How to Design for Optimal Printed Circuit Board Assembly (PCBA) Speed

In sports, there is a well-known correlation between strength and speed. Basically, strength or capacity for power is a determinant in how fast you can move. However, there is a point where this positive correlation between strength (which is closely tied to muscle size) and speed no longer holds and adding size and strength actually slows you down. Similarly, in the not-too-distant past, productivity and efficiency had a comparable relationship in manufacturing.

Today, the most advanced manufacturing facilities are highly automated. This reduction in reliance on the human repetitive capability has increased efficiency and productivity and aligned both of these metrics more closely with process speed. This is certainly true in optimal printed circuit board assembly (PCBA) where equipment capacity and process techniques define productivity and efficiency. Ensuring the boards are well designed and following good design for assembly (DFA) guidelines can minimize the need for human intervention. Let’s review the process to help identify design focus areas to maximize PCBA speed.

The Printed Circuit Board Assembly (PCBA) Process

Transforming your design into the completed physical structure requires three steps: 1) fabrication, 2) component procurement and 3) assembly. Printed circuit board assembly or PCBA is one of the two PCB manufacturing processes. The other stage, fabrication, is performed first. During fabrication, your board design is constructed and prepared for assembly, where the components are securely attached to the board. Although PCBA may be comprised of ten or more steps, the process can be divided into the following major tasks:

• Preparation

Before placing surface mount technology (SMT) components, an initial layer of solder paste is applied to the board’s pads. This is done to promote good flow during soldering and minimize assembly defects. The application method may be manual or automated using a stencil or jet printing.

• Component Placement

For SMT components, accurate placement of components on pads is crucial. Misalignment can lead to bad solder joints or tombstoning, where one side of the component is not attached to the board. Through-hole technology (THT) components are more flexible; however, it is usually advisable to have the component body as close to the board surface as possible.

• Soldering

The most common method for securing SMT components is reflow. For THT components, wave soldering is the preferred method. If both types of components are used, the SMT components are usually placed and soldered first. Only then are the THT components are placed and soldered, which extends the soldering task. Soldering is further extended for two-sided boards, as components are mounted on both the top and bottom surfaces. After the soldering step for each component type, the connections are inspected and if any issues are discovered, then rework is performed to correct them.

• Cleaning

Cleaning is performed to remove any excess debris from the board’s surface(s). Alcohol or deionized water is effective at removing most contaminants.

• Depanelization

Depanelization, which is the separation of the multi-board panel into individual units or PCBs, is the final major task and should not be overlooked. Inefficient panelization or panel design can result in significant waste and additional cost.

The above tasks are performed by your contract manufacturer (CM) and the quality of the manufacturing process depends on your selection of fabrication and assembly services. However, certain design tactics can be employed to optimize PCBA, especially for speed.

Designing Your Boards to Optimize PCBA Speed

Any design decisions made with the objective of improving PCBA can and should be a part of your DFA guidelines. These include options intended to increase the efficiency, quality or speed of the process. In particular, for speed, we can define a checklist of specific tasks that, if performed during design, can optimize the process.

PCB Assembly Speed Optimization Checklist

✔	Planning Stage Prioritize speed when selecting a CM and assembly services.
✔	Sourcing Stage Select components that are and will be available throughout development. Exclude or minimize the use of through-hole components.
✔	Schematic Stage Make sure that your component footprints exactly match the BOM.
✔	Layout Stage Clearly mark component and connector orientations. Use thermal relief in plane connections. Layout a panelization design that minimizes waste and can be scored for quick and easy separation, if possible.
✔	Manufacturing Stage Use your CM’s default materials, solder mask and silkscreen colors and surface finish. Ensure your design package is complete and accurate, including BOM, drawings, and any special information, and is in the best format for your CM.