Engineers encounter innumerable variables when designing a printed circuit board, especially when you consider that even though a printed circuit board may function properly, it doesn’t necessarily mean that it can be manufactured in a way that maximizes quality and reduces cost. Completing a PCB design is a struggle between different constraints which encompass time, budget, and functionality, with manufacturability often seen as an afterthought. However, if engineers involved their manufacturing team and considered a comprehensive design for manufacturing review, many common problems could be avoided. In addition to increased manufacturability, engineers can benefit from reduced cost and time to market.
As a contract manufacturer, we repeatedly see the same mistakes regarding design for manufacturing. These hold back projects and create additional design costs and delay timelines. We’ve compiled a list of the top 15 “gotchas” for PCB design to make sure your board is set up for success when it’s time to transition to manufacturing. This is not a completely comprehensive list, but we always tell prospective customers that if these rules are followed, about 80% of DFM mistakes can be avoided.
This list is based from over thirty years of experience and it’s our hope that they can help you see success with your next project.
1. Minimum Component Spacing from the Edge of the Board
Whenever possible keep components 3.175mm from the edge of a board. SMT machines need this space on the edge of the board to handle the panels on the conveyor and in the placement tray. Neglecting this spacing could lead to damaged parts. Also, we understand that some components will not adhere to these spacing requirements. For example, charging ports and USB ports will most likely mount flush to, or extend past, the edge of a board, but they are often placed after an SMT machine would place components.
2. Proximity of SMT Components to Thru-Hole Components
If you're PCB is using mixed technology, space the SMT and Thru-Hole components at least 3.175mm from each. This is to avoid damaging SMT components with a soldering iron, whether it's a selective solder machine or traditional hand soldering.
3. Include Global Fiducials
Include at least two Global Fiducials on the opposite corner of each board. Providing the fiducials helps an SMT machine locate the component pads more accurately. The addition of a third or fourth fiducial increase the accuracy of a parts placement.
4. Include Local Fiducials with Fine Pitch Parts
Smaller or finer pitch parts benefit from local fiducials. That's because, as the size of a part decreases placement accuracy on the pad is more important. The margin of error is tighter. Adding local fiducials allows the machine to find a more accurate placement area.
5. Part Size and its Relationship to Placement Accuracy
This is a MyData specific requirement but all SMT machines are similar. Use parts that fall in the 0201 through 56X56mm range. Many times, we encounter a PCB with lots of empty space, but designers choose to use 01005 parts for no reason. We can certainly place these small components, but you need to understand that a smaller part increases the likelihood of an error. As a frame of reference, a 01005 is approximately the size of the dot above the letter “i”.
6. Proximity of SMT Components to Other SMT Components
Similarly, to the SMT to Thru-Hole component limitations, there are SMT to SMT component limitations. Avoid placing these components less than 0.3mm from each other. BGAs should have more space around them, at least 0.75mm. If a PCB design calls for SMT components to be placed directly next to each other without adhering to these limitations parts could be placed on top of other parts or lay on another parts pad.
7. Follow IPC Standards
IPC is the standardization body for assembly and production of electronic equipment. It's always good to design a circuit that conforms to these standards. We manufacturer assemblies to these standards.
8. Max Array Size
When possible try to limit the array size to 14”x14”. Larger sizes can certainly be accommodated but it will require additional tooling or engineering support. If you need to use a larger board it will, in most instances, negatively impact PCBA costs.
9. Double-Sided Mixed Technology Boards
If your design is double-sided and requires mixed technology, place the larger and/or heavier parts on the top side of the board. Placing these components on the bottom side could have an adverse effect during the reflow process. As the board passes through the oven these parts could be pulled off the board as the solder reflows from the top sides' heat profile.
10. BGA Placement
Avoid placing BGAs on both sides of a board. The reflow process for BGAs is extremely precise. You want to design a board so that all BGAs in a circuit are reflowed at the same time. If you were to reflow one, then pass the board through the oven a second time, the first BGA on the opposite side could reflow and shift off the pads. If you absolutely must place BGAs on the opposite side of a board never place them over each other. To verify a BGA soldered correctly we use x-ray technology, if the BGAs are aligned perfectly on the top and bottom of the board the x-ray would return inconclusive evidence that the parts soldered properly.
11. Efficient Thru-Hole Layout
Try to place all thru-hole components on one side of the board. This is a good practice because it reduces the amount of manual labor associated with soldering. We would be able to create one profile on the selective soldering machine, prep the board once, and let it run. This saves time and subsequently money.
12. IC's and Passive Placement Orientation
It is best to place IC's and Passives with the same orientation. Mainly, the machine runs more efficiently when the head can continually run at the same angle, picking and placing faster than it would if it had to change the way it picked up a component and placed it. These few seconds per board add up to a substantial saving over the lifetime of a design. Additionally, when all the components are aligned in the same orientation, a misplaced component is caught easily in a cursory visual inspection.
13. Quality of Your Finish
There are a few different finish options. but whenever possible try to use immersion gold. The gold finish is costlier but typically the pads are more consistent in shape and provided better connectivity for the components.
14. Best Way to Depanel BoardsRemember to account for the depaneling process if your boards will be panelized from the board house. It's best to use v-scores it's the most reliable way to depanel a board. Also, provide 6.35mm rails on all four sides with 3.175mm mounting holes and 1.27mm fiducials in each corner. Meeting these requirements will ensure components near the edge of a board will not be damaged during the manufacturing process.
15. The Importance of a Test Program
Finally, take the time to provide a manufacturer with a robust test platform. This is so important. A manufacturer would prefer to test every board that leaves the factory to ensure that any defects are caught as early as possible. Additionally, a robust quality system can help gather data when troubleshooting a failure. Ultimately helping to determine if it's a manufacturing process or a design problem. The sooner a failure is caught the less disruption it will cause in the supply chain.
Seemingly minor details can stack up to major time and cost savings, especially as you scale up. For example, even a design change that reduces reject rates by 0.5% can add up to major cost savings on the scale of thousands of units.
Looking for more? Explore our interactive PCBA Design Guidelines to see examples of where you can optimize your design for manufacturability.