Circuits Assembly Online Magazine - Understanding Outsourced PCB Layout

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The key components include corporate standards and OEM-EMS collaboration.

The process of designing a printed circuit board is key to the overall product lifecycle. This process not only impacts product development, it influences the speed and success with which a company is able to bring products to market. Here are an EMS provider's insights into the stages of PCB layout – a key phase in the design process.

During design layout, an EMS provider gathers necessary input from the customer, including schematics, mechanicals and other critical information, and leverages its experience with a cross-section of product types to lay out the board and conduct the necessary reviews. Once the customer has approved the board layout, the EMS provider outputs a set of files that are either leveraged by the OEM in the fabrication, test and assembly of PCBs, or by the EMS provider if it has been engaged for turnkey services.

The level of outsourcing engagement greatly influences the activities, people and stages involved in layout. In recent years, OEMs have begun outsourcing activities traditionally handled in-house. These activities range from full turnkey solutions – through which EMS providers act as an extension of the OEM throughout the design, manufacture and test of a product – to outsourcing a single aspect of the product lifecycle, such as PC layout, sometimes in conjunction with other engineering services.

OEMs' engineering teams need to engage in PC layout well in advance of circuit design. The OEM may not yet know if there is enough real estate to fit all the necessary components on the board and if it will meet certain criteria such as design for excellence (DfX), test points and signal integrity. This early supplier involvement has become typical of outsourced PC layout during the past three years. (And EMS providers welcome this shift, as the earlier they are engaged, the more value they can provide to the customer.)

It is crucial for EMS providers to have an established, proven set of global guidelines and processes for PCB layout. These can be internal, such as corporate guidelines, or external, such as ISO certifications that many customers require. These processes ensure the same method is used for each design layout, and that the output provided meets customer expectations. Regardless of OEM input, the output is verified in the same manner.

Celestica's PCB layout team follows established ISO 9001:2000 procedures, and is also required to follow an internal corporate design guide that outlines standards and guidelines for:

Design for manufacturability (DfM).
Design for fabrication (DfF).
Design for test (DfT).
Sourcing.
Manufacturing/assembly.

Manufacturing and test guidelines should also be taken into consideration during layout (Figure 1). While many established customers plan their level of outsourcing engagement in advance, others do not plan early enough. In such instances, unless customer-specific guidelines are provided, Celestica leverages its own manufacturing guidelines. OEMs that eventually outsource the assembly find this "turnkey" mindset beneficial.

Click here to see Figures 1 and 2 (688KB PDF).

Stages of Circuit Layout

Once the OEM's engineering team has developed a product concept, the physical aspects of the product must be determined (Figure 2).

The OEM must decide on the product's functionality, how many circuit boards will be required, the physical "envelope" of the product, packaging size, etc. OEM engineers then design the circuit board schematics and mechanical drawings and define the types of components to be used. It is important to note that, as the industry further embraces outsourcing, it is becoming commonplace for the customer to develop the product concept and then engage an EMS provider for the remainder of the product's lifecycle.

Before commencing with layout, a team must be established to manage the process. At Celestica, once a project is defined, a team is quickly established. A program manager, whose responsibilities include monitoring the scope of the project and ensuring that customer requirements are met, leads the team. The rest of the team includes electrical and mechanical engineering, DfM, DfT, physical layout design and components procurement. This comprises the minimum team that should be assigned to new product development; depending on the complexity of the project and customer requirements, the program manager may add additional members.

A formal face-to-face kick-off meeting is often the first step of the layout process. This meeting allows all technical and business authorities involved in the project to interact, and should be held with all first-time design customers, for complex projects, and for design projects involving other departments or suppliers. A design engineer typically represents the OEM customer. The team may also choose to invite representatives from external parties such as PCB fabricators, high-volume printed circuit assembly or testing houses, or EMI testing facilities. The meeting provides a forum to discuss customers' and outside vendors' requirements, expected design results, project timeline and schedules and the end product. This discussion, along with customer input, leads to the development of the design plan.

