ERF Variable

Description

rm_d2c

Reliability margin for all drill to copper measurements other than annular ring. Used in the calculation of the DRA attributes.

DRA (Design Registration Allowance)

is the maximum misregistration permitted by a design while still meeting all other requirements. For example, if a 39 mil hole must be within a 51 mil land, the DRA is +/- 6 mils. Even though this simple example illustrates the concept of DRA, it is not usually that simple in actual design.

RM (Reliability Margin)

is a value that offsets the attained DRA value based on customer requirements. If, in the example given immediately above, a customer requires a 1 mil annular ring on the end product, then your RM is 1.

The standard formula used to calculate the DRA for various tests is:

DRA = (minimum test value) - RM

The formula used to calculate the DRA for annular ring, as an example, would be:

DRA = (annular ring) - RM

In the above example, applying the formula will produce:

DRA = 6 - 1 = 5

This new value of DRA is used as a guideline to determine the difficulty of manufacturing this job and the tolerances that will be forced upon it during the manufacturing process.