What Is PCB Design For Assembly (DFA)?

The ‘Design for..’ term has been increasing in mainstream use in recent years, particularly amongst Makers, hobbyists and emerging businesses. Perhaps this can be attributed to the rise in affordable one-stop-shop manufacturers (and hopefully in part due to the Seeed Fusion DFM manual :D). But there is much confusion as to what it really means in the context of fast prototyping and agile manufacturing. If you are being offered DFM or DFA review services, what does that actually mean?

Traditionally, the term “Design for X” circulated between industrial players – those in the business of churning out hundreds of thousands of products to the masses.

The Wikipedia page for ‘Design for X’ or design for Excellence, defines it as a methodology of set traits or guidelines “where the X in design for X is a variable which can have one of many possible values… including: manufacturability, power, variability, cost, yield, or reliability.”

So, for example, Design for Inspection (DFI) could be concerned with improving aspects of the product to accommodate for efficient identification of errors and faults. Design for Manufacture or Manufacturability (DFM) is concerned with the potential problems that occur during manufacture, before a mass run. Essentially, you could design for anything, and managers should be looking to cover a good number of different aspects to release a good product and avoid costly fixes later.

“The general rule in product development is simple: The later on you find a fault, the more costly it’s going to be to fix. It’s about being proactive and trying to predict issues early on.”

Seeed assembly floor manager

The Value of Design for Excellence

Imagine a big name brand that has its own factory. The owners would be in complete control of all the processes and would have direct experience with what works and what doesn’t. Designers could work hand in hand with technicians on the production floor from day 1 of the project. But with the gradual move towards small batch production and the subsequent appearance of independent, low-cost, pop-up factories, the value of DFX has diminished and become undervalued.

“Many people without a background in manufacturing treat the process like buying a custom printed T-shirt. They’re not prepared for the kind of problems that can occur. I guess that’s the way it’s marketed but it results in a lot of upsets and delays for both the customer and the production line.”

 “Not to name names but customers from abroad are particularly prone to design errors. Wrong footprints, BOM quantities, all that stuff. And then there are those that get aggressive about the price for a handful of pieces.”

“Maybe it’s because parts are not as readily available as somewhere like Shenzhen. The concepts in manufacturing may seem somewhat alien as well. We’re all used to things being dirt cheap.”

The Value of Design for Assembly

Seeed Fusion began by offering free Design for Assembly services as part of their Expedited Turnkey PCB Assembly offering. All expedited orders accepted had the review done at the expense of a day or so added to the lead time. The results are somewhat shocking and relieving at the same time.

Nearly all orders had items of high or moderate concern and around 70% of orders had mismatching land patterns, meaning the part could not be placed.

While the footprint mismatch issue may not seem so scary at first, the implications could mean the difference between making or breaking a project.

Seeed Fusion are no strangers to the consequences of an incorrect footprint. In fact, much of the production delays and hold-ups are due to incorrect land patterns.

“We’re so used to it that we’ve even developed tricks to ‘make them fit’. USB connectors are the worst, for example when the through-hole pins are a long tab like shape but the board only has two tiny drill holes. Sometimes the only way to salvage the boards is to snip off the leads or bend them so the connector can lie flat on the board. It’s definitely not ideal but for prototypes sometimes it’s enough for customers.”

“Then there’s silly stuff for example you have a 0402 land pattern when the part is 0603. If you’re lucky and it’s just a generic resistor then all we need to do is grab a substitute from the shelf.”

“Cell 0” Hand Soldering Team Manager

The consequences of a mismatched footprint can be detrimental to a project if discovered at the last minute. Considering that imported parts can take several weeks to arrive, a single wrong part can effectively double the lead time if it needs to be purchased again. The absolute worst-case scenario may occur if the mismatched part is only found at the last stage of assembly. Typically surface mount parts are solder first, but suppose the technicians find a vital through-hole part does not fit after all the surface mount parts are soldered. If no workaround can be found, the only option is to scrap the boards, fix the design and start again from scratch.

Such design errors constitute a large majority of backlogged orders and production floor delays on behalf of the assembler as well, so it is also in their interest to have a thorough review done at the earliest point possible. Not only does the delay affect the order in question, but also all other orders in the queue, especially if substantial time and manpower has to be invested to resolve the problem.

“It’s a problem innate to agile manufacturing and we’ve been trying to solve it for years, or at least make it less painful. We’ve had close scrapes or missed deadlines in the past.”

“One of the main reasons DFA review was included with the Expedited PCBA service is because it removes the biggest source of uncertainty. If we can make sure everything is correct, then the rest just goes by the book so to speak.”