The next stage should involve obtaining the basic project input requirements or a statement of work from the customer. This may include:

Board outline drawings.
Component specifications.
Design technologies to be used.
Netlists and critical netlists.
Suggested placement drawing.
Fixed component location drawings.
Logic diagrams.
Wiring diagrams.
Schematics.
Test specifications.
Renderings.
Mechanical drawings.
Samples and any other supporting material.

This input is evaluated to determine if it will produce the expected design results and output documentation.

The output is further defined when the design task is outlined more precisely and the customer exposed to other available deliverables. Design output packages may include:

Bills of material (BoM).
Assembly or fabrication drawings.
Schematics.
Wiring, timing or block diagrams.
Gerber photoplot files.
Aperture lists.
N/C drill files or drill bit lists.
Back-annotation files.
Component X-Y placement files.
Valor ODB++ database.
Netlists in various formats.
Signal integrity reports.
Test procedures.
Approved vendor lists (AVL).
Approved manufacturer lists.
PCB databases.
Film packages.

If the customer requests additional output, the design plan must be adjusted accordingly.

The EMS provider's PCB layout group then develops CAD library symbols for each component the customer identifies, determines the physical board size, and generates a netlist based on the board schematics that itemize components and their point-to-point connections. This is a collaborative phase, as OEM and EMS electrical and mechanical engineers work to flag restrictions and other potential issues. In these early stages, the board layout team is in daily contact with engineers.

The board layout team then determines the placement of critical components on the board. Critical components include high-speed parts and those that are sensitive to other components and must be placed and routed in a certain way. Depending on the level of engagement, at this stage, signal integrity analysis services can be leveraged to save the customer time and money. This upfront analysis results in fewer revisions and builds for each layout and is quickly becoming a "must" given the complexity of today's designs. One might say that design analysis is to the electrical design engineer what "spell-check" is to PC users – once they use it, they can't live without it.

Our OEM customers use design analysis extensively, a requirement on all new board developments internal to Celestica. This technology can contribute to the layout process, including the schematics phase, in several areas. This technology permits Celestica to determine the outcome of the customer's board schematic. The ability to manage the complexity of designs and general design constraints is critical, as companies cannot afford multiple design revisions to resolve issues that could have been resolved before the first release build.

The next stage of PCB layout is design verification: providing both parties with visibility into the progress and direction of the project. Verification activities vary, depending on customer requirements (defined in the design plan) and the EMS provider's level of engagement. In-process design reviews are held and approved at milestones specified during the kickoff meeting or in the design plan. All approving authorities receive data in the form of files, plots and reports, which permit them to do in-process checking at specified points.

While the layout is in process, it is sent to various people within the EMS provider for review so that issues can be flagged. Reports are then sent to the customer to review verification performance. Design verification can be conducted through the use of one or more of:

Paper check plots.
System-generated reports.
On-screen reviews.
Database checking tools.
System-specific checking tools.

Following design verification is design validation. As the circuit's owner, the OEM engineer is the only party capable of determining whether the original intent of the electrical circuit performs to specifications. The customer's ability to validate is enhanced by the use of on-screen software applications and reports that show the expected results of the layout techniques as they are used. If design changes are required, the customer must approve them prior to incorporation. An appropriate procedure and form should be used to document all design changes, ensuring their incorporation and disposition. The customer must be notified of the impact of each proposed change with regard to schedule and cost and must approve or withdraw the change request.

Following several review cycles and OEM approvals, the EMS designer prepares all of the documentation requested in the design plan. At this stage, the EMS provider may conduct a final analysis to see if all customer requirements have been met. The design release enables the next stage of product development to take place (i.e., fabrication and assembly).

It is extremely important that OEMs partner with EMS providers possessing the experience, skill set and best practices needed to optimize this phase of product development, shedding light on potential issues and enabling the customer to overcome any barriers to success.