ERF Variable

Description

rm_ar

Reliability margin for annular ring measurements. Determines 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.

Annular Ring Measurement

This section describes the possible modifications of annular ring measurements in cases where breakouts are allowed by the designer of the board.

The normal procedure requires two values:

*     RM_AR, Reliability Margin for Annular Ring as received from the customer (the implementation of the RM_AR does not allow the specification of a breakout).

*     DRA, Design Registration Allowance is generated by the analysis (the maximum misregistration permitted by a design while still meeting all other requirements).

For example, if the minimum annular ring width = 4, RM_AR =1, then DRA=4-1=3.

If the user specifies that the RM should allow a breakout (RM_AR=0), the measurement will be extended to the outer perimeter of the drill hole in the position of a theoretical breakout, as in the following figure:

The breakout angle is defined in the ERF parameter via_breakout_angle or pth_breakout_angle .The range is 0-180, where 0 means no breakout is allowed.

When you do specify a breakout, RM_AR must be specified as zero, and the system will automatically calculate an extended Annular Ring measurement based on the size of the AR and drill. The AR size that is reported is larger than the actual AR size by an amount that depends on the AR and drill sizes (see example below).

The system assumes a drill that is centered within the AR and an AR that is a circle. If the AR is any other shape the system will construct a circle from this shape and plan the breakout on the constructed circular AR. The following figure will clarify:

 

 

Example

The following example describes an annular ring with a 90 0 breakout angle:

Outer radius = 20, Inner radius = 10. The annular ring width = 5. In this case, since RM=0, the minimum annular ring = 5.81 and DRA = 5.81 - 0 = 5.81.