PCB DFM Process

February 18, 2021 Cadence PCB Solutions

Key Takeaways

Artistic concept of automated PCB assembly

Avoid production delays by incorporating good DFM methodologies into your PCB

Design for Manufacturing

Design for manufacturing (DFM) is the practice of optimizing the engineering workflow of producing an electronic device. To name a few, these practices involve changes on the design-side of things, such as proper documentation, and layer stackup information, as well as procurement side, cost optimization, and project lifecycle management (PLM). 

In the race to market new and innovative technology, fresh electronic designs are coming off the drawing boards every day. With this much competition, it isn’t only the latest and greatest designs that will win the day, it is also which circuit boards can be built quickly, efficiently, and for the lowest manufacturing costs. To achieve that goal requires that today’s PCB engineers be well versed in design for manufacturing, or DFM, techniques, and methodologies.

Why DFM is Important in PCB Design

When a prototype circuit board is first being developed, the focus is usually on the functionality rather than the buildability of the board. As such the component placement on the board may not be optimized for assembly, which may result in higher assembly costs. Even during limited production runs for prototyping, a lot of manufacturing costs can be saved by designing the board for automated manufacturing.

These problems are magnified however when the board is ready to be moved into its production phase. Components that are not optimized for manufacturing will have to be re-positioned to avoid the necessity of manual assembly. As you can guess, having to solder even one component manually on a run of a thousand boards can quickly escalate manufacturing costs and delays. Another problem is with components that weren’t selected for long-term production. If their price is too high or their availability is limited, the board will encounter additional manufacturing problems.

Although problems like these are often worked out during new product introduction (NPI), this can create new difficulties for the development of the board. Having to replace sensitive components simply to make the board more manufacturable could end up affecting the performance of the circuitry. Circuit boards that require extensive layout changes in order to incorporate DFM changes, may end up going through a complete redesign and re-validation of their functionality.

In order to avoid all of these problems, PCB designers need to incorporate good DFM practices during their initial circuit board design. This will reduce the need for changes later on, and preserve the layout of sensitive circuitry that is fine-tuned for performance in the prototype. The first step in understanding DFM in circuit board design is to be aware of the manufacturing problems that can happen without them.

3D layout of PCB in Allegro PCB Designer

The 3D layout of a PCB where following good DFM practices is essential 

The Problems that Can Happen if DFM Requirements are Not Followed

To get a printed circuit board assembled as quickly and as efficiently as possible, it is important to design it according to the manufacturers’ DFM requirements. Before we start looking at those, however, let’s first examine some of the more common problems that can happen if those requirements are not followed.

Component Selection that is Not Optimized for Price and Availability

Electronic components can vary between sources in price, which can have an unexpected impact on the assembly of the board. Although this may have little effect on the cost of a prototype, that cost can quickly escalate once the board goes into production. For instance, paying an extra 10 cents for a part that is used 10 times in a design is a negligible increase of one dollar on a single board. For a production run of 10,000 boards, however, that cost has now increased by $10,000. When you consider all of the parts on the board, even a difference of one penny can have a huge impact on your assembly budget.

Price isn’t the only concern when it comes to procuring components though. Different vendors and distributors may offer variations in their part availability as well. The parts may be at the lowest price, but if they aren’t available when the board is in production you could face expensive manufacturing delays. Even worse is when the components selected for the design are at the end of their product life-cycle. Parts that are flagged as not recommended for new designs (NRND), end of life (EOL), or obsolete (OBS) will eventually be unavailable. These parts will have to either be procured from other resources for much higher prices, or the board will have to be redesigned to accommodate newer parts.

Design for manufacturing process

Automated Assembly Failures

The components placed on a PCB that are not optimized for automated assembly may cause some of the following errors in the manufacturing process:

Poor solder joints: Components that are not rotated correctly for wave soldering may form bad solder joints. This can happen to both thru-hole and surface mount (SMT) parts. Component locations also matter as some parts may prevent a good application of solder from being applied to others creating more bad connections. 

