Design Guide

The data given in the Design Guide for a variety of thicknesses is dependent upon the metal being etched and the process variables. Since this does not express the ultimate capability of photoetching, you should consult our engineering department if your requirements fall outside these guidelines.

DIMENSIONS
For dimensions such as slots, corners, etc. there are a few guidelines for designers which express practical limitations as the dimension under consideration approaches, the thickness of the metal. The most common roles are as follows.

1. RELATIONSHIP OF HOLE DIAMETER TO METAL THICKNESS.
Generally, the diameter of a hole cannot be less than the metal thickness. This relationship, however, varies as the metal thickness changes. A more exact relationship is illustrated in Table 1.

Table 1.
Metal Thickness (T) Smallest Hole Diameter (D)
.001" - .005" At least metal thickness
.005" or over At least 110% metal thickness

Some practical hole sizes attained from sample thicknesses are shown in Table 2.

Table 2.
Thickness (T) Practical Hole Size (D) Tolerance Attainable
0.0020" .0060" ±.001"
0.0050" .0060" ±.002"
0.0070" .0080" ±.003"
0.0100" .0120" ±.004"
0.0200" .0260" ±.005"




5. RELATIONSHIP OF BEVEL TO METAL THICKNESS.
An etchant attacks the material laterally as well as vertically. Assuming that the material is being etched equally from two sides, it can be easily seen that a bevel is produced. As a general rule, when etching from two sides, the bevel B is approximately 20% of metal thickness. See figure 2.

MATERIALS
Parts can be manufactured in material from .0005 to maximum thickness allowable with practical tolerances as determined by the chart below. Copper, copper alloys, stainless steels, kovar, nickel, lead, molybdenum, spring steel, nickel/silver and tungsten are typical of the metals capable of being photoetched.

TOLERANCES

1. CENTER-TO-CENTER DIMENSIONS.
Photofabricated parts will duplicate the center-to-center dimensions which exist on production artwork. Due to the nature of artwork preparation, Table 4 gives practical center-to-center dimension tolerances for finished parts.




2. RELATIONSHIP OF LINE WIDTH & SPACES TO METAL THICKNESS.
The width of metal between holes is not a problem in photoetching. However, when this space is the remaining surface area in a large field of slots or holes, there are limitations as to how small the metal width between the holes can be. This relationship is shown in Table 3.

Table 3.
Thickness (T) Spaces between holes (W)
Less than .005" At least metal thickness with min of .003"
Over .005" At least 1.25 times metal thickness

3a. RELATIONSHIP OF INSIDE CORNER RADIUS TO METAL THICKNESS.
The smallest corner radius is approximately equal to the thickness of the metal, (i.e., for metal thickness of .002" corner radius would be .002").

3b. RELATIONSHIP OF OUTSIDE CORNER RADIUS TO METAL THICKNESS.
Outside corners tend to etch more sharply than inside corners. Therefore radii of less than metal thickness are obtainable. As a general rule outside radii are considered to be at least .75 metal thickness, T.

4. FINGERS AND SLOTS.
Max resolution is .005" wide slots and .005" wide dingers on .002" thick material. For greater thicknesses, slots and fingers can be 1.25 times material thickness.

Table 4.
C/C Dimensions (inches) Tolerance Attainable
1.0" or less ±.0005"
1.0" - 3.0" ±.0010"
3.0" - 6.0" ±.0020"
6.0" - 10.0" ±.0030"

2. ETCHED DIMENSIONS.
Because of the many parameters involved in determining etching tolerances, it is impossible to give a definite, absolute rule which will affect the etching tolerances are: type and thickness of metal , size of production run, and number of critical dimensions per piece.

For purposes of establishing some general rule of thumb, a tolerance of +/- 15% of the metal thickness is usually acceptable. Table 5 below gives tolerances to be expected for varying metal thicknesses.

Table 5.
Thickness, (T) inches
.001"
Empirical
.002"
±.0010"
.005"
±.0010"
.010"
±.0015"
.015"
±.0020"
.020"
±.0030"
.040"
±.0050"









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