DFA

Printed Circuit Board (PCB) Design for Assembly (DFA) is critical to customers who outsource PCBs for design and manufacturing. If correctly performed, it shortens the production cycle, minimizes development cost, and ensures a smooth transition into production from prototype stages.

When designing a printed circuit board (PCB), bringing the future into the present is critical to ensuring your board can be successfully assembled, prototyped and produced. By planning ahead, you can reduce the production cycle, minimize development costs and ensure a smooth transition from the prototyping to production phases of product development. The institution of specific steps that target the assembly process into a PCB development DFM regimen is known as DFA. But before laying out guidelines that constitute good PCB DFA, let’s take a look at why it’s important for PCB product development.

PCB manufacturing includes fabrication, components sourcing and acquisition, and assembly. Of the three, assembly is typically the most time-consumption and expensive process. The primary objective of PCB assembly is to securely attach the correct components to the fabricated bare PCB. Good PCB DFA, therefore, incorporates steps into the design process that support and enhance the PCB assembly manufacturer’s ability to meet this primary objective.

In PCB design and development, there are two assembly stages: PCB prototype assembly and PCB production assembly. Both of these should be included as part of DFA during PCB design.

 

DFA during PCB Prototype Assembly

 

The assembly process for prototyping strives to ensure the correct components are securely attached and clearance and tolerance restrictions are adhered to. PCB DFA questions that should be asked during design phrase include:

 

• Do components match their pads?
• Are components adequately spaced from each other?
• Are component markings and identification accurate and clear?
• Are drill hole rules applied?
• Are solder mask guidelines applied?
• Is thermal relief adequate?
• Are PCB edge clearance rules applied?

 

DFA during PCB Production Assembly

 

All of the above considerations for PCB prototype assembly apply to PCB production assembly, as well. However, we also have to take into account high volume production, testability and component availability and reliability. To aid the process, the following DFA questions need to be asked:

• Does board shape allow panelization?
• What type of quality control testing should be used?
• Will components be readily available for future runs?
• Are thermal relied options included?
• Will the board be subjected to vibration?

 

Successfully addressing these questions, whether for PCB prototype or production assembly, ensures that your DFA efforts are enhancing your manufacturer’s ability to meet the primary objectives: securely attaching your component in order for your board to meet this performance objectives in its intended environment. Nearly as important to your PCB design and development process is the speed at which you get your boards back from the manufacturer. Thus, the secondary objective of PCB DFA is to institute actions during design that aid your PCB manufacturer and supplier in performing their role as quickly and efficiently as possible.

 

How is DFA Important in PCB Design and Development?

 

When applying DFA during the design process, the most important question to ask yourself as “How will this decision impact the assembly of my PCB?” If your decision makes the process harder, longer or more expensive, then you should reconsider. Below are some specific PCB DFA actions that you can take, and why these actions are important. Taking these steps can ensure the above questions will be answered and the DFA solutions can be implemented within your PCB design. Following these guidelines will assist your PCB and assembly manufacturer in efficiently producing your boards with securely connected components to meet your design and development needs.

 

What to Do?	Why Is It Important?
Choose readily available components and validate their continued production.	This will prevent or minimize possible future production delays.
Apply component spacing guidelines.	The placement of components impacts whether your board can be assembled, what soldering techniques can be used and what type of thermal dissipation is needed. It may also affect signal integrity.
Use component manufacturer recommendations for footprints.	This should prevent pad mismatches and ensure that accurate markings and identification are present.
Adhere to spacing clearance and tolerances that fall within your manufacturer’s capabilities.	This ensures manufacturability of your PCB design.
Adhere to size, spacing and tolerances for drill holes that fall within your manufacturer’s capabilities.	This ensures manufacturability of your design.
Consider the operating environment of your PCB.	For boards that will be subject to excessive movement or vibration, thru-hole components may be preferred, as they are more securely attached than SMDs.
Follow good trace routing techniques.	Traces misalignment with pads or vias may result in inadequate solder connections and rework.
Apply PCB edge guidelines.	PCB shape and component placement can impact what panelization can be used.

