The most fundamental approach to design for manufacture in PCBs involves the use of design rules and design rule checking within PCB design software. Design rule checking (DRC) entails examining a design to determine if it complies with the manufacturing capabilities of a PCB fabricator. Usually, a designer obtains the highest tolerances supported by a PCB fabricator from the fabricator and inputs these tolerances into their design program. They then perform a design rule test on their proposed design. PCB design software often includes integrated design rule checks and is not typically considered an additional service. More advanced software for design for manufacture analysis is also available to identify complex and less obvious design flaws. PCB fabricators often offer DFM software checks as an additional service to customers. The reason for this distinction is due to the added cost of high-end DFM software and the extra training required to use it.
- Starved thermals
Starved thermals occur when the thermal relief traces connected to a pad are not adequately connected to the associated copper plane. This problem is often observed when multiple vias are placed in proximity to each other. Although the spacing between vias may pass a basic design rule check, the attached thermal relief traces may be interrupted, leading to the affected vias being inappropriately isolated from their assigned copper pours.
2. Acid traps
Acid traps occur when two traces are joined at a highly acute angle, causing the etching solution used to remove copper from the blank board to become “trapped” at these junctions. This can cause traces to become disconnected from their assigned nets and leave them open-circuited. While it is still advisable to avoid acute angles, acid traps are less of a concern than in the past due to fabricators switching to photo-activated etching solutions.
3. Silvers
Silvers occur when small portions of a copper pour are connected to larger portions of the same copper pour through a narrow trace. This can result in unintended shorts if these small portions break off during fabrication and “float” to other parts of the board. Although silvers should be avoided in designs, they are not as prominent an issue as in the past due to fabricators switching to photo-activated etching solutions.
4. Insufficient annular ring
Vias are made by drilling through pads on either side of a board and plating the walls of these holes to connect the two sides of the board. If the pad size called out in the design is too small, the via may fail due to the drill hole taking up too large of a portion of the pads. This issue is commonly detected in the DRC process, but missed drill hits can still occur in prototyping boards.
5. Via in Pads
While it may be convenient to design a via to be positioned within a PCB pad, via in pads can cause issues during board assembly. Vias can draw solder away from the pad, resulting in improper mounting of the component associated with the pad.
The image below shows difference between via in pad PCB and normal PCB.
6. Copper too close to board edge
Placing copper layers too close to the edge of a board can cause those layers to short together when the board is cut to size during the fabrication process. While this error should be caught using design rule checking (DRC) features typically available in PCB design software, a PCB fabricator that performs a DFM check will also identify this issue.
7. Missing solder mask between pads
In tightly spaced, small pin pitch devices, it is common for there to be no solder mask between pins due to standard design settings. The absence of solder mask can lead to solder bridges forming more easily when the fine pin-pitched component is attached to the PCB during assembly. Professional PCB assembly services can prevent missing solder mask between pads, ensuring high precision for small pin pitch devices.
8. Tombstoning
Tombstoning occurs when small passive surface mount components lift up on one end and “tombstone” during the reflow process of soldering to a PCB assembly. This can significantly affect PCB yields and drive up production costs. Tombstoning can be caused by incorrect landing patterns and imbalanced thermal relief to the pads of the device. Design for manufacture (DFM) checks can effectively mitigate tombstoning issues. A sample and schematic of tombstoning are shown in the image below.
