The photo etching process relies on the ability of the etching fluid to enter and exit the line or hole or slot that is being etched. The effect of the etchant is dependent upon having adequate access to the metal that needs to be dissolved.
As a result, there are limitations that are directly related to the thickness of the metal. To make a hole, we spray the etching fluid at spots of bare metal on both sides of the sheet that are not protected by photoresist. The etchant erodes the metal from both sides and the hole is created at the moment the etching action breaks through.
If we try to make a hole that is too small for the metal thickness, it is like overfilling a bucket. The etchant being sprayed can't displace the etchant that is already in the cavity, and the etchant in the cavity loses effectiveness.
The need to allow enough room for the fluid action of the etching solution gives rise to the first rule of thumb for etching, which is that the minimum dimension of a hole or slot must be at least 115% of the metal thickness. This allows the necessary clearance for the etching fluid to circulate through the area being etched.
Dimensional tolerances are also driven by metal thickness. And the same effect that pertains to hole sizes applies to overall dimensions. The minimum dimensional tolerance for etched features is +/-15% of the metal thickness. However, locational tolerances will be +/-.001" to drawing nominal. So, a pattern of holes, for example, may vary in size up to +/-15% of the metal thickness, but the locations of the centers of those holes will be within +/-.001.
What is not well understood about the differences between etching and other forms of metal fabrication is that there are no mechanical forces being applied to the work piece. There is no deflection or wear of the cutting tool and no mechanical force being applied by a hold-down device. In photo etching, the sheets of metal are being trundled through a slow-moving conveyer and sprayed on both sides with heated etching solution.
Another important thing to remember is that just because, for example, the process can hold +/-.002 on .010 material, if your application works at +/-.005--leave it that way. The more generous the tolerances, the larger the sheet we can run, and the more cost effective it will be.For more information, download FREE Design Guidelines: