, can add appropriate throttle valve in the extraction pipeline to do control.
(2) etching quality is often limited by the pooled effect (pudding), (because the fresh liquid is obstructed by water, can not effectively and copper surface reaction called the pooled effect) which is why the front part of the PCB board often have over etch phenomenon, so the equipment design needs to consider the following points.
a. The board is a thin line face down, a thicker line faces up.
b. Nozzle up and down to adjust the pressure of the spray liquid to compensate for the difference according to the actual operating results.
c. The advanced etching machine can control when the board enters the etching section, the first few groups of nozzles will stop spraying for a few seconds.
d. There are designs of vertical etching to solve the problem of unevenness on both sides, but it is less used in China.
3. Supplementary additive control
Automatic replenishment of ammonia, usually with a very sensitive specific gravity meter, and sensing the temperature at the time (because of the difference in particular gravity at different temperatures), set the upper and lower limits, above the upper limit when the addition of ammonia, until below the lower limit to stop. At this point, the location of the detection point and the location of the ammonia addition is very important, so as not to add too much ammonia due to the detection delay and waste costs (because it will overflow off)
4. Daily maintenance of equipment
(1) Do not allow the etching solution to have sludge (light blue monovalent copper sludge), so the composition control is very important, especially the PH, too high or too low may be caused.
(2) Keep the nozzle from being blocked at all times. (Filter system should be kept in good condition)
(3) Specific gravity sensing adding system should be calibrated periodically.
Tin (lead) stripping
The tin (lead) stripping step is purely for processing and has little added value, but the following points need to be noted to ensure the quality of the product.
(1) Stripping tin (lead) solution is usually available in two-liquid type or single-liquid type and many others from suppliers, stripping method is semi-soluble and fully-soluble, and solution composition formulations are fluorine system/H2O2, HNO3/H2O2, etc.
(2) Regardless of the formulation, the following potential problems may occur during operation.
a. Attack the copper surface
b. Stripping is not exhausted to affect the post-process
c.Waste liquid disposal problem
In order to ensure stable product quality, stripping tin (lead) requires good equipment design and pre-processing tin (lead) plating thickness control and management of the potency of the liquid. How to judge the good and bad of etching? Mainly by the following bases.
l Abrupt edge
l Side etching
l Etching factor
l Over-etching
l Etched surface finish
l Clarity of line spacing
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