Solder shorts: Surface mount pins that are very close together may form thin slivers of solder between them. These solder slivers can create intermittent shorts that are very difficult to find and correct.

Tombstoning: Discrete SMT parts that are unbalanced thermally between their pins may have problems going through the solder reflow oven. This imbalance may unseat one of the pins and pull the part up until it stands up like a tombstone.

Floating parts: SMT components going through solder reflow may float out of position if their pads are the wrong size. This potentially could cause interference with other circuitry.

Any PCB components that are not placed correctly for automated assembly may require manual assembly instead. This will significantly slow down the manufacturing of a circuit board and cost more in time and money.

Design for Usability

Another common problem is when a circuit board is not designed for full system assembly and accessibility. By placing a connector on the board where it is difficult to access, it may slow down the manufacturing of the entire system as well as create additional problems later on. In some cases, board connectors, switches, and other mechanical interfaces get placed where they are obscured by other system-level components during assembly. This can force an entire redesign of the board.

The goal is to lower your production costs and increase throughput by designing the board to be manufactured without any errors. Next, we’ll look at some ways for PCB designers to be able to do that.

Plugging in a connector on a PCB

Not only does a PCB have to be designed for automated assembly, but for human assembly too

Incorporating DFM Methodologies in Your PCB Design

Design for manufacturability is usually thought of as belonging only to PCB layout. In truth, however, good DM methodologies should apply to all parts of the design. The objective is to design a circuit board that can be assembled without any errors, and that process starts with the parts that are selected in the schematic.

DFM Starts in the Schematic

When capturing a schematic, the PCB designer will select the components needed for the board. Often this is done based on the part’s functionality, but there are other factors that should be considered as well. These include the component’s price, availability, and its life-cycle status. As we pointed out earlier, one of the major DFM problems that can happen is when a selected part is too expensive or not available to use. To make sure that you avoid this DFM problem, designers should scrub their bill of materials (BOM) to make sure that the parts they have selected are available and at a good price.

One way that you can help yourself with this is to involve your PCB manufacturer early on in your design. They will be able to help you with your component selection by discerning which components are available or not as well as which distributor offers the best price. They may also be able to recommend other circuit design options that can leverage alternate components that are less expensive or more readily available.

Preparation for PCB Layout

Preparing the PCB database for layout is an essential part of starting the design, but it is also very important for good DFM. In this early stage of the design, there will be some important decisions on how the board will be set up, which will also have a direct bearing on how the board will be manufactured:

PCB Layout Flow Chart

Board layer stackup: Stackups are usually defined for signal integrity and thermal management, but the manufacturability of the board needs to be taken into account as well. One example of this is deciding what board thickness is required for the optimum drill aspect ratio for the vias being used. This may require using different board materials or changing the board layer count. Mechanical outline: In order to create an accurate board outline model within the CAD system, a detailed mechanical drawing of the shape and size of the board is required. It should also include details on where to place fixed components and other mechanical features like mounting holes and slots. To avoid costly redesigns and assembly errors, good communication between all members of the design team is essential. Library parts: To avoid soldering problems during assembly, the latest versions of the library parts to be used in the design should be on hand. Using industry-standard or vendor specified footprints will help to guarantee that the components on the board will be easily assembled with automated manufacturing equipment.

3D layout of power supply components on a printed circuit board

For the best power integrity, a tight placement is preferred, but it can also cause DFM problems 

Place and Route PCB Components According to DFM Requirements

PCB component placement and routing need to be laid out with the shortest signal paths in high-speed and power supply circuits to achieve the best electrical performance. This requires that the components be very close to each other, which can in turn cause problems for automated assembly equipment. It is important that the PCB designer create a balance between the electrical and manufacturing needs of the board when laying it out. Here are some of those requirements that designers should be aware of:

Automated installation Wave soldering Solder reflow

In order for the pick and place machines to install parts on the board, they need consistent clearances between the components. Your manufacturer can give you precise details on what these clearances should be. Also, remember to leave enough room around larger components that will have to be manually assembled to the board for soldering equipment.

