PCB Design Perfection Starts in the CAD Library - Part 11

Part 11 - PCB Design Perfection Starts in the CAD Library

Placement Courtyards

 

The IPC-7351B standard focuses on five two major aspects of the CAD library:

  1. Land (pad) Size and Spacing calculations
  2. Placement Courtyard Excess boundary
  3. Land Pattern Naming Convention
  4. Zero Component Orientation
  5. Padstack Naming Convention

 

All of the other aspects of the CAD library section such as silkscreen and assembly outlines, polarity markings, reference designators, centroid marking, etc. are considered user definable drafting items. This includes all of the rules that pertain to these items such as line widths, silkscreen to land spacing, polarity sizes, reference designator height, etc. They are not part of the IPC-7351B standard.

 

The placement courtyard boundary is defined in the IPC-7351B standard, but the line width used to create the closed polygon outline shape is user definable. There are industry recommendations like the IPC-2610 series for Fabrication and Assembly documentation. The IPC LP Calculator uses 0.05 mm as the default line width. The default solder mask and paste mask values are mentioned in the IPC-7351B as to be 1:1 scale of the land size, but this is only a recommendation.

 

See Courtyard Determination in Figures 1 and 2 below to see the 3 outlines defined by IPC-7351B as:

  1. Maximum Component Boundary
  2. Minimum Placement Courtyard
  3. Courtyard Manufacturing Zone

 

Figure 1 - Plated Through-hole Courtyard Excess Determination

 

Figure 2 - Plated Through-hole Courtyard Excess Determination

 

Here are the IPC-7351B standard spacing rules from the Maximum Component Boundary to the Minimum Placement Courtyard Excess:

  1. Least Environment = 0.1 mm
  2. Nominal Environment = 0.25 mm
  3. Most Environment = 0.5 mm

 

There are different Placement Courtyard spacing rules for Grid Array packages based on ball size:

  1. Ball size above 0.50 mm = 2 mm
  2. Ball size between 0.50 mm & 0.25 mm = 1 mm
  3. Ball size below 0.25 mm = 0.5 mm

 

One of the key aspects of the placement courtyard is that it allows room for rework. In the case of the BGAs, the larger the ball size, the larger the heat nozzles and removal equipment is for getting around and under the component to unsolder the contacts.

 

An important note to make here is that if you have no intentions of ever reworking (removing and replacing) a BGA component the 2 mm placement courtyard is not necessary and a 0.5 mm courtyard excess is okay. This also is relevant to the "Most Environment" where the minimum courtyard excess is 0.5 mm on all SMT land patterns — i.e. the military and medical instrument industry might use the Most Environment and require "Class 3" High Reliability Electronic Products where continued high performance is critical and product downtime cannot be tolerated. The "Class 3" fabrication is very expensive and if a component on the PCB fails, rework becomes necessary to reduce cost. Rework requires additional land pattern placement courtyard excess to allow adequate space for rework equipment.

 

Alternatively, the "Least Environment" for high density PCB Layout, such as handheld devices, has a courtyard excess of 0.1 mm with no room for rework equipment. So if a component fails in your cellular phone, it will not be sent back to the shop for rework, but instead, it will be discarded.

 

The placement courtyards can be placed next to each other so the outline overlaps; however, you need to discuss this with your assembly shop if they require an additional manufacturing zone for their process. The placement courtyard round-off snap grid is 0.5 mm. The question of "why don't you place the silkscreen outline outside the land pattern boundary" is a FAQ and this is the answer – "It is important that all silkscreen outline data be located inside the placement courtyard".

 

This rule includes the post assembly inspection dot, which sometimes gets partially placed outside the placement courtyard. This is why in the upcoming IPC-2614 for Board Fabrication Documentation and drafting standards the "post assembly inspection dot" will become the "post assembly inspection line" and it will look like this to keep them inside the Placement Courtyard outline. See Figure 3. Also, the line in most cases has higher visibility on the assembled PCB. The post assembly inspection marker is used for post assembly QC for possible component inverted (incorrect) assembly orientation. The polarity marker on the component should align with the post assembly Inspection silkscreen marking.

 

Figure 3 - Post Assembly Inspection Lines inside Placement Courtyard

 

The placement courtyard or courtyard excess is the smallest area that provides a minimum electrical and mechanical clearance of the maximum extremities of the land pattern and/or the component body. However it is the responsibility of the user to verify the land patterns used for achieving an undisturbed mounting process including testing and an ensured reliability for the product stress conditions in use.

 

For many through-hole parts and connectors, the placement courtyard will follow the contour of the component body outline and land pattern. See Figure 4.

 

Figure 4 - PTH  Resister Courtyard

 

Most enterprise CAD tools, like Mentor Graphics Expedition Enterprise, have a different DRC checking feature that the user can define the component type to component type spacing rules. In this case, the placement courtyard excess should be turned off and the placement courtyard would be identical to the maximum component boundary. The maximum component boundary is a combination of whichever land pattern features, land (pad) or component body which protrude the most from the center.

 

Figure 5 – Enterprise CAD Tool Placement Courtyard

 

The Enterprise CAD tool placement courtyard is not defined in the IPC-7351 standard. This concept is based on the Enterprise CAD tools ability to determine various components body to body spacing that are user definable rules within the CAD tool options.

 

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