PCB Land Patterns
The standard features that comprise a land pattern.
Alternate methods of land pattern creation.
Steps to follow for land pattern creation to minimize work duplication and the chance of error in translating manufacturer’s data.
Well-designed and legible PCB land patterns result in a functioning board that’s easy to read
Repeatable design focuses on atomizing the smallest possible design elements so they can be reused across a number of different projects if the need arises. In the electronics system design world, this takes the form of PCB land patterns, which outline where parts interface on the board and provide some general outlines of the shape of the parts.
The need for accurate land patterns is undeniable: electrical performance and long-term reliability follow from proper solder fillets (or other forms), and the interconnected nature of a PCB means failure at one component could cascade elsewhere, up to catastrophic failure in certain cases. Even in less disastrous instances, improper land pattern design can result in excruciating troubleshooting and complete redesigns of boards. Understanding the features of land patterns and how best to organize their creation to reduce duplicated work is key for any design team.
Analyzing PCB Land Patterns
A simplified image of a chip land pattern
The PCB land pattern may occupy a small section of board design, but there is no worse feeling than having a board be rejected due to soldering issues caused by an incorrect land pattern. As the interface between the board and component, the land pattern represents a crucial connection point. Land patterns that adhere to the manufacturer’s datasheets (some alterations to support the assembly process are allowed) support performance and reliability, while incorrect land patterns serve as a detriment.
The land pattern is comprised of a few core elements that convey part information:
- Pads - The areas of mechanical and electrical connectivity between board and components. These can come in the form of etched copper on the outer layer for surface mount devices (SMD) or drilled and electroplated holes for through-hole (TH) technology. Pads containing vias for thermal purposes may be present, but layout is generally better served by including these at the board level rather than the component level. The pads are the most important feature of the land pattern, and any unintentional deviations from manufacturer specifications can result in components that are unable to solder down, or more insidiously, lose connectivity and eventually detach from the board entirely.
- Component body - The body represents the physical extent of the part once installed. Body sizes can vary from simple chip packages that occupy only a small area between pads, and connector receptacles, which extend past the board edges to allow ease of mating with plugs. While the land pattern is a 2D space allotted to a component on the board, its vertical height may be of importance to the enclosure or for vertical connectors. 3D body files provided by manufacturers can utilize the component body shape to provide a 3D view of the assembly – and importantly – serve as a secondary check of the body dimensions.
- Place bound - Sometimes alternatively referred to as the courtyard, this artwork layer represents the “breathing room” provided for every component to prevent overcrowding and any resulting solderability issues, especially rework. Adequate spacing can also help prevent thermal and electrical performance issues by improving airflow and increasing conductor air gap, respectively.
- Holes and slots - Not every drilled feature supports electrical connectivity: depending on the size of the component, additional pins or pegs may be provided to aid in mechanical support. Pins can also be used to prevent a backward installation on some reversible parts.
- Silkscreen - Pin 1 indicators, polarity markers, reference designators, and other visual items are provided to aid the assembly and readability of the PCBA for diagnosing errors.
Implementing Land Patterns Into the Greater PCB Design Flow
A handy chart to determine how to approach land pattern creation
The process of creating land patterns can differ tremendously. The size of the design team will likely be the foremost determinant of how land pattern creation is approached:
- Smaller teams may have individual layout personnel create their parts.
- Larger teams may have a dedicated part creator, known as a librarian, that exclusively handles part creation.
- Still larger teams may supplement librarian work with part creation services, typically for more complicated land patterns or custom designs. Alternatively, design teams may prefer a comprehensive solution to library maintenance via an outside contractor.
Regardless of the implementation, design teams should follow a few simple steps for part creation:
Check the central library to see if the part has already been created. This library should be for verified parts only to reduce the time spent on duplicating work. Failing this, matching land patterns may be sourced from older designs provided there were no assembly issues, but designers should adopt a “trust but verify” mentality.
Locate the datasheet, which should be furnished by the engineering team. Online repositories can be used if the datasheet is not provided, including the manufacturer’s website. The manufacturer may even provide the land pattern itself.
Is the part a standard package construction? If so, it can probably be handled with a wizard tool that will accept the manufacturer’s specifications and output a land pattern.
More complicated components may require manual part creation. This is the most involved method of part creation, and also carries the highest risk of error. When applicable, designers should adapt verified land patterns to minimize the amount of new work that needs to be done. For example, a new connector may only need the addition/removal of a few pins and adjustment to the package body or a swap between horizontal and vertical connectors that use the same hole and pad sizes.
Land pattern creation is not complete without verification: an incorrectly dimensioned land pattern can easily necessitate a redesign of the board. Internal verification processes should focus on matching critical measurements like pad and hole sizes/location, pitch, the component body, and distance to the board edge, if applicable. Misapplied place bounds and silkscreen information is unlikely to affect the soldering process but can flag false positives in the design rule check or failure during operation owing to backward installation. For these reasons, unverified land patterns should never be used.
Limit Rework and Revisions With a Comprehensive ECAD Tool
PCB land patterns represent the overlap between the bare board and components, and errors in manufacturing or abbreviated field service may result if proper best practices are not adopted. Land pattern verification needs to enact stringent checks to prevent defects from arising, but more comprehensive defect detection at the board level is also necessary to avoid incurring excess costs during production. Cadence’s PCB Design and Analysis tools, like the Constraint Manager, support users with customizable DFM and are fully integrated with OrCAD PCB Designer.
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