Both plated thru-hole (PTH) and surface mount (SMT) components need to be placed perpendicular to the direction of the board going through the solder wave. This will help to equally distribute the solder around all of the pins of the part. Larger SMT parts should also not proceed smaller parts through the wave to eliminate the possibility of shadowing the smaller parts from the full effects of the solder wave. Additionally, the placement of PTH and SMT can affect your solder, such as when they are placed on the same side of board and a reflow process would precede the solder wave. 

The two pins of smaller discrete SMT parts need to be thermally balanced for solder reflow. When one pad is connected to a large area of metal, it can act as a heat sink and cause the solder paste on that pad to reflow more slowly than the other resulting in tombstoning the part. Pad size is also a concern, as pads that are too large may cause the part to float out of alignment, while pads that are too small may not hold enough solder paste to reflow into a solid connection.

PCB footprints

Care must be taken in creating custom PCB footprints that are not covered by industry or vendor specifications:

  • Thru-holes that are too large for their pins can disperse solder too quickly through them resulting in a bad solder connection.

  • SMT pads that are too large may have an excess of solder paste which could bridge to a neighboring pad and cause a short.

  • Pads that are too close together are also susceptible to solder bridging. This is often seen in fine-pitch components with insufficient solder mask between the pads to prevent this.

Design Finalization and Documentation

In addition to place and route, there are many other tasks that are done during PCB layout which can affect the manufacturability of the board:

PCB Test: In order to verify the accuracy of the assembly process, circuit boards normally include testpoints that are accessible to the probes of a test fixture. If the testpoints aren’t included in the original design however, the board will have to be redesigned to add them before it can graduate to production. Redesigns like this are not only costly and time-consuming, they may also introduce new DFM problems that didn’t exist before. 

Silkscreen and board markings: Silkscreen markings are important for PCB test, rework, and debug. Without the necessary information on the board, manufacturing can be slowed down while rework and test technicians look for specific parts or polarity markings.

Drawings: Without complete and accurate information time can be lost while manufacturing details are verified, or even worse, errors are made in the production of the board.

As we have seen, there are many different DFM requirements that have to be met in order to achieve error-free circuit board manufacturing. In order to meet these requirements, it is essential that the PCB design tools in use are equipped with features and functionality that are specifically made for DFM. Next, we will look at how some of these features in a PCB design system can help.

Tweezers and a soldering iron used for manual assembly of a PCB

By following good DFM practices, you can avoid manual PCB assembly processes

How PCB Design Tools Can Help Achieve Good DFM in Your Design

Now that we’ve looked at some of the manufacturability problems that are common in PCB designs and how to prevent them, the next step is applying good DFM practices in our designs. To accomplish this requires a PCB design system that gives you the tools you need to design according to the rules, as well as powerful features to enhance and simplify your work. One of the best tools you can use for this is Allegro PCB Designer from Cadence. Here are some of the features in Allegro that can help.

Get the Correct Components with the Unified Parts Search Feature

Components that are out of date, too expensive, or simply not available can add a lot of expense and delay to the manufacturing of your circuit board. To avoid these unexpected assembly costs, you need access to the latest parts along with their data sheets and part information. Traditionally this has required a long part procurement process before the parts are available for use in the design.

Allegro has improved this search process through the use of its Unified Parts Search tool. With it, PCB design engineers can search approved library sources for component information, and then download the data into their own schematic for immediate use. This gives designers access to vendor information such as datasheets, symbol and footprint models, and 3D step models as well. With Unified Parts Search PCB designers can be assured that they are working with the latest vendor-approved parts in their designs.

