A couple of years ago, my grown son decided that he wanted to go skydiving. Thankfully, it all went well—he landed in one piece, and we didn’t tell his mother about it until after he was safely back on the ground. One of the things he relayed to me about the experience was how much effort the skydiving company put into ensuring his safety. He told me how he went through all kinds of training for the jump before they would let him proceed. Then, they thoroughly checked his gear and his parachute, and he spent time with his jump partner making sure he knew exactly what was going to happen and when.
To ensure the protection of their customers, the skydiving company took no chances and made sure that everything was checked and double-checked before the plane left the ground. That’s good advice for any venture in life, and it shouldn’t surprise you that quality contract manufacturers go through the same effort as they prepare to manufacture your printed circuit board.
There are a lot of steps and processes going on during PCB assembly, and your CM wants to make sure ahead of time that your board won’t have any design for manufacturing (DFM) issues that could cause problems. Here are the PCB DFM checks your CM should perform to ensure your PCBA is manufactured correctly.
PCB DFM Check: The First Step in Manufacturing Your Circuit Board
When your contract manufacturer first receives your design data, it should go into a thorough design review. How the data is processed by the CM will be determined by what format you choose to send it in.
Data that is received in a PCB design CAD database format will go directly into engineering for review. Data that is received in other formats must be imported or translated into the tools that the CM is using. This process takes longer and is less reliable, which is why your CM would rather get the original CAD database instead. Also, if the CM ends up making any design changes, they can do that in the original data format much easier as well.
The component engineers at the CM will review your bill of materials, looking for parts that are not recommended for manufacturing, have long lead times, or are going end of life (EOL). At the same time, the design engineers at your CM should be reviewing your data for DFM issues, including signal integrity, manufacturability, and testability.
Potential DFM Problems and How They Are Resolved
During the CM’s component review, the parts on your board will be reviewed and matched up to the master component vendor inventory database. This database is constantly updated and gives the component engineers real-time data on part availability. Based on their conclusions, they will make any recommendations necessary for part replacements. If an exact match is not found, as can be the case in a part that has gone EOL, they will recommend functionally similar parts.
The design engineers meanwhile are reviewing your design for its functional performance. They will make recommendations for board materials and layer stackup changes if impedance controlled routing can be improved. They will also examine the component placement and routing for signal integrity improvements as well.
At the same time, the CM’s manufacturing engineers will review your board for assembly and test issues that can impact the manufacturing. Some of the items they are looking for include:
- Component spacing: Parts that are too close together can cause problems for automated pick-and-place machines, and make rework difficult. Connectors that are placed incorrectly can make intra-system cabling difficult to work with.
- Component location and rotation: How your parts are placed can affect the soldering of the board. Larger parts that precede smaller parts through the wave can shadow those parts and cause bad solder joints.
- Chip component pad size: Incorrect pad sizes on chip components or pad sizes that are different due to wide traces can cause uneven heating of those pads during reflow soldering. This can lead to chip components standing up on one end, a condition known as “tombstoning.”
- Acid traps: Traces routed with acute angles can be difficult when cleaning etching chemicals off the board, causing those traces to fabricate at a smaller width than expected.
- Drill optimization: Some designs use too many drill sizes, which should be consolidated to reduce the expense of the board fabrication.
- Missing solder mask between pads: This can result in solder bridging and potentially short together two pads that shouldn’t be connected.
- Testability: This covers many areas of PCB design, from component-to-board-edge spacing to full coverage of testpoint pads for in-circuit test. It also includes building or modifying a test fixture for the board if needed.
Once all of the design reviews are completed, a report of what this board needs to pass the DFM check should be prepared for you. At this point, you can make the corrections yourself, or enlist the aid of the CM’s engineering staff to do the work.
How You and Your CM Can Best Partner Together for PCB DFM Checking
As you can see, there are a lot of areas where potential DFM problems could be found in a PCB design. There are some things that you can do though that will help to reduce or eliminate these kinds of problems before you send the board out for manufacturing:
- Engage your CM early on in your design cycle. A good CM will want to work together with you while you are still in the design phase to make sure that your board will be prepared as much as possible for manufacturing.
- Ask questions. If you aren’t sure about a specific issue, speak up.
- Trust the recommendations of your CM. When you work with a trusted CM, you can rely on their experience in building circuit boards, and their understanding of board materials, layer stackups, and DFM requirements.
Ultimately, your success guarantees your CM’s success. Partnering with a CM that will do everything to ensure that your board is built the way you intended it to be, including conducting a thorough DFM check, is a critical step to a successful project.
