The process from the light board to show the line graphics is a more complex physical and chemical reaction process, now the typical process of PCB processing using the “graphic plating method”, the need to retain the line graphics to protect, and then the next step in the etching. corroded copper
The practice is to pre-plate a layer of lead-tin resist on the graphic part of the PCB, and then chemically corrode the rest of the copper foil, known as etching.
Currently, tin or lead-tin is the most commonly used resist layer, used in the etching process of ammonia etchant. Ammonia etchants are commonly used chemical solutions that do not have any chemical reaction with tin or lead-tin. Ammonia etchant mainly refers to ammonia/ammonia chloride etching solution.
In the printed circuit board outer layer of the process, can also be used instead of metal coating photographic film as a resist layer.
The etching process is also divided into positive and negative processes, the positive process uses fixed tin to protect the line, and the negative process uses dry film or wet film to protect the line. With the traditional etching method, the line or pad edge is deformed. For every 0.0254mm increase in height of the top line, the edges are skewed. To ensure adequate spacing, the gap of the wires is always measured at the nearest point of each pre-set wire.
To prevent the protective layer from falling off and to meet the process spacing requirements for chemical etching, conventional etching specifies that the minimum spacing between wires should not be less than 0.127 mm. Considering the phenomena of internal corrosion and edge biting that can occur during the etching process, the width of the wires should be increased. This value is determined by the thickness of the same layer. The thicker the copper layer, the longer it takes to etch the copper between the wires and under the protective coating. Above all, two figures must be considered for chemical etching: the etch factor – the number of copper etches per ounce; and the minimum gap or pitch width per ounce of copper. corroded copper
Some examples of etching failures
1. Phenomenon: peeling off the corrosion protection layer during etching
Causes:
(1) mold degreasing is not clean
(2) ink did not dry
(3) poor corrosion protection ink or ink expired
(4) etchant acidity is too strong
(5) The etching temperature is too high
(6) Etching time is too long
Solution:
(1) strengthen the mold degreasing work
(2) Before etching, the ink must be dried through
(3) Replacement of the anti-corrosion ability of the ink or replace the new ink
(4) Dilute the etchant to reduce acidity
(5) Reduce the temperature to the process-specified range
(6) shorten the etching time
2. Phenomenon: etching depth is too deep
Causes:
(1) etchant acid concentration is too high
(2) The etching time is too long
(3) The etching temperature is too deep
Solution:
(1) dilute etching solution, reduce acid concentration
(2) shorten the etching time, often check the depth of the etching
(3) reduce the etching solution temperature to the process-specified range
3. Phenomenon: The etching surface is coarse, with bad luminosity
Causes:
(1) The etching solution in the HNO3 content is too low
(2) The etching solution in the H3PO4 content is too low
(3) The etching temperature is too high
Solution:
(1) dilute the etching solution, and reduce the concentration of HNO3, while adding H3PO4 and H2SO4
(2) Add H3PO4
(3) Reduce the etching temperature corroded copper