 

Although this list is not exhaustive, it is a good starting point for a smoother PCB assembly process. You might encounter additional issues if your design requires special single source components or your board placement dictates using a specific shape. As size, spacing and clearance tolerances are highly dependent on the equipment and processes used, you should include your PCB manufacturer early in the process to ensure that proper specifications are instituted. Following these specifics and maintaining clear communication with your PCB assembly manufacturer throughout the process is a part of good DFA implementation.

Applying good DFM for your PCB design is more than just following rules and guideline to simplify your PCB fabrication process. Good DFM includes instituting good practices that support board assembly as well, and PCB DFA should be an important part of your overall DFM strategy.

 

Rules for Preventing Components Headaches at PCB Layout

 

Preventing components headaches at printed circuit board layout involves a number of key design considerations and precautions as following:

• Carefully evaluating PCB CAD design tools because each commercial version has trade-offs. Some allow parts or components to be easily designed on the PCB, while others make it more difficult. One tool in particular creates a component in a one-step process, but provides limited access to component pins. Another tool takes three to four steps to create a component. However, it incurs more time, yet is more flexible. Radial components at the CAD level pose potential issues due to the possibilities of CTE mismatch. To avoid problems, it’s best to select the proper CAD tool for a given PCB layout. But regardless of the tool used, PCB layout requires that IPC design guidelines 7351 must be stringently followed when designing through-hole components on a PCB.

 

• Keeping in mind fabrication tolerances. Sufficient margin on the pads of a through-hole component must be maintained. Otherwise, “breakout” can occur if requirements aren’t maintained for the annular ring. Also, correct coefficient of thermal expansion must be carefully maintained for plastic- and ceramic-based leadless chip carrier components, especially Pb-free, to avoid component thermal damage during high temperature cycles.

 

• Making sure component mechanical specifications are carefully read and correctly implemented. It’s vital that mechanical footprints on the PCB’s physical surface are viewed from top side. A few component manufacturers do inadvertently specify their components with a bottom side view without noting them as such. By placing a component specified this way on a PCB, the pin-out is changed 180° and PCB functionality is adversely affected. Checking out component height requirements is also important. Heights of components must comply with the PCB enclosure’s mechanical dimensions.

 

• Placing special attention to the PCB application. For example, the end product may be subjected to harsh environments with considerable vibration. Special component placement and robust PCB design must be applied to guard against these conditions.

 

• Not overlooking component life cycles. A big headache can occur when specifying an obsolete, soon-to-be-obsolete, or non-existent device.

 

• Taking special pre-cautions when replacing an obsolete component. Replacements or substitutions must be carefully evaluated to assure specs are correct and pin-to-pin compatibility.

 

• Following proper guidelines for component placement. Components must be placed at least 1.5 millimeters or more from the PCB’s edge. A pick and place line requires components to be as much as three to five millimeters from the PCB’s edge. A PCB undergoes different manufacturing cycles, and each machine has its own limitations. Hence, it is important to follow set guidelines to avoid damaging expensive components like ball grid array (BGA).

 

• Creating press fit components with very tight tolerances, ensuring there’s no wiggle room.

 

• Assuring PCB design-for-assembly (DFA) engineers are highly trained and experienced.

 

• Checking that high voltage or current traces are substantially distant from one another to avoid thermal problems.

 

• Providing thermal relief so that components can be properly soldered on a PCB.

 

• Applying heat sinks to dissipate heat from thermal intensive components.

 

How MADPCB Helps You with PCB DFA?

 

• Informs you of potential problems you can avoid by employing DFA guidelines.
• Makes available DFA guidelines that you can upload to your design program.
• Provides you with tolerance and clearance requirements.
• Assists you in understanding the importance of DFA and how it should be applied.
• Works with you to quickly correct any DFA issues with your design.

 

If you neglect to apply PCB DFA as part of your PCB design, you may be setting yourself up for future problems with PCB board assembly, such as longer production cycles, increased costs and unexpected hiccups when converting from prototyping to production. Prepare your PCB board for a successful by applying DFA before sending it off for manufacturing.

At MADPCB, we work with you to simplify the process of incorporation good DFA. Furnish accurate information for your DFM, employ state-off-art component validation software and enable you to view and download DRC files. Additionally, if you’re an Altium user, you can easily add these files to your PCB design software.

If you are ready to proceed and have your design manufactured, try our quick quote to send us your CAD and BOM files. If you want more information on DRC, DFA, DFM, DFT or how to apply best practices into our design, contact us.