Etch outer layers

We have now plated the panel with 25 microns of copper through the hole and an additional 25 – 30 microns on the tracks and pads. The copper is covered with a thin layer of tin as an etch resist.  Now we will remove the unwanted copper foil from the surface.

We do this on a single continuous process line.  The first step is to dissolve and wash off the resist which covers the unwanted copper.

Then we remove the unwanted copper using a powerful alkaline solution to etch away the exposed copper. The process is carefully controlled to ensure that as we etch down we don’t etch sideways as well. This means that the finished conductor widths are exactly as designed. But designers should be aware that thicker copper foils need wider spaces between the tracks.

Finally we strip off the thin tin coating which protected the copper image. So now you can see that only designed copper pattern remains. As the boards emerge from the line they are stacked automatically.

Etch outer layers

Top Side Versus Bottom Side Problems, and Leading Edge Issues

It is important to understand that the majority of etch quality problems are restricted to the top side of the panel being etched. This is because of the “puddling” effect of the etchant on the top of the printed circuit board. The top side of a printed circuit board panel builds up a thick puddle of etchant, that absorbs the energy of the sprays, and makes replenishment of the etchant at the surface of the metal a difficult, and relatively slow process. Puddling causes different etch patterns on the top versus the bottom of the PCB. Puddling is also the reason why the top side of the leading edge of panels get etched more thoroughly, or more commonly, over-etched. The mechanism for this is that the leading edge has not had time to develop a “puddle” of liquid on it when it is first introduced into the etch chamber, and thus etches quickly. The final part of the panel, however, will have a puddle on it as soon as it enters the etch chamber, and thus etches more slowly.

The top versus bottom side difference in etch quality can be compensated for by adjustment of spray pressures on the top and bottom spray manifolds. There are some innovative designs now being proposed to compensate for the leading edge problem, that shut off the first few spray bars for a time after a panel enters the etch chamber.

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