A design is only as good as its manufacturability – no matter how innovative or expertly a PCB layout, board designs succeed based on their ability to seamlessly translate to production with few (if any) adjustments. This design for manufacturability (DFM) mentality permeates the industry, but there are many cases where designers emphasize other board aspects to the detrimental expense of manufacturing. While it’s certainly possible (and highly probable) for designers to have to revise boards based on production guidelines, the better approach in terms of time and cost is to anticipate these issues through industry quality control standards and direct communication with the manufacturer. A conscious and deliberate layout will also minimize the chance of DFM issues in PCB production making their way to end-users, improving customer satisfaction and limiting the need for costly recalls.
The Different Levels of PCB DFM
The Primary DFM Issues in PCB Production
There are many design problems that, if not accounted for, can complicate PCB design. These can range from board materials and stackup configurations that are incompatible with the needs of high-speed design to important silkscreen information covered by components or mounting hardware. Design issues that impact manufacturing can be especially troublesome as they undermine performance and long-term reliability.:
- Incorrect PCB footprints and land patterns – Surface mount land patterns or pads that are too large can cause their components to float out of alignment during solder reflow. These parts may cause clearance problems or even short together. Pads too small may not form a strong enough solder joint, resulting in intermittent or future total board failures. Pads that differ in size on small two-pinned parts like SMT capacitors and resistors or have more metal on one than the other may cause an imbalance in heating during solder reflow. This imbalance could cause one pad to reflow before the other and pull the part with it. This effect is known as tombstoning because the part will stand up on one pad instead of lying down as it should.
- Component location and rotation – Components not positioned correctly for wave soldering can cause problems with poor solder joints. If a larger part proceeds a smaller part into the wave, an effect known as shadowing can cause an improper solder filet formation between the pins of the component and the pads on the PCB. These poor solder joints are prime candidates for later failure. Rotated components go through wave solder parallel to the wave instead of perpendicular to it and are also susceptible to bad solder joints.
- Component clearances – PCB component placement requires enough clearance between them to facilitate automated placement and testing. Additionally, part placement must be accessible for rework or functions such as plugging and unplugging connectors. There also needs to be a minimum amount of clearance from placed parts to the board’s edge to prevent damage to components when individual boards are broken out of their assembly panels and for testing purposes.
- Insufficient solder mask coverage – Small surface mount pads, such as those on fine-pitched parts, need a solder mask between them to protect them from solder bridging. If left unprotected, these pads can form solder slivers between them, which are difficult to find and correct due to their size and may create an electrical short. These shorted pads could produce intermittent problems, such as sending false signals that can be very difficult to diagnose.
- Errors in the bill of materials (BOM) – Component errors in the BOM may result in the incorrect part preparation for manufacturing. If not caught in time, these parts could get assembled onto the board. In the case of parts that have a slight difference in values or tolerances, this could cause intermittent problems that are difficult and expensive to find and correct.
Preventing DFM At the Source
Knowing the problems is the first half of the battle: with an idea of what to look for, designers can set up review processes to ensure the detection of issues before sending board files out for manufacturing. It is also a great idea to understand the design rules before laying the board out to avoid problems like these in the first place. Knowing a contract manufacturer’s capabilities early on can help.
The best manufacturing outcomes come from working with a CM during the layout phase of board design. This way, they can offer advice and guidance on the different DFM requirements and answer any DFM-related questions to ensure the board is suitable for manufacturing. Also, a good CM will have engineering expertise on hand so that their answers are specific to a board beyond general guidelines.
Your Contract Manufacturer Makes DFM Easy As ABC
At VSE, we have built our business by helping PCB designers prepare their boards for efficient and error-free manufacturing. We have a staff of engineers who will start with a design review of a board to check for DFM issues. Our component engineers will inspect the BOM for parts availability and prices, and we will alert designers to any errors or replacements we recommend. Likewise, our design engineers will evaluate boards to ensure readiness for manufacturing and will collaborate on identifying problems requiring approval for correction. We’ve worked alongside designers and manufacturing partners for over forty years to produce life-changing and life-saving devices.