In the planning stage of a new project, we work on the technical and financial aspects of PCB mass production. In this process, PCB manufacturing cost should be less, and there should be no revisions of the design otherwise the profit margins will be not good. To avoid the revision phase, we should be careful and design the PCB layout that ensures smoothness in PCB fabrication. When the supplier receives the CAM data, they modify the small mistakes that incur additional cost and time.
With this article, we will learn about the potential rules that the designers should follow in the PCB layout stage. These DFM guidelines for PCB will improve the electrical, mechanical, and thermal behaviour of the overall PCB.
What is DFM in PCB?
DFM is the abbreviation of Design For Manufacturability. It is a topic that helps in reducing manufacturing issues. In the design rules, we define the clearances with tolerance acceptance. So in PCB DFM, we make the boards ready for practical application. Tolerances should match the real system. During the PCB layout stage, designers follow the list of PCB DFM checks. The process is useful in optimizing design to the production stage of a PCB. It will improve cost, and quality and educate the designers to follow the DFM checks every time they want the designs to be fabricated. PCB DFM is a teamwork of inter-department groups who are part of a complete product design.
What are the Common PCB DFM Issues?
Some PCB DFM issues can be addressed during the board layout, while other issues arise when the supplier checks the CAM data using the DFM tool. When we ignore the weightage of DFM errors, quality, and cost is compromised. We face backlash during bulk orders from customers. Considering the risk of design failure in a highly competitive market will be a huge downfall for the company. Let us explore various reasons why PCB DFM failure occurs.
1. Footprint Shape
Every electronic component added in the schematic sheet requires corresponding footprint creation in the PCB library. The part will be soldered during board assembly. IPC 2221 standard defines the correct geometry of each packaging whether it is SMT or through hole. We can use the built-in IPC footprint wizard tool to avoid the incorrect size and shape of the footprint. This is important for a more acceptable PCB DFM.
2. Component Placement
In the PCB DFM checklist, designers should ensure the correct location of parts on the board. The clearance with other board features, rework accessibility for connectors or power supply ports on the edges. Components should positioned at a distance from the board edge to avoid any damage during PCB handling.
3. Soldering Process
After the PCB DFM checklist is certain and the component placement is done, the next step is soldering. When the bare board is carried to the reflow area for assembly, component alignment with the printed footprint is important to avoid the tombstoning effect. In the tombstoning effect when a big part is placed before a small part like a 0402,0201 resistor, inductor, or capacitor in the wave soldering machine, then due to uneven pad shape or large pad shapes it will make the part float away from their position in the reflow. This will lead to extra solder deposited between pins which causes pin shorting. Also, one pad gets the solder and the other pad gets lifted in the air. This causes issues in the solder joint of the part and manufacturing errors, which will lead to failure in the PCB DFM.
4. Open Copper
During PCB layout, designers should delete the extra copper trapped between the via pattern, pins, and parallel traces for accurate PCB DFM. The free-floating unconnected copper is called a sliver. Due to the copper islands and sliver construction, there are multiple issues in the signal integrity of the PCB.
- Antenna Formation: During PCB manufacturing, the open copper elements on the board form a noise-emitting antenna as they are not connected to the ground. They cause a signal disturbance with other component signal interference. If the open copper element is big they do not float on the board, forming antennas. So careful PCB DFM is necessary.
- Sliver Formation: At the time of the etching process, solder masks or copper traces that are thin in size and shape do not get completely removed. They attach to other boards by floating anywhere in the reflow machine. The slivers can be of other conductive metals like tin. They highly disturb the board’s working operation and cause PCB DFM failure.
5. Uneven Copper Shape
Acid Traps are described as an acid collection in the copper traces. Traces bent at acute angles less than 90 degrees create acid traps. During board etching, the board is washed with acid, and at these acute angles, more acid will remove other copper traces and features which is a big issue in PCB DFM, causing missing connections. It is better to set the design rule in the eCAD tool before beginning the layout, to allow only 90-degree angles for the bent. Oversight of acute traces will lead to problems.
6. Unplated Vias/Holes
Vias and holes are drilled in the PCB. We must plan the PCB DFM well. Because they connect the multiple layered boards to carry electrical current throughout. Conductivity is done using the plating process by making electroless copper deposition. Due to bad material quality, air bubbles, and improper process, we are left with unplated/uncoated holes which affect the board connectivity. Hire a well-qualified supplier having a history of fewer rejections in the manufacturing report because they can cope with PCB DFM more professionally.
7. Via in Pad
It is not a good practice to place vias on the pads of the footprint in the PCB DFM. It builds an issue in board manufacturing as well as the thermal management of the individual components. A drill on the pad will cause weaker solder joints as the less area available for soldering.
8. Solder Masking
Small package components like controllers and processors have a very fine pitch. If the designers miss the step of adding a solder masking layer, then in reflow and board assembly all the pins will be shorted with one another. A thick boundary of masking in the PCB DFM is necessary to surround the footprint pads to avoid short circuits, it also protects the board against external rusting and damage.
How to Avoid DFM Errors in PCB Design?
The ultimate goal of any designer is to get their project into the manufacturing stage. To make the task easier and well-optimized, it is important to pursue DFM guidelines for PCB and best practices that exist in the industry. It will enhance the board’s electrical, and mechanical performance. Rules make layout a simple task even if the circuit is highly advanced. With the final layout checks the output data sent to the manufacturer can be confirmed as a design that is free from PCB DFM errors and issues. Now we will learn about the DFM guidelines for PCB to be undertaken
- PCB Library of the component footprints should be well aligned with the IPC standard dimensions.
- PCB Panelling should be of uniform shape, to avoid board wastage when separating PCBs from panels.
- The board shapes should be regular, to avoid cutting problems during manufacturing.
- Perform design rule check and double-review your design before final submission to the supplier.
- Distribute the copper traces and polygon area in an even form.
- Keep the Th-hole parts on one side, and the SMT parts other side of the board. It will avoid the extra manufacturing cost of the selective soldering process.
- Make use of the teardrop method when the junction is made with via and traces
- Connect the SMD pads of IC outside on the board area. Below pad routing will not allow optical detection to inspect any shorting.
- Overlay layer text should not overlap each other, and should be visible as it indicates crucial information about component markings and orientation.
- Thermal vias should not be left without metal plating on exposed copper areas.
- Calculate the required trace width to suffice the current rating. Based on the trace width the gap between other conductive parts should be calculated and added to the design rule settings of the eCAD tool.
Summary
Following the PCB DFM guidelines will truly create precise prototypes of good quality. It does not matter if the design is for a simple project or an industrial-grade high-speed complex board. Keeping the DFM guidelines for PCB in mind for each design will gain the customer’s trust. It is a benefit for the designer and the company for future orders. These procedures control the manufacturing process. It lowers the overall cost of board fabrication. Always keep up with the advanced technology and update the PCB DFM checklist.
