PCB Design Perfection Starts in the CAD Library - Part 17

PCB Design Perfection Starts in the CAD Library

Mounting Holes

 

Note: All numeric values in this article are in millimeters

 

Mounting holes are on every PCB design, but there is very little documentation about this subject matter. A Google or Wikipedia search on “Mounting Holes” renders no solutions to the PCB designer. Another issue that interferes with standardization is Imperial Unit ASNI hardware and ISO Metric hardware. So we’re going to have to explain both unit systems for clarity. But first let’s start with the basic fundamentals that both unit systems have in common.

 

Mounting hardware normally consists of these 4 items (See Figure 1) –
  1. Phillips Head Screw
  2. Hex Nut
  3. Flat Washer
  4. Lock Washer
 

Figure 1

 

There are 4 types of mounting holes –

  1. Supported – Plated through with annular ring
  2. Supported – Plated through with annular ring with vias
  3. Unsupported – Non-plated and with copper pads
  4. Unsupported – Non-plated and with no copper pads

 

The supported mounting hole usually gets tied to the GND plane without a Thermal Relief (a direct connection is best) and the supported hole w/vias gets both the main hole and the vias tied to the GND plane. Due to the fact that mounting hardware never gets soldered to the PCB, there is no reason for a Thermal Relief pattern and you connect all holes (including vias) directly to the plane. The unsupported (non-plated) hole has no connection to a GND plane layer and they require an outer layer keep-out defined that compensates for the hardware tolerances. See figure 2 for an illustration of the slop tolerance of a flat washer and the necessary copper keep-out sizing.

 

Figure 2

 

There are two primary reasons for adding vias to the supported mounting hole. The first was to insure that if the screw threads stripped the copper plating from the main hole that the vias would still provide adequate ground connections. The second reason was for additional support to prevent the PCB from crushing when too much torque was used to tighten the nut. The average via hole size for mounting holes is 0.5 mm. See Figure 3 for a supported mounting hole with vias.

 

Figure 3

 

See Table 1 for the most popular PCB hardware sizes for metric unit technology.

 

Table 1

 

In Tables 2 and 4 there are 3 different padstack configurations for each metric screw size for land (pad) size calculations.

  1. No Washer – Pan Head clearance
  2. Flat Washer

 

The land (pad) diameter is equal to the hardware diameter and a placement courtyard is added to compensate for the slop tolerance indicated in Figure 2.

 

Note: These Land (pad) and Placement Courtyard padstack values are in the “Least” material values. You can add 0.25 mm for “Nominal” or 0.5 mm for “Most” Land (pad) and Placement Courtyard environments. The hole sizes are for a loose fit.

 

Table 2

 

Table 3

 

See Table 3 for the most popular PCB hardware sizes for ANSI standards.

 

Table 4

 

Table 5

 

Table 6

 

The “Loose Fit” mounting holes are normally used on large boards greater than 100 mm (4”) and the “Tight Fit” mounting holes are commonly used for smaller board sizes.

 

There are some differences in hardware manufacturer’s feature sizes, so make sure that the hardware you use is adequately covered with the correct pad size and/or keep-out.

 

There are 3-Tiers for the Mounting Hole family, but the only difference is the “Placement Courtyard Excess”.

  • Least – 0.1 mm annular
  • Nominal – 0.25 mm annular
  • Most – 0.5 mm annular

 

Coming Up

Additional brief topical articles will appear in future newsletters. You can also read more detail in my blog, which can be found at:http://blogs.mentor.com/tom-hausherr/

 

Written by Tom Hausherr CID+

EDA Library Product Manager

Mentor Graphics Corporation

Reprinted by permission from iConnect007