Selecting a resistor from the Unified Part Search, and placing it on the schematic

With Unified Part Search, Allegro designers can find the part they need and use it immediately

Advanced CAD Functionality to Create Specific PCB Footprints

Even with the ability to download library parts directly into your PCB design system, there will still be times that you will need to make custom parts. There may not be an approved footprint yet, or you may need a modified version of the footprint for unique design requirements. In these situations, you will need a CAD system that gives you the ability to easily create schematic symbols and footprints for layout.

Allegro has a lot of different functionality to help with the creation of library parts. Not only do you have a host of drafting tools available to you, but there are also part creation wizards that can save you time and work. An incorrect part can lead to all kinds of DFM errors in your design, and it is essential that when you build your own parts that they are made correctly.

Real-Time DFM Rules While You Design

At one time a simple set of clearance rules worked for PCB design, but that is no longer the case. Different components, different areas of the board, and different manufacturing techniques all stipulate that multiple DFM rules have to be obeyed. This can be overwhelming for a designer to keep straight in their head, and most tools aren’t set up to track all of these different rules. Fortunately, Cadence has you covered.

The DesignTrue DFM technology in Allegro PCB Designer is made for complex DFM rules and constraints. As part of the Cadence Constraint Manager system but separate from signal-based constraints, the DFM rules can be set up to specifically monitor different metal to metal clearances as well as non-spacing conditions such as holes and silkscreen. Everything that you need to get your board through your manufacturer’s DFM requirements can be set up and checked for in Allegro.

Cutting Edge Place and Route Tools

To meet your manufacturer’s DFM requirements, it helps to have the best component placement and trace routing tools available to work with. Allegro offers a multitude of features and functions to help you instantiate and refine the placement of your components on the board. Not only can you easily place your components according to your DFM rules and constraints, but you can quickly edit that placement as needed.

Once placed, the connecting nets can then be quickly routed with a variety of tools that will give you manual or automatic control over your trace routing. These tools also work in conjunction with the design rules and constraints to not only govern your board's electrical performance but to keep the design within your DFM tolerances as well.

Direct Data Transfers with Manufacturers Using the IPC 2581 Open Standard

In PCB layout, good DFM starts with the right board layer stackup configuration. The needs of the circuitry have to be balanced with the manufacturing requirements and costs in order for the board to be produced without errors. Circuit board manufacturers have built their careers on engineering these stackup configurations, but there has always been the problem of getting this data into the PCB design before the layout commenced.

Allegro PCB Designer has provided a solution for this problem with its IPC-2581 bi-directional interface. IPC-2581 is an open standard used to communicate PCB manufacturing data between design and manufacturing. PCB designers can pull in their manufacturer’s recommended stackup data directly into their designs instead of recreating the stackup manually. Then when the design is completed, the IPC-2581 interface can be used to send back the fabrication and assembly files to the manufacturer.

Open Vendor Communication through Cadence’s Customer Partner Portal

Communication is often one of the leading culprits in DFM errors, and Cadence has addressed this problem by providing a partner program to open up communication between designers and manufacturers. With the DesignTrue DFM Partner Program, PCB designers can initiate communications with a partner manufacturer before the design is started. This way you can send your design’s requirements directly to a leading manufacturer, and they can respond with an Allegro ready set of rules and constraints that can be read directly into your design environment.

Full DFM Support Using Allegro PCB Designer

With the need to improve both time to market and production yields, designing a printed circuit board to meet the DFM requirements of your manufacturer is no longer an option. Every PCB that is submitted for manufacturing should be fully DFM compliant for error-free fabrication and assembly. This may be a change in direction for some designers, but Cadence has paved the way down this path with the full services and features of their software.

To stay ahead of the DFM curve, you need the best PCB design tools available to keep yourself aligned with the requirements of your manufacturers. Allegro PCB Designer has the tools and functionality you need to fully set up your design for error-free manufacturing.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

 

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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