Seeed Fusion Product Manager

As a result of the positive customer feedback and a major increase in efficiency on the assembly line, Seeed Fusion decided to provide DFA review for all PCBA orders for free. Since then, almost all customers have seen faster delivery and greater reliability. One could only imagine the number of orders saved from complete disaster and the time and money saved.

What does DFA cover that DFM doesn’t?

In traditional engineering circles, Design for Manufacturing can cover all aspects of building the hardware of a product, including assembly. However, to better relate Design for X in modern PCB turnkey manufacturing practices, we distinguish DFM and DFA in terms of what is analysed and by who:

DFM reviews are limited to just what is contained in the Gerber files and other supporting documentation, and typically go through a checklist of abnormal patterns to find. While there may not be shorts or missing solder mask openings, the PCB engineer does not necessarily have knowledge of PCB design and assembly, or know what parts are intended for the design.

For DFA, at the bare minimum, the Gerber files and BOM files are needed and possibly the pick and place file and schematic. The review is more complete and in-depth, combining all parts of the design as a semi-finished device. DFA reviews require a qualified electrical engineer with PCB layout and production line experience to fully understand the documentation and the consequences. Hence, few suppliers liberally offer this service for free.

“Essentially, it’s what every beginner and even seasoned PCB designers want. An expert second opinion. We get quite a few people dumping all their files on us asking if we could go through them for any problems but it’s no simple feat. It can be a good half or whole day’s work for a complex design. And even if nothing is found after all that, at least you have peace of mind that your design got the all-ok.”

“But don’t go thinking you’ve got the all-clear. There are also things that DFMs catch that DFAs don’t. It’s expected when different factories and expertise are involved at different stages. They’ll have different tools at their disposal, different production processes and different interests.”

The DFA Checklist

Specifics will vary from supplier to supplier, but Seeed’s professional DFA engineers currently provide the following checks and are not limited to:

Footprint Verification: Footprints verification is the most significant and by some opinions, the most laborious and tedious. Perhaps the reason why such errors are the most prevalent. Certain databases and programs available to DFA engineers are designed to make this quicker and easier which is, in essence, comparing the PCB land pattern to the manufacturer’s specification. In times of great supply shortage, where last minute component substitutions are made, footprint verification has become even more essential.

Overall Component Layout: As well as the individual components, the relative spacings between components and their position on the board as a whole, considering the properties and functions of the components and their effects on assembly are analyzed. This includes spacing between components (especially important if requiring wave soldering), the placement of fragile components on vulnerable areas on the board, weight considerations to avoid board warp, access to functional components (e.g. SD card socket) or fitting into enclosures if included and taking into consideration potential re-work that may be required on the boards.

PCB features: According to the types of components to be populated, engineers will determine whether the PCB board requires specific features or modifications. For example, the choice of surface finish for a particular part or pad pitch to improve first-pass yield and long-term reliability, additional treatment or technologies to improve solder joint quality, how to preserve solder mask dams, pad and via design, etc.

Completeness and Correctness of Data: Involves checking whether all the necessary manufacturing files are present, whether the BOM quantities and MPNs are clear and consistent, if enough information is available to determine the locations and orientations of all components, verifying the pick and place file, clarity of silkscreen text, etc.

Panelization Considerations: For small batch assembly, panelization of individual boards into a larger panel with tooling margins is almost always required for machine assembly. Designers must take into consideration features such as overhanging parts, size of the panel, connection type, component weight etc. that may impact machine assembly. DFA engineers are best positioned to anticipate such problems and inform the PCB manufacturer how to design the panel. The customer does not need to take this into account at all. Failure to panelize correctly could result in certain parts or entire boards requiring hand soldering, warped boards, broken panels, which leads to delays and additional rework costs.

The value of Design for Assembly cannot be underestimated. Whether you are a seasoned designer or a tentative beginner, everyone is prone to making mistakes that may become very costly along the line whether in terms of time or money.

Now you can take advantage of Design for Assembly services from Seeed Fusion, courtesy of Seeed’s own engineers for free. Get peace of mind and ensure your project’s success with a professional assembly partner.

Seeed Fusion also offers many other value-added services to maximise yield, cut down lead times and reduce costs. Functional testing verifies the entire turnkey assembly process and ensures your boards work as intended straight off the production line, and the Seeed Open Parts Library offers thousands of parts at low cost with fast lead times. Get an instant online quotation with Seeed Fusion now!

Seeed Fusion also have a number of sponsorship program available including free prototypes for makers/business users, send us an email via [email protected] if you wish to participate.

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June 2021