I have often been told about engineers who do not read, much less follow, directions when assembling an item. Regrettably, I am no exception to this observation. There have been numerous occasions when I have emptied the contents of a newly acquired product onto the floor and immediately begun trying to piece it together, oblivious to the included instructions. Simply following directions would have spared me from the embarrassment that followed.
In stark contrast to my usual method of assembling home products, contract manufacturers (CMs) follow a well-defined set of steps for board fabrication and PCB assembly. PCB assembly or PCBA, the process of securing your components to the board, employs a sequential procedure that includes board preparation, component placement, component attachment and board finishing. This process, at its best, is characterized by collaboration and the incorporation of CM techniques and equipment capabilities into your design decisions. Let’s see how your board design and component choices can optimize your CM’s PCBA of your design.
PCBA Optimized Board Design
Design for manufacturing (DFM) is the utilization of guidelines to determine specifications for the layout of your board based on your CM’s equipment capabilities. DFM includes specifications that target board fabrication, PCB assembly or both. The particular design decisions you make to improve the assembly of your PCB can be categorized as design for assembly (DFA). Just as DFM is intended to ensure manufacturability and improve the efficiency of PCB manufacturing, the objective of DFA is to assure PCB assembly is as smooth as possible. The effectiveness of your application of DFA determines the level of PCB assembly optimization.
How to Design Your Boards for PCBA Optimization
PCBA optimization is primarily determined by and positively correlated with the design choices made for your board parameters, as shown below.
|BOARD DESIGN CHOICES FOR OPTIMAL PCBA|
|Design Parameter||General Choice||Optimal Choice|
|Component footprint||Use general package footprint||Use component manufacturer specifications for footprint|
|Spacing and clearances||Use minimum spacing||Use maximum spacing and clearances between pads and surface elements|
|Solder mask dam||Use default||Use maximum solder mask dam based upon design layout|
|Solder mask expansion||Use zero solder mask expansion||Use positive solder mask expansion around pads and open vias and negative solder mask expansion for tented vias|
|Silkscreen markings||These are optional||Use clear reference IDs for components|
|May not be included for all footprints||Make sure pin 1 markings and polarity indicators are clear to ensure proper component orientation|
|Thermal relief||Use single default size path||Use multiple paths and higher copper weights to dissipate heat more efficiently|
To improve the optimization of your PCBA, follow the recommendations in the last column as much as possible.
DFM for High-Speed Digital PCBs
PCBA Optimized Component Selection
SMD with Fanout
Although the impact of component selection on PCBA is often undervalued, your component choice can significantly affect the assembly process. For example, utilizing the component lifecycle to minimize the selection of components drifting towards obsolescence avoids delays and minimizes the chances of PCB redesign prior to assembly. Your choice of component package type can reduce PCBA steps when either through-hole or surface mount devices (SMDs) are used (and not both). Additionally, using double-sided assembly can allow for DFA specifications to be relaxed. Along with these decisions, your PCBA may be optimized by following the recommendations below:
How to select components for PCBA optimization
- Verify component availability.
Obsolete or soon to be obsolete components will extend the PCBA process.
- Use either through-hole or SMDs.
Through-hole components and SMDs are mounted differently and therefore require different steps. By using one or the other, the number of PCBA steps can be reduced.
- Avoid components that tend to take on moisture, if possible. (If components are required, be sure to identify them for your CM).
Components that take on water require baking before they can be soldered, which takes additional time. Failing to bake them may result in an explosion.
- Select components that can withstand PCBA temperature increases.
The soldering process usually adds 80° C or more to the board temperature. Temperature sensitive components may not be able to withstand this increase in heat.
- If using components that are sensitive to X-ray radiation, make sure to identify for your CM.
X-ray inspections are used for a deeper level of internal observation. However, some components may suffer radiation damage during exposure.
The PCBA process is best optimized by forging a symbiotic relationship with your CM to ensure that your design leverages the expertise and capabilities of your CM while incorporating your design objectives.
|Tempo‘s Custom PCB Manufacturing Service
Tempo Automation’s open, transparent approach to PCB manufacturing is built upon this foundation. And to help you get started on the best path, we furnish information for your DFM and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.
If you are ready to put our industry-leading PCB assembly process to work for your design, try our quote tool to upload your CAD and BOM files. If you want more information on PCB assembly and how to employ DFA, contact us.