PCBA Design for Optimal Manufacturing Process Flow

May 1, 2020 Cadence PCB Solutions

Key Takeaways

  • What is the manufacturing process flow for PCBAs?

  • The most common challenges to the manufacturing process flow.

  • How to optimize the manufacturing process flow for your boards.

 

Automated assembly of PCBAs with SMT components

PCB assembly manufacturing process flow

 

Believe it or not, there was a time not that long ago when it was considered standard for PCB designers to be virtually in the dark about how their boards actually were made. Yes, I agree this seems a bit counterintuitive today as most designers and developers recognize what is aptly articulated by a well-known Chinese proverb: “When one is prepared, difficulties do not come.” 

For your board builds, these difficulties are manifested as longer turnaround times, increased costs, lower quality, unreliability and sometimes the inability to be manufactured without significant redesign. And these will come if you are in the dark and do not design to aid the fabrication and assembly processes that result in the manufacture of your boards. 

Board fabrication and assembly is a manufacturing process flow that consists of process windows for virtually every step that if properly targeted will yield the best quality and reliability PCBAs. Let’s explore this process in more depth which will enable us to define a design method to help achieve optimization.

What is the Manufacturing Process Flow For PCBAs?

The importance of designers understanding and incorporating the principles and process of PCB manufacturing into the design cannot be overstated in terms of its relationship to the quality of the boards your contract manufacturer (CM) produces. This approach to PCBA development is based on collaboration and the use of DFM guidelines that are correlated with your CM’s equipment and techniques and has resulted in faster turnarounds, higher quality and better reliability. 

These results; although, far superior to what was previously acceptable, can be further enhanced for your specific design by recognizing that DFM specifications are typically ranges and not absolutes.

The building of a PCBA is a multi-faceted and well-defined process that consists of two distinct stages: Fabrication and assembly.

  • PCBA Fabrication is the mechanical construction of a printed circuit board (PCB) from a design file(s) that consists of the following steps:

    • Creating an image of the circuit layout on the board layer(s).

    • Etching or removing unwanted copper from the layers.

    • Drilling vias and mounting holes.

    • Adding copper or plating the vias.

    • Adding soldermask to protect the non-copper areas of the board.

    • Printing the silkscreen layer that consists of labels, component markings and reference indicators, polarity symbols and other information.

    • Adding a surface finish to protect the copper areas of the board from oxidation and other contamination prior to assembly.

 

  • PCBA Assembly is the board manufacturing stage during which the components, connectors and other devices are mounted. This is accomplished by performing the following steps:

    • Putting down the solder paste foundation.

    • Placing surface mount devices (SMDs) on their pads.

    • Securing the SMDs by solder reflow.

    • Reworking any SMDs that do not have secure connections.

    • Placing through-hole components.

    • Securing through-hole components using wave soldering.

    • Doing final soldering for any components not secure.

    • Cleaning the board to remove any contaminants from the assembly.

    • Depaneling or separating the boards into individual units.

    • Adding a conformal coating to protect the boards during transport, storage and/or deployment in a hazardous environment.

 

The successful movement of your board through the fabrication and assembly steps listed above resulting in a usable PCBA is the manufacturing process flow. 

Challenges to the Manufacturing Process Flow

As evident from the listings in the previous section, there are many potential failure points during the manufacturing process. Failure can be almost eliminated from the first stage of manufacturing by following guidelines to ease PCB fabrication. For errors discovered during the final stage of building your boards, you will most likely have to make design changes. These redesigns can be avoided by following good DFM and DFA guidelines; such as for component package spacing shown below.

 

DFA spacing example

 

The utilization of DFM and DFA is essential for a smooth, efficient manufacturing process flow. However, how it is implemented determines the level of optimization of the process flow. Or the challenge to achieving optimal manufacturing process flow is how to align your design intent with your CM’s equipment and processes to ensure high-level reliable performance and board integrity over the lifecycle of your PCBA. 

Optimizing the Manufacturing Process Flow for Your Boards

Many of your CM’s DFM and DFA guidelines are in fact ranges that define the minimum and maximum parameters for the function of a piece of equipment. An example of this is drill hole size tolerance. If you choose a hole size that falls anywhere within the range and tolerance, your CM can bore it. Although your board would be manufacturable, the manufacturing process is not necessarily optimized. 

To achieve this, the parameter should be chosen such that it places the least amount of stress on the equipment’s capability and provides the greatest room for error. Typically, this means that you choose parameters that fall in the center of the process window for a manufacturing step, if at all possible. The more you can apply this to the manufacturing steps or process windows for the fabrication and assembly stages the more optimal the manufacturing process flow.

This strategy is more easily implemented if you have the right software tool that not only provides you with the ability to implement DFM (DFF if limited to fabrication) and DFA, but also DFT and structure the rules for its usage as shown below.

 

Managing DFF, DFA and DFT constraints

OrCAD DFF, DFA and DFT setup

 

With Cadence’s PCB Design and Analysis program, you have advanced DFM, DFA and DFT capabilities at your fingertips. This enables you to real-time DFM integration throughout your design that will lead to the best manufacturing process flow for your boards.  

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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