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 foreigners 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
Recently, Seeed Fusion began offering free Design for Assembly services as part of their Expedited PCBA service offering. Almost all expedited orders accepted to have 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, like 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.”
“Then there’s silly stuff like you have a 0402 land pattern when the part is 0603. If you’re lucky and it’s just a common 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. For example, all the SMD parts, ICs and all are already reflow soldered but a vital through-hole component doesn’t fit. 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 is 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
What does DFA cover that DFM doesn’t?
DFM reviews are limited to just what is contained in the Gerber files and other supporting documentation. While everything may appear fine to the PCB engineer, they can’t be sure what’s going on the boards or how.
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. Where a DFM review requires a trained individual to identify certain patterns and features from a checklist of items, 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.
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.
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.