When you have your printed circuit board design manufactured, you want to get the best quality board built without any unpleasant surprises in cost. There are however some unintentional errors that can happen in design that could drive those costs up. Here are the top 5 PCB assembly cost drivers that you should consider while still in design:
- Clean Bill of Materials (BOM). Validating that all parts on the board are available, low cost, and will give you the performance you expect.
- Accurate PCB footprints. Making sure to use industry-standard PCB footprints for your components.
- Good component placement. Placing your parts in observance of design for manufacturing (DFM) rules to reduce manufacturing errors.
- Designed for test and rework. Following standard design for testability (DFT) rules so that the board can be easily verified, reworked, or repaired.
- Clear documentation. Not only do you need clear documentation for manufacturing, but for debug and field technicians as well.
A problem in any one of these areas can result in higher than expected manufacturing costs, while multiple problems could potentially shut down production of the board. Let’s take a closer look at these areas so that you can control the PCB assembly cost drivers to ensure that you end up with the highest quality PCB manufacturing for the best price.
A Clean BOM, the First PCB Assembly Cost Driver
Starting with a clean PCB bill of materials is essential to controlling the overall cost of the board. If components are chosen that are expensive or have long lead times, it will cost you more in time and expense to manufacture the board. Incorrect parts could end up costing you more time and money in debugging, while parts that are going end of life could potentially force a redesign of the board.
BOM scrubbing may seem unimportant, especially for design engineers that are working with familiar parts, yet these parts can change over time. CAD libraries may not always be up-to-date, and there may be newer and better options available that the design team isn’t aware of. To avoid these problems, it is always the best practice to make sure that the BOM is scrubbed before manufacturing begins.
Bad Footprints Lead to Rework and Board Failures
PCB CAD footprint models that are incorrect may result in a number of different manufacturing failures. Wrong pad sizes can cause SMT parts to float off the pad during soldering, while bad pad spacing can cause faulty solder joints or problems inserting thru-hole pins into their holes. Component outlines that are drawn too large may not allow for a compact board placement, while outlines that are too small may result in an inability to assemble the board.
It is important, therefore, to use the correct footprint pattern for the component that will be soldered there. When possible, industry-standard footprints should be used. Manually created footprints must be verified to avoid having to redesign the board later, and all of the necessary data must be included for layout including attributes and STEP files.
Following DFM Rules Will Guarantee a More Manufacturable PCB
In the rush to get a prototype board designed and built, it is not unusual to gloss over the manufacturing requirements for the board. Prototypes often get more manual attention than a production board, and any imperfections are fixed by hand. The problem, however, is that this board will then often be put into production without any corrections, and manufacturing problems will start popping up. You may see bad solder joints, tombstoning parts, and connectors, and other hardware that are difficult to reach and work with.
To avoid this, it is best to use good DFM practices in the initial layout. You will also want to review the design for DFM rule violations to make sure that your board is ready for full production from the start. This will save you the time and expense of having to go back and redesign the board as well as re-validate that design once the corrections are completed.
Rework Happens, is Your Design Ready for It?
At some point, your assembled circuit board is bound to need some rework. This may be because of an assembly problem, debug, or a design change. To avoid additional clean-up work to repair any collateral damage due to the rework, plan ahead for some extra space in your design whenever possible. This would include giving smaller parts enough clearance to their associate processors and other large components, as well as moving parts that are potentially hidden by taller components.
At the same time, don’t forget that your PCB needs to be designed for test. This includes testpoints for ICT or flying probe, and or probe points for manual test. Boards that are not set up for test will require additional manufacturing steps for validation, which will add to their overall expense.
Make Sure That Your Documentation is as Good as Your Design
Lastly, take the time to prepare good documentation. Documentation that is unclear or unreadable will force your contract manufacturer to slow down their process to get answers from you. It may even result in bad boards being built. To dodge these problems, take your time in document creation and use all of the resources available to you in your PCB design CAD system. These could include templates and automated features to simplify drawing creation, or BOM and other manufacturing file generators.
Another very useful resource to you in all of these areas is to enlist the help of your PCB contract manufacturer. Here at VSE, we have over 30 years of industry experience helping our customers to find and avoid these problems, and we can help you do the same.
